learning objectives€¦ · hierarchy of tropical cyclones in each of the seven basins (tropical...

LEARNING OBJECTIVES After reading this chapter, students will:

• Be able to identify the seven ocean basins where tropical cyclogenesis occurs.

• B_e able to identify the generic naming conventions used to describe the hierarchy of tropical cyclones in each of the seven basins (tropical depression, tropical storm, hurricane, etc.).

Gain an appreciation for the history and protocol for generating the lists of specific names used in each of the seven ocean basins.

Be able to assess the six criteria for tropical cyclogenesis in the setting of real eather data.

463

• tropical cyclone • landfall • eye • storm surge • hurricane • typhoon • supertyphoon • severe tropical

cyclone • severe cyclonic

storm • tropical depression • tropical storm • eye wall • tropical disturbance • major hurricane • easterly wave • tropical wave • African easterly

wave • Middle Level African

Easterly Jet • horizontal wind

shear • harmattan winds • lntertropical Front • Cape Verde storm • extratropical low-

pressure system • subtropical cyclone • right-front quadrant • mesovortex • dropsonde • spiral bands • eye-wall

replacement cycle

I I

464 CHAPTER 11 Tropical Weather, Part II: Hurricanes

• Understand the genesis, movement and weather associated with easterly

waves. 1

· • Gain an appreciation for why tropical cyclogenesis is rare in the South At antic

Ocean .

• Understand the positive feedback cycle, Conditional ln~tabilitrot t~e Second Kind, and its role in the development, maintenance and mtens1f1cat1on of tropical cyclones. _ . .

• Understand the underpinning science of storm surge and its relat1onsh1p to landfalling tropical cyclones .

• Understand the physics that explains the existence of a hu_rricane's eye and gain an apprec'iation that winds in the eye are not always 1_1ght. _ _ _

• Understand the role of subtropical high-pressure systems 1n providing steering currents for tropical cyclones. _

• Gain an appreciation for the role that the National Hurrican~ Ce_nter plays in warning the public, and be able to interpret some of the advisories and products issued by NHC.

On March 28, 2004, a rare hurricane over the South Atlantic Ocean ~truck the east coast of Brazil near the to:'111 of Torres abo;ut 800 km ( 500 mi) south of Rio de Janeiro ( see Figure 11.1). Wind speeds estimated by the U. ~. National Hurricane Center at 78 kt (90 mph) unofficially ranked the hurricane as a Category 1 on the Saffir-Simpson Damage Potential Scale ( see Tabl~ 11.1 ). Locally dubbed Hurricane "Catarina" beca~se it came ashore in the· Santa Catarina province of Brazil , the storm killed at least two people , destroyed 500 homes, damaged 20,000 others, and left 1,500 people homeless.

TABLE 11.1 Saffir-Simpson Hurricane Damage Potential Scale

Category Wind (knots; mph) Storm Surge (m; ft)

1: Minimal 64-82 74-95 1.0-1.7 4-5

2: Moderate 83-95 96-110 1.8-2.6 6-8

3: Extensive 96-113 111-130 2.7-3.8 9-12

4: Extreme 114-135 131-155 .3.9-5.6 13-18

5: Catastrophic > 135 > 155 > 5.6 >18

ii@iiJjli• A visible satellite image of a rare hurricane off the east coast of Brazil on March 26, 2004 (courtesy of NASA).

Before meteorologists had reliable satellite imagery (prior to the mid- l 960s ), hurricanes could have formed over the South Atlantic Ocean and nobody would_have ever known (unless they hit land, of course) . But if the last four decades are any indication , there weren't many. Indeed, Catarina was the only hurricane ever observ~d in the South Atlantic Ocean during the modern satelh_;e era Somewhat understandably, Catarina caught Brazi -

. . h b only two ian forecasters by surprise. There ave een . h s thAtlant1c other tropical cyclones observed over t e ou

04 Ocean since the 1960s, in April 1991 and January 2_0 . . h · umver-For the record tropical cyclone is t e generi c,

' th t form over sal name given to low-pressure systems · a ,11 · I h · hapter you warm tropical or subtropical seas . . n t _ is c ' eft of

discover why the South Atlantic 1s virtuall y her . es . h Atl t. c sometnn tropical cyclones , while the Nort an i

teems with them.

CHAPTER 11 Tropical Weather, Part II: Hurricanes 465

4i@liJjf fj (a) Radar reflectivity shows the eye of Hur'.i~ane Ike ~circula~ blue r~gion) starting to move over the Texas Coast at 0654Z on September 13, 2008; (b) Doppler veloc1t1es associated with Hurricane Ike at 0404Z on September 13, 2008. Fast winds

in the lower troposphere blowing toward the radar at Houston, TX (shades of blue and a little purple) were representative of the strong, onshore surface winds that produced a destructive storm surge along the upper Texas coast (courtesy of NOAA).

On animations of satellite images , tropical cyclones display a noticeable cyclonic circulation. They also have an organized area of convection (thunderstorms) around or near their center . Figure 11.2a is a radar reflectivity image from Houston , TX around 07002 on September 13, 2008. A few minutes later, Hurricane Ike made landfall (the storm's center crossed over land) on the northern tip of Galveston Island as a Category-2 hurricane . Maximum sustained winds were approximately 95 kt (about 110 mph). Note the yellow and orange echoes around the western flank of Ike 's well-defined eye , which is the roughly circular island of generally light wind s at a hurricane's core. In this chapter , you'll learn that thunderstorms organizing around (or near) the center of a tropical cyclone are necessary for the lowpressu re system to intensify.

Tropical cyclones attract a great deal of attention from meteorologists, and for good reasons. They destroy property and take human life-tolls that can reach staggering proportion when these powerful storms pass over highly populated, low-lying coastal areas that are vulnerable to storm surge- the wind-driven rush of the sea into coastal areas as a strong tropical cyclone arrives. Figure 11.2b is an image of Doppler velocities around 04002 on September 13, 2008 , a few hours before the ~eflectivity image in Figure 11.2a. This velocity image llldicates a swath of fast winds in the lower troposphere blowing toward Houston and the Texas Seaboard ( shades of blue and a bit of purple). These onshore winds produced a formidable storm surge that destroyed parts of th

e coast (see Figure 11.3). From space , the swirling

j~@hJIJii Photographs of a portion of the upper Texas •••

11••••

1•·-•·•-•-• Coast before (top) and after (bottom) Hurricane

Ike. The yellow arrows point out features that appear in each image (courtesy of USGS)


(a) (b)

@j@jjJIII• (a) Spiral Galaxy Messier 101 bears a striking resemblance to some hurricanes. This awesome image is a composite of about 50 individual exposures from the Hubble Space Telescope (courtesy of NASA and ESA); (b) Hurricane Ivan on September 5, 2004 (courtesy of NASA).

galactic-arm appearance on satellite images, together with an eye staring menacingly into space, absolutely invites ,investigation by the scientifically curious (see Figure 11.4).

Giii.liii~~~~.::;..t!!:.,...~"""'llllll .. Check out the satellite loop of Hurricane Isabel (2003) on the companion CD, and

also a movie of the angry ocean surface under Isabel taken from a hurricane hunter aircraft.

As you probably already know, the most intense tropical cyclones that form in the tropical Atlantic and northeast Pacific Oceans, with sustained surface winds of at least 65 kt (74 mph), are called hurricanes . How about the rest of the world?

TROPICAL CYCLONES: A GLOBAL PERSPECTIVE There are seven ocean basins in which tropical cyclones routinely form: We recommend that you glance at Figure 11.5 while you read through the following list. To get your bearings, we point out that the shaded areas in Figure 11.5 mark the common breeding grounds for tropical cyclones . The red arrows indicate the directions that tropical cyclones typically move within and beyond these breeding grounds.

1. Atlantic Basin (the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea)

2. Northeast Pacific Basin (from Mexico to the International Dateline)

3. Northwest Pacific Basin (from the International Dateline to Asia, plus the South China Sea)

4. North Indian Basin (includes the Bay of Bengal and the Arabian Sea)

5. Southwest Indian Basin (from Africa to about 100°E longitude)

6. Southeast Indian/Australian Basin (100°E longitude to 142°E longitude)

7. Australian/Southwest Pacific Basin (142°E long itude to about 120°W longitude)

Figure 11.5 contains a lot of information that we will explore in this chapter. One observation that we'd like to make right off the bat is that tropical cyclones do not form on the equator. Also note the absence of trop ical cyclones in the South Atlantic Ocean. Catarina was a rare bird indeed .

Strong tropical cyclones go by different names in the other five basins. In the Northwest Pacific Basin , forecasters use typhoon to label a tropical cyclone whose maximum sustained winds have reached the hurricane threshold of 65 kt (74 mph), and supertyph oon if wind speeds reach 130 kt (150 mph). In the Sout~west Pacific Ocean west of l 60°E longitude and also in

the Southeast Indian Ocean east of 90° longitude (gets a little dicey, eh?), strong tropical cyclones with hurricane-


60N

45N

30N

15N

EQ

15S

30S

45S

60S

0 30E 60E 90E 120E 150E 180 150W 120W 90W 60W 30W

Ni@!ililFI The primary global breeding grounds for tropical cyclones (shaded areas) and the typical tracks they take (red arrows).

force winds are known simply as severe tropical cyclones (Austra lian forecasters refer to "weaker" hurricanestrength storms simply as cyclones). Continuing on our world tour, we note that forecasters call a strong tropical cyclone in the North Indian Ocean a severe cyclonic storm. Finally, in the Southwest Indian Ocean, the generic tropical cyclone is the chosen designator.

From "hurricane" to "typhoon" to "severe cyclonic storm," the labels that meteorologists attach to strong tropical cyclones vary widely across the globe. As it turns out , the lists of names that forecasters use to distinguish storms that form in a particular basin also vary across the globe. Before we delve into these naming conventions, we first need to trace the life cycle of a typical tropical cyclone because, in some ocean basins, assigning a name depends on the cyclone's stage of development.

Naming Tropical Cyclones: Ham with Chicken Livers and Mushrooms On the way to becoming a hurricane, an intensifying tropical low-pressure system evolves from a tropical depression to a tropical storm. A tropical depression is a tropic al cyclone that has an observable cyclonic circulation on satellite imagery and whose maximum sustained winds are 33 kt (38 mph) or less. A tropical storm is a tropical cyclone with maximum sustained Winds of 34- 63 kt (39- 73 mph). Of these different stages of development, only hurricanes are distinguished by the characteristic eye in the center of the circulation.

Globally each year, approximately 80 to 90 tropical cyclones reach tropical storm intensity, with about two-thirds of these attaining the threshold of 64 kt (74 mph) to qualify as a hurricane ( or typhoon, etc.). In order to efficiently communicate information about these systems and avoid confusion when more than one is active at the same time, meteorologists give them a number-letter tag when they become a tropical cyclone. For example, Tropical Depression 2-E would be the second tropical cyclone of the season in the Northeast Pacific Basin. This generic naming convention never changes.

Most tropical cyclones receive a name once they reach tropical storm intensity. Table 11.2 shows the six alphabetized lists ofnames currently used by the National Hurricane Center for the North Atlantic Basin. The lists of names are recycled. So, for example, the 2009 list will be used again in 2015. The only exception occurs when hurricanes or tropical storms are extremely destructive or deadly. In these cases, their names are "retired " from the list because using the name again for a future storm would be inappropriate and /or insensitive. The World Meteorological Organization, · a specialized agency of the United Nations headquartered in Geneva, Switzerland, then chooses a new name as a replacement. A recent example of a retired name is Dean in 2007, which will be replaced by Dorian in 2013. Prior to the 2008 hurricane season , approximately 70 names had been retired from the North Atlantic lists of names, including Agnes (1972), Andrew (1992), Tropical Storm Allison (2001),

J


TABLE 11.2 Atlantic Basin Tropical Storm and Hurricane Names

2009 2010 2011

Ana Alex Arlene

Bill Bonnie Bret

Claudette ' Colin Cindy

Danny Danielle Don

Erika Earl Emily

Fred Fiona Franklin

Grace Gaston Gert

Henri Hermine Harvey Ida Igor Irene

'~1Joaquin Julia Jose

/

? Kate Karl Katia

Larry Lisa Lee Mindy Matthew Maria Nicholas Nicole Nate Odette Otto Ophelia Peter Paula Phillippe Rose Richard Rina

Sam Shary Sean

Teresa Tomas Tammy

Victor Virginie Vince Wanda Walter Whitney

Isabel (2003) , Charley, Frances, Ivan and Jeanne (2004), Dennis, Katrina, Rita, Stan and Wilma (2005), and Dean, Felix and Noel (2007).

The custom of naming tropical storms and hurricanes has an interesting history. Contrary to popular belief, the practice of naming Atlantic hurricanes dates back a few hundred years to the West Indies (the three main groups of islands that comprise the West Indies are the Bahamas, the Greater Antilles, and the Lesser Antilles) . In the nineteenth century, islanders began to name hurricanes after saints. Indeed, when hurricanes arrived on a day commemorating a saint, locals christened the storm with the saint's name. For example, fierce Hurricane Santa Ana struck Puerto Rico on July 26, 1825. Hurricane San Felipe (the first) and Hurricane San Felipe (the second) hit Puerto Rico on September 13, 1876 and September 13, 1928, respectively .

During World War II, Navy and Army Corp forecasters informally named Pacific storms after their girlfriends or wives . The spirit of this informal naming convention apparently started a trend in the United

2012 2013

Alberto Andrea Arthur

Beryl Barry Bertha

Chris Chantal Cristobal

Debby Dorian Dolly

Ernesto Erin Edouard

Florence Fernand Fay

Gordon Gabrielle Gustav

Helene Humberto Hanna

Isaac Ingrid Ike

Joyce Jerry Josephine

Kirk Karen Kyle

Leslie Lorenzo Laura

Michael Melissa Marco

Nadine Nestor Nana Oscar Olga Omar Patty Pablo Paloma

Rafael Rebekah Rene

Sandy Sebastien Sally

Tony Tanya Teddy

Valerie Van Vicky William Wendy Wilfred

States. From 1950 to 1952, meteorologists named trop ical cyclones in the Atlantic Basin according to the phonetic alphabet (Able, Baker, Charlie, etc.). Then, in 1953, the U.S. Weather Bureau switched to an alphabetized list offemale names. In 1979, the National Weather Service amended their lists to also include male names.

In 1959, forecasters monitoring the central Pacific Ocean started to use female names to designate tropical storms and hurricanes that formed near Hawaii. The rest of the Northeast Pacific Basin followed suit in 1960. In 1978, the lists of names for tropical storms and hurrican es in these basins were amended to include male names as well.

In the Northwest Pacific Ocean, forecaste rs began using female names for tropical cyclones in 1945. In tandem with the 1979 change in the United States, they also revised their lists to include male names. On January 1 2000, however, in a dramatic departure from tra-

' . · · · the dition , forecasters from the nations and ternton es m . f Northwest Pacific Basin agreed to use a markedly di -ferent naming convention. From that day forward, ~e

. 1 r 1e-new lists did not (for the most part) contam ma e 0


male names. Instead, they included Asian words that referred to flowers, animals, birds, trees, foods, etc., while other names are simply descriptive adjectives .

For example, consider Typhoons Ketsana and Parma, which patrolled the Northwest Pacific Basin in late October 2003 (see Figure 11.6) . Ketsana (contributed by Lao People's Democratic Republic) means "a kind of tree," and Parma ( contributed by Macau, China) means "ham with chicken livers and mushrooms"! Hungry for a tropical cyclone, anyone? Table 11.3 shows the names for this basin - you'll note that they do not appear in alphabetical order. Rather, the contributing nations are listed in alphabetical order. This list of countries determines the order in which names are assigned .

Before October 2004 , tropical cyclones that formed in the North Indian Ocean were not named from a traditional list. For this basin, forecasters simply used a label consisting of a two-digit number and letter that the cyclone received once it attains tropical depression status. For example, "Tropical Cyclone 02A" would be the second tropical cyclone of the season to form over the Arabian Sea. "Tropical Cyclone O 1 B" would be the first tropical cyclone of the season to form over the Bay of Bengal.

One of the most infamous named storms in the North Indian Ocean was Severe Cyclonic Storm Gonu, which developed over the Arabian Sea in early June 2007 (see Figure 11. 7). On June 4, Gonu's maximum sustained winds reached 140 kt (161 mph), compelling forecasters to upgrade Gonu to a super cyclonic storm. As of this writing, Gonu stands as the most powerful tropical

, A visible satellite image of Typhoons Parma and - ~ ~ Iii Ketsana on October 24, 2003. The southern tip of Japan appears in the upper left of the image (courtesy of NASA)

@i@Mil•• A visible satellite image of Super Cyclonic Storm Gonu at 09Z on June 4, 2007. On this date,

Gonu's maximum sustained winds reached 140 kt (161 mph), making it the strongest tropical cyclone ever to develop in the North Indian Ocean (courtesy of MODIS Rapid Response Project at NASA/GSFC).

cyclone ever to develop in the North Indian Ocean. As Gonu moved toward the Arabian Peninsula, the storm weakened considerably as it drew dry air over the desert into its circulation.

The notion that dry air circulating into Gonu would cause the storm to weaken is part of a much broader discussion about the conditions that are favorable ( or unfavorable) for the genesis and development of tropical cyclones. Now that we've played the "name game," let's investigate these conditions.

RECIPE FOR HURRICANES: SIX INGREDIENTS IN JUST THE RIGHT MEASURE So far you've learned that tropical cyclones have an observable cyclonic circulation on loops of satellite imagery.

~ C~e~k out th~ companion C~ to watch a ~ stnkmg satellite loop of a hurncane.

There are also organized thunderstorms concentrated around or near their centers . ·w e will elaborate on the role that organized convection plays in the intensification of tropical cyclones later in this chapter. First, however, we will discuss the six ingredients required for the genesis of tropical cyclones. For students interested in a tropical cookbook, here 's the recipe:


TABLE 11.3 Northwest Pacific Tropical Storm and Typhoon Names

The names for tropical storms and typhoons that develop in the Northwest Pacific Basin are Asian words submitted by countries in the region. The names are compiled into "running lists": After all the names in one list are used, the name of the next storm that develops is simply the first word in the next list.

NORTHWEST PACIFIC BASIN

Contributor II

Cambodia Damrey Kong-rey China Haikui Yutu DPR Korea Kirogi Toraji

HK, China Kai-Tak Man-yi Japan Tembin Usagi

~Lao PDR Bolaven Pabuk

Macau Sanba Wutip Malaysia Jelawat Sepat

Micronesia Ewiniar Fitow Philippines Malaksi Danas RO Korea Gaemi Nari Thailand Prapiroon Wipha U.S.A. Maria Francisco Vietnam Son-Tinh Lekima Cambodia Bopha Krosa China Wukong Haiyan DPR Korea Sonamu Podul HK, China Shanshan Lingling Japan Yagi Kaziki Lao PDR Leepi Faxai Macau Be bin ca Peipah Malaysia Rumbia Tapah Micronesia Soulik Mitag Philippines Cimaron Hag ibis RO Korea Jebi Neoguri Thailand Mangkhut Rammasun U.S.A. Utor Matmo Vietnam Trami Halong

1. Sea-surface temperatures of26.5°C (80°F) or higher and a relat ively deep layer of warm water beneath the ocean surface

2. Conditional instability through a deep layer of the troposphere

3. Moist air in the middle troposphere

Ill IV V

Nakri Krovanh Sarika Fengshen Dujuan Haima Kalmaegi Mujigae Meari Fung-wong Choi-wan Ma-on

Kanmuri Koppu Tokage Phanfone Ketsana Nock-ten Vongfong Parma Muifa Nuri Melor Merbok Sinlaku Nepartak Nanmadol Hagupit Lu pit Talas Jangmi Mirinae Noru Mekkhala Nida Kulap Higos Omais Rake Bavi Canson Sonca Maysak Chanthu Nesat Haishen Dian mu Haitang Noul Mindulle Nalgae Dolphin Lionrock Banyan Kujira Kompasu Washi Chan-horn Namtheun Pakhar Linfa Malou Sanvu Nangka Meranti Mawar

Soudelor Fanapi Guchol Malave Malakas Talim Goni Megi Doksuri Morakot Chaba Khanun Etau Aere Vicente

Vamco Songda Saola

4. Weak vertical wind shear above the newly forming tropical cyclone

5. A genesis location that lies at least 5 ° latitude away from the equator

6. A group of initially disorganized showers and

thunderstorms

CHAPTER 11 Tropical Weather, Part 11: Hurricanes 471

li@i;Jii•:j fhe long-term average of sea

surface temperatures (in °C) in the Atlantic Basin from June 1 to November 30 (courtesy of NOAA).

Average Sea-Surface Temperature (°C) - June to November

22 23 24 25 26

Don't get nervous. Although these ingredients might seem daunting at first, they all relate to the two characteristics that all tropical cyclones display-organized convection around or near their centers and an observable cyclonic circulation on satellite imagery. Keep these two traits in mind as we explore each of the six ingredients.

High Sea-Surface Temperatures: The Foundation for Organized Thunderstorms Figure 11.8 shows average sea-surface temperatures (SSTs) over the North Atlantic Ocean from June 1 to November 30, the period that corresponds to official hurricane season in this basin. Note the corridor of water temperatures of 26.5°C (80°F) or higher that extends from the west coast of Africa across the tropical Atlantic to the Caribbean Sea and the Gulf of Mexico (roughly, the yellows and oranges). This corridor marks "hurricane alley," where tropical cyclones routinely develop during hurricane season.

The heart of Atlantic hurricane season is August to October, when there's a marked upturn in the frequency of tropical cyclones. In light of Figure 11.9, which shows the daily frequency of Atlantic hurricanes and tropical storms, it's obvious that hurricane season peaks during the second week of September. Not coincidentally , seasurface temperatures in the North Atlantic are also at their highest during this month ( see Figure 11.10).

High sea-surface temperatures ensure high evaporation rates and thus high dew points in the lower troposphere. High water temperatures also help to destabilize the lower troposphere by providing a "warm bottom."

27 28 29

Any way you slice it, high SSTs pave the way for deep convection, which newly forming and intensifying tropical cyclones require around their centers.

To close the deal on the pivotal role that high sea-surface temperatures play in the genesis and development of tropical cyclones, we point out that Hurricane Charley rapidly intensified to a Category-4 storm (110 kt to 125 kt in just three hours) as it bore down on the west coast of Florida on August 13, 2004 (see Figure 11.11 ). Figure 11.12 shows a satellite-based analysis of seasurface temperatures on August 12, 2004. Note the very warm water just off Florida's southwest coast. Indeed,

;: 0 ;: 0 ~ 0 ~ 0 ... N ... N ..... N ... N Qi

WI 1111 0. .... .... > u u C ::, ::, Qi u u 0 Qi Qi ~ < < v> 0 0 z Q Q

110 I'.!• 100 : :

> 90

~ ' 80 70 ~

0. 60 e 50

~ 40 30

... o :

20 1:: 10 E '

::, : 0 :z

Hurricanes and Tropical 5 tor ms

• Hurricanes NOAA

l"ll!lll""'!l"W!la The daily frequency of hurricanes (yellow) and If \M'lilllf P both hurricanes and tropical storms (red) in the Atlantic Basin, per 100 years. Climatologically, the peak of the tropical season in the Atlantic Basin is around September 10 (courtesy of NOAA).


FIGURE 11.10 The long-term average of sea

surface temperatures (in °C) in the Atlantic Basin during September (courtesy of NOAA).

Average Sea-Surface Temperature (°C)- September

22 23 24 25 26

SSTs in the neighborhood of 32°C (almost 90°F) lay in the path of Hurricane Charley. The stage was set for rapid intensification once Charley moved over these very warm waters ( other factors also likely contributed to Charley's rapid intensification).

Not only must sea-surface temperatures be high for tropical cyclones to form and develop, but a fairly deep

FIGURE 11.11 A composite radar image of Hurricane Charley along Florida's southwest coast at 21 Z on August

13, 2004, just after the storm came ashore near Cayo Costa with maximum sustained winds near 125 kt (145 mph) (courtesy of WSI Corporation).

27 28 29

layer of water below the surface must also be warm. Let's investigate.

Warm Water Below: When the Wind Stirs the Sea A newly forming but slowly moving tropical cyclone can sputter and fade before it ever really develops. To see how, let's start on the high end of tropical cyclones by focusing our attention on Hurricane Nora, which patrolled the Northeast Pacific Basin from September 16-26, 1997, off the southwest coast of Mexico (see Figure 11.13).

From midday on September 18 to early on September 20, the storm essentially stalled just off the coast. Dur-

Sea-Surface Temperature (°C)-August 14, 2004

26 27 28 29 30 31 32

FIGURE 11.12 A satellite-based analysis of sea-surface temperatures on August 12, 2004. Note the very

warm water (shown here in yellow) off Florida's southwest coa5t

(courtesy of NOAA).

33


FIGURE 11.13 A visible satellite image of Hurricane Nora off the west coast of Mexico in September 1997 (courtesy of NOAA).

ing this time, Nora's maximum sustained winds decreased from 90 kt (104 mph) to 65 kt (75 mph). Why would this weakening occur? With Nora's winds continuing to agitate and mix the same area of sea, cooler water upwelled from depths of several tens of meters (while warmer surface water also mixed downward). In response to the upwelling, sea-surface temperatures in this region decreased by as much as 2°C (3.6°F), creating a pocket of anomalously cool water off the southwestern coast of Mexico that persisted for days (see Figure 11.14). As a result of cooler surface water, evaporation rates lowered, the atmosphere stabilized a bit, and organized, tall thunderstorms around Nora's eye began to collapse. In other words, the hurricane weakened.

With this example in mind, you can imagine what might happen if a newly forming tropical cyclone encountered tropical seas where the layer of warm water beneath the surface was relatively shallow. Indeed, as winds began to strengthen, cooler water would quickly upwell to the sea surface, in effect limiting further development. After all, if fully developed hurricanes such as Nora take a big hit, imagine the effects on a newly forming and rather fragile tropical cyclone. So you can see why tropical meteorologists stipulate that high sea-surface temperatures must be accompanied by a relatively deep warm layer below the ocean surface in order to promote genesis of tropical cyclones.

30N ·

20N

Pacific Ocean

~-IOM- ClRES/ Cllrnot,,- OlognosUoa Cent.er ..._,.,,

Mexico

Sea-Surlace Temperature Anomaly (°C) , October 1, 1997

-2 -1 0 2

FIGURE 11.14 The departure of sea-surface temperature (in °C) from average on October 1, 1997, still

reflected the cooling of the ocean surface along the track of Hurricane Nora. The more southern pocket of cooler-than-average water marks the region where Nora stalled. The second pocket of relatively low sea-surface temperatures just south of Baja California reflects the upwelling of cool water associated with Nora's re-intensification on September 21 when 115-kt (130-mph) winds really stirred up the sea (courtesy of NOAA).

Incidentally, hurricanes and tropical storms that pass over the cool wake of a previous storm can also lose strength. Again, the organized convection around the center of such a tropical cyclone would likely weaken, with the degree of weakening depending on how cool the water was and how long the storm stayed over the cool wake.

These arguments about the role of water temperatures in the intensification ( or weakening) of tropical cyclones assume, of course, that the other ingredients on the tropical cyclone recipe don't affect the storm's intensity. Speaking of other ingredients, let's move on to the second item-the atmosphere must _be conditionally unstable through a deep layer of the troposphere .

Conditional Instability: Paving the Way for Tall Thunderstorms Truth be told, high sea-surface temperatures go handin-hand with a conditionally unstable troposphere. In other words, the two ingredients are not independent. That 's because high SSTs ensure high evaporation


rates, which , in tum -, ensure that the air above the warm ocean is teeming with water vapor. In Chapter 8, yo~ learned that high dew points in parcels ~ear the earth s surface mean that net condensation readily occurs aft~r a relatively short ascent. Such rising parcels can remam positively buoyant to great altitudes because lar~e releases oflatent heat keep parcels warmer than their environment through a deep layer of the troposphere. Such ascent to great altitudes is pivotal for tall thunderstorms to form and organize around the center of a newly form-

ing tropical cyclone. _ . Although high sea-surface temper~tures and_condit1onal

instability are not independent , we hst Ingred1_ent #2 separately here because it gives us the opportumty to sho~ you how tall thunderstorms around the center of a tropical cyclone help the cyclone to develop.

By the time air parcels reach the tops of thundersto~s (we're talking very high altitudes here), most of their water vapor has been depleted. Hold _this thought fo_r a moment. Figure 11.15 indicates that air parcels reach~ng the tops of thunderstorms near the center of a developmg tropical cyclone take one of two routes. Some parc_els move outward away from the developing center, creatmg upper-level divergence, which helps to maintain the low weight of the central air columns.

Check out "Hurricane Structure" on the ei ~ ~ =--11) companion CD; it shows the movement of

air parcels within a hurricane.

Alternatively , some parcels sink into the central column of the tropical cyclone. Because these parc~ls have very little water vapor , they warm by compr~ss10n at a rate close to dry adiabatic. In response, the air h_eats up dramatically. As a result, the central air column m a developing tropical cyclone has the highest average temperature and, accordingly , the lowest average column

FIGURE 11.15 Some air parcels reaching the top

of thunderstorms around the center of a newly forming tropical cyclone sink into the central air column. Because they are practically devoid of water vapor, these sinking air parcels warm dramatically at a rate close to dry adiabatic.

density and weight. In other words, there is relatively

low pressure at the surface. The taller and more organized the thunderstorms be-

come around the center of a tropical cyclone, ~he greater the compressional warming during descent m the central column. Thus, as thunderstorms around the center of a tropical cyclone grow taller , the surface pressu~e tends to decrease. Figure 11.16 is a color-enhanced mfrared satellite image of Super Cyclonic Storm ?onu at l SZ on June 4, 2007 . The circular yellow shadmg around the

FIGURE 11.16 A color-enhanced infrared satellite image of Super Cyclonic Storm Gonu at 18Z on June

4 2007 (from the Meteosat-7 geostationary satellite). The yello~oF) shading which represents cloud-top temperatures of -soac (-11f

' · th ye wall o or lower, marks the tops of tall thunderstorms in e e Gonu (courtesy of NOAA and the Naval Research Laboratory).


eye of Gonu represents cloud-top temperatures of -80 °C (-l 12°F) or lower, which, in turn, indicates a ring of very tall thunderstorms around the eye, formally called the eye wall .

As ferocious as the tall , eye-wall thunderstorms might seem, the updrafts that sustain these thunderstorms are relatively tame. To explain this apparent paradox, recall from Chapter 9 that the strength of a thunderstorm's updraft depends largely on the degree of positive buoyancy--essentially , the difference between the temperature of a rising air parcel and the temperature of the environment. Over warm tropical seas , the environmental lapse rate in the middle troposphere is pretty close to the moist adiabatic rates that rising air parcels typically follow there. As a result , positive buoyancy is often surprisingly small inside eye-wall thunderstorms. Thus, the updrafts that sustain them tend to be rather weak.

Relatively weak updrafts in the eye wall help to explain the general lack of lightning around the eye of a hurricane. These weak updrafts are rather inefficient at separating electrical charges- that is, creating regions with large positive and large negative charges that are necessary to produce lightning. As an example , the eye wall of Hurricane Andrew, the last Category-5 hurricane to hit the United States, produced cloud-to-ground lightning at a rate less than ten strikes per hour between the time it was over the Bahamas and the time it made its second landfall in Louisiana ( see Figure 11.17). There were even hours without any strikes at all! By comparison, thunderstorms 100 km (60 mi) or more from a hurricane's core can produce cloud-to-ground lightning at rates as high as several hundred strikes per hour.

Even though they have relatively weak updrafts, tall eye-wall thunderstorms are integral cogs in the powerful hurricane machinery (more details later in the chapter) . For now, just keep in mind that high sea-surface temperatures, a relatively deep layer of warm water below the surface, and conditional instability through a deep layer of the troposphere, are primary ingredients for cooking up a tropical cyclone. There's one other thermodynamic ingredient (related to temperature and moisture) crucial to the development of a tropical cyclone.

Ingredient 13: A Moist Middle Troposphere The color-enhanced water vapor image in Figure 11.18a shows Super .Cyclonic Storm Gonu over the Arabian Sea at 18Z on June 4, 2007. At the time, Gonu's maximum sustained winds were 140 kt (161 mph) , making it the strongest tropical cyclone ever to form over the North Indian Ocean. In this image, the blue shading indicates dry air in the middle and upper troposphere. Within the next several hours, as Gonu moved westward toward the Arabian Peninsula and started to draw dry, mid-tropospheric air into its circulation , the historic storm took a big hit. By 1230Z on June 5 (Figure 11.1 Sb), Gonu's maximum winds had weakened to 105 kt (121 mph). The tropical cyclone continued to weaken as it ingested more and more of this low-humidity air from high over the deserts of the Middle East. The message here should be loud and clear- dry air in the middle troposphere is poisonous to hurricanes.

Logically , if hurricanes take a hit when they ingest dry air in the middle troposphere, imagine the effect of

FIGURE 11.11 The track of Hurricane Andrew,

August 16-28, 1992. Positions are shown every six hours. The color of the dots corresponds to the tropical cyclone's intensity, from tropical depression to tropical storm to the five Categories on the SaffirSimpson scale (courtesy of Wikipedia).


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(a) (b)

FIGURE 11.18 (a) A color-enhanced water vapor image of Super Cyclonic Storm Gonu over the Arabian Sea at 18Z on June 4, 2007 (from the Meteosat-7 geostationary satellite). Maximum sustained winds were 140 kt (161 mph) at the time. The blue

indicates dry air in the middle and upper troposphere over the Arabian Peninsula and other parts of the Middle East; (b) A color-enhanced water vapor image of Gonu about 18 hours later. By this time, the storm had weakened after ingesting dry, mid-tropospheric air over the Middle East. Maximum sustained winds had dropped to 105 kt (121 mph) (courtesy of the Naval Research Laboratory).

dry, mid-level air on a newly forming and somewhat fragile tropical cyclone. It can't be good for development. To understand why, consider that the seed for most tropical cyclones is typically a group of disorganized showers and thunderstorms that meteorologists call a tropical disturbance . Until thunderstorms become better organized around a single, well-defined center of cir -culation, the system's development will be limited. That's because the disorganized warming associated with air sinking around disorganized thunderstorms near the core of the disturbance tends to cause pockets oflow pressure to jump around like a hot baked potato being tossed around the dinner table.

When a newly forming tropical cyclone mixes in, or entrains, dry air from the middle troposphere, chances are that it will fizzle. Why is that? Mid-level dry air entrained into thunderstorms enhances evaporational cooling, so parcels become more negatively buoyant. This leads to stronger downdrafts , which are able to penetrate farther down into the lower troposphere, bringing cooler, drier air along with them. This downward intrusion acts to snuff out new convection in the vicinity of the core of

the disturbance. To the extent that new convection is pivotal for the development of the disturbance into a tropical cyclone, the die is cast and the disturbance fizzles ( or at least doesn't develop any further).

Now that we've discussed all three of the thermodynamic ingredients, let's move on to the dynamic ingredients ( related to air motions) needed for the genesi s and development of tropical cyclones.

Vertical Wind Shear: A Little Goes a Long Way Figure 11.19 is a multi-channel ( combination visible/ infrared) satellite image of Tropical Storm Nichola s at l 5 l 5Z on October 20, 2003. On this type of image, the yellowish shades represent low clouds, while bright white typically marks the tops of cumulonimbus clouds (a duller, bluish white usually indicates cirrus clouds).

Note the swirl of yellowish low clouds that marks the low-level circulation of Tropical Storm Nicholas (keep in mind that all tropical cyclones have a cyclonic ci~culation that's typically observable on loops of satellite un-

CHAPTER 11 Tropical Weather , Part II: Hurricanes 477

FIGURE 11.19 A multi-channel satellite image of Tropical Storm Nicholas in the Atlantic Ocean on October 20,

2003. Yellowish tones indicate clouds with low tops, while high cloud tops appear as white or whitish gray. At the time, Nicholas was northeast of Guyana in South America (courtesy of NOAA).

agery). A single multi-channel satellite image can occasionally accomplish the same result when a tropical cyclone's low-level circulation becomes "exposed." This happens when thunderstorms near and around the center of circulation move away from the storm's inner core. Given that we can observe Nicholas 's "exposed" lowlevel circulation on Figure 11.19, we can infer that the storm was in a "highly sheared environment" at this time (we'll explain what this means in just a moment). First, a final point about Nicholas. As you have already learned, the lack of deep convection around the inner core of a tropical cyclone suggests that the system is doomed. Not surprisingly, the National Hurricane Center downgraded Nicholas to a tropical depression on October 23.

If you had an inking that a "highly sheared environment" referred to vertical wind shear, then you are right on the money. As you recall from Chapter 9, vertical wind shear is a change in wind speed and/or wind direction with increasing altitude. In Nicholas' case, it was northwesterly shear that exposed the storm's low-level circulation and set the stage for the tropical cyclone to dissipate. Consider the analysis of winds at the 200-mb level (about 12,500 m) on October 20, 2003, in Figure 11.20a, three hours before the satellite image. Note the northwesterly Winds (represented by arrows) in the vicinity ofNicholas.

These high-altitude winds essentially helped to push the deep convection southeastward away from the low-lev 1 circulation around the center of Nicholas. e

Meanwhile, at the 850-mb level (about 1500 m) the winds blew pretty much from the southeast at m;dest speeds compared to 200 mb (see Figure 11.20b). With modest southeasterlies at 850 mb and stronger northwesterly winds at 200 mb, the low-level circulation associated with Tropical Storm Nicholas got separated from the tall thunderstorms that previously formed around the center of the tropical cyclone. Such is the disruptive power of vertical wind shear.

In the context of hurricane genesis, the most relevant aspect of wind shear is the change in wind speed with altitude. To calculate the vertical wind shear in the layer between 850 mb and 200 mb, we simply subtract the wind vector at 850 mb from the wind vector at 200 mb. Thus, wind shear has the same units as wind speed-perhaps mis or mph. Calculating the direction of the wind shear is a bit more complex because it involves breaking down the winds at 850 mb and 200 mb into their east-west and north-south components.

We '11 dispense with the details and show you Figure 1 l .20c, which is the average 850-200 mb wind shear for October 20, 2003. As a general rule, values of vertical wind shear less than 10 m/s (in white here) are sufficiently low to allow genesis and development of tropical cyclones; in other words, these values correspond to weak vertical wind shear. Clearly, the northwesterly vertical wind shear on this date was strong enough to separate Nicholas' deep convection from the storm's low-level center of circulation.

Figure 11.21 shows the long-term average 850-200 mb vertical wind shear over the Atlantic Basin during the heart of hurricane season, from August 15 to October 15. In the areas in white, vertical wind shear is less than 10 mis, indicating that favorable wind-shear conditions prevail during this time ( on average) over much of the basin. Given the region's favorable thermodynamic environment, tropical cyclones commonly form and intensify here during this time. Keep in mind that Figure 11.21 shows average conditions during a two-month period-wind shear over any specific region at any specific time can be more or less favorable for tropical cyclogenesis (tropical cyclqne formation and/or intensification), depending on the prevailing weather pattern.

The bottom line here . is that vertical wind shear over a tropical disturbance should be weak if the disturbance is ever going to develop into a tropical cyclone. This debate about vertical wind shear becomes moot, however,


/) I \_ 200-mb Winds (m/s}

5 10 15 20

850-200 mb Wind Shear (m/s)

10 14 18 22 26 30 (c)

25 30 35 40

FIGURE 11.20 The wind direction (arrows) and wind speed (color-coded in m/s) at 12Z on October 20,

2003 in the vicinity of Tropical Storm Nicholas at the (a) 200-mb level,,and (b) 850-mb level; (c) The average 850-200 mb wind shear for October 20, 2003 (courtesy of NOAA).


Average 850-200 mb Wind Shear (m/s), August 15- October 15

10 14 18 22 26 30

FIGURE 11.21 The long-term average vertical wind shear between 850 mb and 200 mb from August 15

to October 15. Arrows represent the direction of the wind shear, while the magnitude of the wind shear is color-coded in m/s (courtesy of NOAA).

if the tropical disturbance is too close to the equator, which leads us to our next ingredient.

Rotation Needed: Stay Away from the Equator! Throughout the last few sections, we focused our attention on the two basic characteristics that all developing tropical cyclones display: an observable low-level circulation, and organized thunderstorms around the center of that circulation. We haven't yet given much ink to the cyclonic circulation itself, primarily because we defined a tropical cyclone using the words "low-pressure system," so the two already go hand-in-hand.

But there's an underlying assumption here that is not obvious: For the circulation to form in the first place, the Coriolis force must be strong enough to deflect the

120E 140E 160E 180 160W

air to the right ( or to the left in the Southern Hemisphere) . Without a sufficiently strong Coriolis force, air would move almost directly toward the center of the low rapidly adding mass to the air column over the low and thus causing air pressure to increase. In effect, a tropical disturbance too close to the equator would not have the opportunity to develop a low-level circulation and become a tropical cyclone.

Figure 11.22 dramatically illustrates our point. Here, the Indian and Pacific Oceans were partitioned into boxes measuring one degree latitude by one degree longitude. Then the number of times that a tropical cyclone passed through each box during the 30-year period from 1972 to 200 I was counted. This chart displays that frequency on an annual basis, with the dark red splotches marking the m·ost active areas for tropical cyclones, dark blue indicating relatively low activity, and white representing areas that, for all practical purposes, had zero activity. Note the absence of tropical cyclones on and near the equator .

As a generally steadfast rule, tropical cyclones do not form within 5° latitude of the equator - the Coriolis Force is simply too weak there. If you look very closely at Figure 11.22, you can see an exception to this rule along the southern tip of Malaysia near 1.5°N latitude, l 00°E longitude . That narrow blue swath represents the track of Typhoon Vamei in late December 2001, which spun up only about 160 km ( 100 mi) from the equator (see Figure 11.23a). Despite an almost-nil Coriolis force at 1.5°N latitude, Typhoon Vamei was so close to the equator that its winds howled in both hemispheres simultaneously (they were stronger in the Northern Hemisphere, of course). How could this formidable circulation develop at such a low latitude?

140W 120W 100W · BOW

FIGURE 11.22 The average annual frequency

of tropical cyclones over the Indian and Pacific Oceans from 1972 to 2001. The dark red splotches mark the most active areas for tropical cycl,bnes; dark blue indicates very low activity, while white represents areas virtually unaffected by tropical cyclones. Note the absence of activity on and near the equator (courtesy of Chris Cantrell, Joint Typhoon Warning Center).


FIGURE 11.23 An infrared satellite image at 0335Z on December '~ } 27, 2001, shows Typhoon Vamei centered at 1.5°N

latitude near Singapore. At this time, Vamei's circulation spanned both hemispheres. Surface streamlines (in black) indicate how the lay of the land conspired to produce a low-level spin that, when combined with an arriving group of showers and thunderstorms, set the stage for the very unusual development of a typhoon not far from the equator (courtesy of CRISP/National University of Singapore).

The best answer is simply: "the lay of the land and water." Prior to the formation ofVamei, persistent northnortheasterly winds blew over the narrowing South China Sea (streamlines are shown in Figure 11.23b). This funneling of the air led to a strengthening of the wind, like toothpaste squirting from the tube after you squeeze it. Moreover, the Malaysian Peninsula channeled the air into a cyclonic pattern, creating an "artificial" source oflowlevel cyclonic circulation. Note the winds cyclonically swirling near the southern tip of Malaysia in Figure 11.23b. This "artificial" low-level spin, in tandem with a timely cluster of showers and thunderstorms that migrated over this region, set the stage for an unusual storm that defied all the textbooks. Vamei was compact by typhoon standards-the area of open water where it developed was only about 500 km (310 mi) in breadth. But Vamei will live in infamy in Singapore, where government officials told residents that the bad weather was the result of an "an unusual cluster of severe thunderstorms." At least they got the "unusual" right. Scientists calculated that the odds of such a storm developing are about one in every 100 to 400 years .

Speaking of longshots, can a tropical cyclone ever cross the equator? To our knowledge, none has, but a few have come close, particularly in the North Indian Ocean ( check back to Figure 11.22). Is it possible? We would say "yes" because a tropical cyclone's large cyclonic circulation would not initially be affected by the weak change in the Coriolis force as it crossed into the opposite hemisphere. But there's another factor related to the variation of the Coriolis force with latitude that works against any crossover. Although this "other factor" ( called the Beta effect) is beyond the scope of this textbook, we at least wanted to address the issue because we often get questions about hurricanes crossing the equator from inquisitive students.

With these extreme but interesting issues put to rest, let's fa k generic. What is the primary source for the groups o disorganized showers and thunderstorms that can move to th~odynamically favorable environments and increase the chance for tropical cyclones to spin up over the North Atlantic and Northeast Pacific Basins? And, lest we forget, why aren't there any hurricanes in the South Atlantic Ocean, save for that rare storm that hit Brazil in March 2004? Let's investigate.

Easterly Waves: Out of Africa Recall that a group of disorganized showers and thunderstorms over the tropics is formally called a tropical disturbance. Think of a tropical disturbance as a spark that can ignite a favorable thermodynamic environmen t and pave the way for a tropical cyclone to form.

Over the North Atlantic Ocean, approximately 60 percent of the tropical storms and "minor" hurricane s (Categories 1 and 2 on the Saffir-Simpson Scale) are initiated by tropical disturbances that move westwa rd from Africa. These African disturbances also initia te nearly 85 percent of all Atlantic hurricanes that reach Category-3 strength or higher, which the National Hurricane Center classifies as major hurricanes . If that's not enough to convince you that these tropical dis~u~bances from Africa are worthy of our scien tlf 1c

scrutiny, consider that some meteorologists believe that nearly all the tropical cyclones that form in the N or~heast Pacific Basin owe their existence to tropica l disturbances from Africa.

These tropical disturbances that move westwar d from Africa are commonly called easterly waves. Forrnall~, an easterly wave is a tropical disturbance ( disorg anized showers and thunderstorms) that has a cyclonic circulation in the lower part of the middle troposphere, and that


FIGURE 11.24 (a) A visible satellite image of the eastern tropical Atlantic and Africa at 12Z on September 1, 2004. An easterly wave that . ha_d _iust em_erged off Af'.ica would be the catalyst for the seedling low-lei/el circulation of Hurricane Ivan (courtesy of

Dundee Satellite Receiving Station); (b) An infrared satellite image from September 7, 2004 shows Hurricane Ivan approaching the Caribbean. The remnants of once-Hurricane Frances, which also developed from an easterly wave, were moving into the southeastern United States at the time (courtesy of NOAA).

originates north of the equator over Africa and then moves westward across the tropical North Atlantic Ocean (and sometimes into the Northeast Pacific Ocean). Meteorologists sometimes call an easterly wave a tropical wave, though, more formally, it is an African easterly wave . The bottom line here is that easterly waves often serve as the catalyst for the seedling lowlevel cyclonic circulation that's needed for the genesis of tropical cyclones.

During the period from June to October, an easterly wave comes off the west coast of Africa into the eastern tropical Atlantic every three or four days. On September 1, 2004, for example, a strong easterly wave had emerged from Africa (see Figure 11.24a). This tropical wave eventually served as the catalyst for the low-level circulation of Hurricane Ivan, which is shown approaching the eastern Caribbean a week later in Figure 11.24b.

On average, approximately 60 easterly waves emerge from Africa each year. Given that the long-term average annual number of named Atlantic storms is ten ( approximately six of which become hurricanes), it stands to reason that most easterly waves do not initiate a tropical cyclone . At the very least, some easterly waves produce clusters of thunderstorms that affect the Caribbean Islands or the Bahamas .

. What causes easterly waves? This is a good question whose answer requires a deeper understanding of the dynamics of the atmosphere over Africa. Figure 11.25 shows the average wind direction and speed at the 600-mb level (an altitude of about 4000 meters) during the month of August. The narrow ribbon of fast 600-mb

winds stretching across Africa and indicated by the black arrow is the Middle Level African Easterly Jet (MLAEJ for short); the 600-mb pressure level lies in the rnidtroposphere, thus the "Middle Level" in the name of this seasonal jet. During August, the core of the MLAEJ spans from roughly 30°E to 30°W longitude.

As it turns out, the MLAEJ is a major source for African easterly waves. Most form on the south side of the axis of the MLAEJ, where large horizontal wind shear imparts a cyclonic circulation. In the context of the MLAEJ, horizontal wind shear is a change in speed of the easterly winds over some specified horizontal distance in and near the jet. Think of a newly

Average 600-mb Wind (mis), August

10 15 20 25 30 35 40

FIGURE 11.25 Average wind direction (arrows) and speed (color-coded in m/s) at the 600-mb level (about

4000 m) during the month of August, showing the average position of the Middle Level African Easterly Jet as the black arrow (courtesy of NOAA).


FIGURE 11.26 Easterly waves form on the cyclonic shear side (southern side) of the Middle Level African Easterly Jet.

forming easterly wave as a cyclonically circulating e.¢:dy tl~at spins up as a result of this shear (see Figure 11.26).1 Given the pivotal role that easterly waves play in the genesis of tropical cyclones over the North Atlantic and Northeast Pacific Oceans, it behooves us to learn more about their source.

During North Africa 's hot summer, trade winds in the Southern Hemisphere cross the equator and penetrate to the southern edge of the Sahara Desert. As it turns out, this cross-equatorial flow is a component of Africa's summer monsoon (see Figure 11.27). At the southern edge of the Sahara, moist and relatively cool air associated with this monsoon meets hot and dry air flowing from the northeast over the Sahara Desert (these northeasterlies are the infamously scorching harmattan winds ). Figure 11.28 shows average surface temperatures in August and a few arrows that represent these hot winds and the cooler southwesterlies . Note that average temperatures approach 40°C ( 104 °F) over the western Sahara Desert. Also note the narrow ribbon of

relatively large temperature gradient that stretches roughly east-west across Africa. Local African meteorologists call the boundary between these two contrasting air masses the Intertropical Front .

The bottom line here is that a front stretches eastwest across Africa, with hotter air to the north and cooler air to the south- in essence, a reversal of the "normal" north-south temperature gradient. This front is relatively shallow, with north-south temperature gradients vanishing near 600 mb. This lower tropospheric thermal gradient produces a height gradient that max-1m1zes ound 600 111b with higher 600-mb heights to the north a lower 600-mb heights to the south (as shown in Figure 11.29). It follows that there are fairly

~ strong winds that blow from the east just below the 600-mb level. These strong easterlies mark the Middle Level African Easterly Jet, a unique source for easterly waves.

Atlantic tropical storms and hurricanes that originate as easterly waves within 1000 km (600 mi) or so of the Cape Verde Islands are called Cape Verde storms (these islands lie just off the west coast of Africa). It takes some time and distance from land for an easterly wave to develop into a hurricane. Indeed, few easterly waves intensify into hurricanes over the eastern Atlantic Ocean. To our knowledge, only a handful of tropical cyclones have been classified as a hurricane east of25°W longitude in the deep tropics. The farthest east that a tropical cyclone has been classified as a major hurricane in the Atlantic Basin was roughly 30°W longitude, where Hurricane Frances rapidly intensified in 1980. The season for Cape Verde storms typically runs from about August to early October, essentially the heart of hurricane season .

With the six ingredients for the genesis and development of tropical cyclones in hand, we 're finally ready to completely address the reasons why the South Atlantic Ocean is virtually devoid of tropical cyclones.

FIGURE 11.21 Dramatic seasonal shifts in

wind direction occur in a zone from Africa to Southeast Asia and northern Australia, qualifying the highlighted area as the world's major monsoonal region. This region occupies a large portion of the tropical eastern hemisphere.

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FIGURE 11.28 Average surface temperatures

across Africa in August, in °c. Note the narrow ribbon of relatively large temperature gradient that marks the lntertropical Front. Arrows show how relatively cool, moist winds crossing the equator during Africa's summer monsoon meet hot and dry northeasterly winds from the Sahara Desert (courtesy of NOAA).

Average Surface Temperature (°C), August

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FIGURE 11.29 Average 600-mb heights during

August over Africa and the adjacent eastern Atlantic Ocean. With high heights to the north and low heights to the south (a reverse of the "normal" north-south gradient), a ribbon of strong easterly winds (the MLAEJ) sets up in the large gradient of 600-mb heights (courtesy of NOAA).

Average 600-mb Heights {m), August

4400 4420 4440 4460

THE SOUTH ATLANTIC OCEAN: THE BASIN WITHOUT A HURRICANE SEASON

Now that we have easterly waves under our belts, we've taken a big step towards understanding why the South Atlantic Ocean is virtually bereft of tropical cyclones. The Middle Level African Easterly Jet , which is the major source of tropical waves, is strictly a Northern Hemisphere feature. In other words, the tropical region of Africa south of the equator doesn't have any source for easterly waves. As a result, there aren't any tropical

4480 4500

waves over the South Atlantic Ocean to provide the seedling low-level circulation needed to kick-start tropical cyclones.

Moreover, check out Figure 11.30, which shows the vertical wind shear between 850 mb and 200 mb during what would theoretically be· the "peak" of hurricane season in the South Atlantic Basin , February to April. Notice that wind shear is a bit too strong (greater than 10 mis), on average, over the tropical Atlantic off the east coast of Brazil. So, although sea-surface temperatures there are favorably high ( see Figure 11.31 ), this region


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Average 850-200 mb Wind Shear (mis), Feb -Apr

10 14 18 22 26 30

FIGURE 11.30 The average vertical wind shear between 850 mb and 200 mb during the three-month period from

Febr:uary to April. The only region over the South Atlantic Ocean with favorcible ~ ind shear (less than 10 m/s, here in white) consistently resid;s'at low latitudes east of 20°W longitude (courtesy of NOAA).

has at least two strikes against it: no easterly waves and, typically, unfavorable wind shear.

But then came Catarina. With the odds stacked against it, how did this hurricane ever develop off the east coast of Brazil in late March 2004? In particular, what fea-

EQ

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22

South America

Average Sea-Surface Temperature {°C), Feb-Apr

23 24 25 26 27 28

ture provided the group of disorganized showers and thunderstorms (Ingredient #6), and what happened to the typically unfavorable vertical wind shear?

In a nutshell, a 500-mb low approached a stationary ridge of high pressure cut off from the mid-latitude westerlies (see Figure 11.32). This 50 -mb low was accompanied by a corre~peri ·ui::;;-~>--ruce low-pressure system with classic cold and warm fronts (not shown).

This stag11qnt 500-mb pattern provided three key ingredients . First, winds aloft were weak, indicated by the relatively lax 500-mb height gradient off the coast of Brazil. As a result, vertical wind shear was unusually weak there. For proof, focus your attention on Figure 11.33, which shows the average 850-200 mb wind shear during the two-day period March 25-26, 2004. The white area off the coast of Brazil represents favorable wind-shear values less than 10 mis.

Second, because the 500-mb low cut off over warm water, the troposphere off the Brazil Coast destabilized, as relatively cold air aloft associated with the 500-rnb low stacked above warm, moist air near the ocean surface. This set-up paved the way for thunderstorms to erupt and to begin organizing around the surface low's center of circulation. Finally, the 500-rnb low's rather broad circulation drew relatively moist air from the middle troposphere over the Amazon eastward into the brewing storm. With all six ingredients corning together, a rare South Atlantic hurricane developed, and the rest, as they say, is history .

29

FIGURE 11.31 Average sea-surface temperatures from February to April over the

South Atlantic. Recall that sea-surface temperatures of 26.5°G (80°F) or higher are generally required for genesis of tropical cyclones. Contrary to popular belief, SSTs are high enough to support the genesis of tropical cyclones over the South Atlantic Ocean ( courtesy of NOAA).


20S

30S

40S

500-mb Heights {m) 122 March 25, 2004 500-mb Heights (m) 12Z March 26, 2004

(a} 5280 5400 5520 5640 5760 5880

(b) 5280 5400 5520 5640 5760 5880

FIGURE 11.32 The 500-mb height analyses at (a) 12Z on March 25, 2004, and (b) March 26, 2004, show a trough cutting off from strong mid-latitude westerlies (black arrow) (courtesy of NOAA).

We point out that in making the transition from a midlatitude low-pressure system (with fronts) to a tropical cyclone (no fronts), there was likely a time when Catarina had both tropical and extratropical characteristics. For the record, an extratropical low-pressure system forms outside the tropics in an environment where there are temperature gradients . When a system simultaneously has both tropical and extratropical characteristics, it is sometimes called a subtropical cyclone . In this case, no such christening was forthcoming because it caught Brazilian forecasters a bit off guard. Nonetheless, the storm made a complete transition to a purely tropical cyclone by the time the eye came ashore on March 28, 2004.

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Average 850-200 mb Wind Shear (mis), March 25-26, 2004

10 14 18 22 26 30

I FIGURE 11.33 The average wind shear between 850 mb and 200 mb during the two-day period March 25-26,

2004. Note the area of favorably weak wind shear off the coast of Brazil, where the white area represents values less than 10 m/s (courtesy of NOAA).

Subtropical cyclones also form over the North Pacific and North Atlantic Oceans. On April 20, 2003, for example, the National Hurricane Center christened Subtropical Storm Ana while the mongrel system was south of Bermuda. The subtropical cyclone rapidly made the transition to a tropical cyclone and officially became Tropical Storm Ana at OOZ on April 21. Figure 11.34 is a multi-channel satellite image of Tropical Storm Ana. Ana was the first documented tropical

FIGURE 11.34 Tropical Storm Ana at 2oz on April 21, 2003. Ana was just to the southeast of the island of Bermuda

at this time (courtesy of Ray Sterner, Johns Hopkins University).


storm ever to form over the North Atlantic basin dur-ing April. .

Although we've previously given you an inkling of how tropical cyclones intensify , we will now formalize the intensification process.

A TROPICAL CYCLONE INTENSIFIES: CONDITIONAL INSTABILITY OF THE SECOND KIND During our discussion of the thermodynamic ingredients needed for the genesis of a tropical cyclone, we pointed out that development hinged on thunderstorms organizing around the center of a tropical disturbance. Within the updrafts of these storms, the large releas e of latent heat inside rising parcels allows parcels to stay positively buoyant to great altitudes. When moisturedepleted air parcels reach the top of eye-wall thunderstorms, some parcels gently subside over the core of the disturbance. The resulting large compressional warming in the central air column increases its average temperature, and, thus , decreases its average density and weight ( refer back to Figure 11.15). In a nutshell, surface pressure decreases.

For the newly forming tropical cyclone to intensify , the process must kick into a higher gear. Meteorologi sts call this higher gear "Conditional Instability of the Second Kind."

Latent Heat Feedback: Amplifying the Storm Let's carefully examine the amplification process of a strengthening tropical cyclone by starting at the beginning. As low pressure forms at the ocean surface, air converges toward the center. If the low develops far enough away from the equator , the deflection by the Coriolis force creates a cyclonic circulation.

As air parcels spiral inward toward the center of low pressure (see Figure 11.35), they expand. Ordinarily, expansion produces cooling (similar to what happens when a parcel of air rises-it also moves toward lower pressure). Of course, any low-level cooling would be detrimental to further development because it would tend to stabilize the lower troposphere . But the tendency for inward-spiraling par9els to cool as they expand is offset by warm tropical seas, which supply more-than-ample heat enegy. Thus, as

/ i{p arcels converge around the center of the low, they re/ main warm and moist, setting the stage for their long as-

/ cent within thunderstorms organizing around the eye. Meanwhile, as surface pressures decrease in response to

compressional warming over the low's center, the horizon-

FIGURE 11.35 Air spiraling into the center of a hurricane speeds up as it nearly conserves angular momentum.

tal pressure gradient strengthens. In response, wind speeds increase, especially lower-tropospheric winds around the core of the newly forming tropical cyclone. That's because the horizontal pressure gradient is largest there (the large horizontal temperature gradient between the core of the developing tropical cyclone and the surrounding thunderstorms creates a large pressure gradient). Stronger winds then translate to increased low-level convergence and higher evaporation rates. As a result, the number and intensity of thunderstorms increase.

Now the stage is set for an amplifying feedback loop. With the number, strength, and organization of thunderstorms increasing, and the supply of water vapor growing, the release oflatent heat amplifies around the core of the storm and surface pressure further decreases. In turn, surface convergence and wind speed increase, systematically boosting the number and strength of thunderstorms, elevating evaporation rates and upping the release of latent heat. If the pressure continues to decrease, the system graduates to a tropical storm and eventually a Category- I hurricane. If other conditions are favorable (for example, weak vertical wind shear and high sea-surface temperatures) , the positive feedback loop continues and the hurricane intensifies. This positive feedback loop is called Conditional Instability of the Second Kind, or CISK for short. This process cannot continue unchecked, of course. Ultimately, sea-surface temperatures impose an upper limit on the potential intensity of hurricanes.


Check out the nifty flash animation on the companion CD to see how a hurricane in

tensifies in concert with this positive feedback loop.

To see firsthand how an increase in the number and organization of thunderstorms around the core of a tropical cyclone translates to intensification , let's consider Hurricane Katrina , which first made landfall in southern Florida in late August 2005 as a Category-I storm and then twice made landfall in Louisiana as a Category-3 hurricane (see Figure 11.36). Given the failed levees and severe flooding in New Orleans (Figure 11.37) and the overall devastation along the central Gulf Coast , Hurricane Katrina ranks as the costliest and one of the deadliest hurricanes ever to strike the United States.

As with all hurricanes , Katrina began as a humble tropical depression . Figure 11.38a, a color-enhanced infrared satellite image at 0615Z on August 24, 2005, shows the very cold cloud tops (in red) of several tall , disorganized thunderstorms over the Bahamas associated with Tropical Depression 12. At the time , the maximum sustained winds ofT.D. 12 were 30 kt (35 mph) . In just about four days, the tropical depression intensified into a Category-5 hurricane (see Figure 11.38b) , with maximum sustained winds of 150 kt (173 mph). Note the highly organized ring (in red) of tall eye-wall thunderstorms that encircles the eye of Katrina at that time . The message here should be loud and clear-the strength of a tropical cyclone goes hand-in-hand with

FIGURE 11.36 The track of Hurricane Katrina in late August 2095. Positions are shown every six hours. The

color of the dots corresponds to the tropical cyclone's intensity, from tropical depression to tropical storm to the five categories on the Saffir-Simpson scale. Katrina made three landfalls: one in southern Florida on August 25 as a Category-1 storm, and two in Louisiana on August 29 as a Category-3 hurricane (courtesy of Wikipedia):

an increase in the number and organization of thunderstorms around the eye.

We can go one step further. Research meteorologists have found that a hot tower forming around the eye of a hurricane means that the storm is twice as likely to intensify within the subsequent six-hour period . A hot tower forming around the eye of Hurricane Bonnie in August 1998, visualized in Figure 11.39 by NASA's TRMM satellite (Tropical Rainfall Measuring Mission),

FIGURE 11.37 An aerial view of one of the

breached levees in New Orleans, LA, on August 30, 2005, the day after Hurricane Katrina made landfall (courtesy of FEMA).


(a) (b)

FIGURE 11.38 · · 12 (TD 12) t 0615Z on August 24 2005. Maximum (a) A _colr~n~anced i~~a~e~3~a~~~; i:o~~~~: :~~i;~~ln~:~i~~~~;all thund.ersto~ms, whose very cold ~loud tops are sustaine win s were . . or 5 Hurricane Katrina with maximum sustained winds of 150 kt

indicated in red; (b) By August 2~, T.D. 1 ~ had develo1p5eZd l~to ca;~g h(g~ly organized tall th~nderstorms (ring of red) around the eye of the (173 mph). This color-enhanced infrared image at 18 s ows e hurricane (courtesy of NOAA and the Naval Research Laboratory).

FIGURE 11.39 A hot tower forming around the

eye of Hurricane Bonnie on AuguSt 22, 1998, was a precursor for intensification. The top of the hot tower was almost 18 km (11 mi) above the ocean surface (courtesy of NASA's Tropical Rainfall Measuring Mission).

FIGURE 11.40 The observed surface winds around Hurricane Katrina at 09Z on August 29, 2005. Wind

speeds are color-coded in knots, and arrows indicate wind directions. Measurements came from a variety of sources, including aircraft reconnaissance, ocean buoys, and Doppler radar. The hurricane was moving north at the time (courtesy of NOAA's Hurricane Research Division).

was a clue to Bonnie's upcoming intensification from a Category-1 to a Category-3 hurricane as it passed north of the Bahamas.

The strongest wind speeds associated with a hurricane are typically not evenly distributed around the eye of the storm . Figure 11 .40 shows the surface wind speeds ( color-coded, in knots) and wind directions (arrows) around the eye of Hurricane Katrina at 0900Z on August 29, 2005, a few hours before Katrina made its first landfall in Louisiana. At the time, the maximum observed wind speed was 99 kt (114 mph) to the northeast of Katrina 's eye on the storm's forward right flank (looking in the direction of movement). Here, in this right-front quadrant, the storm's forward motion adds to the speed of the winds generated by the strong horizontal pressure gradient ( see Figure 11.41 ). You can think about this boost in wind speed in much the same way as a moving train adds to the total speed of a robber running atop the train toward the engine-the robber's total speed equals his running speed plus the forward speed of the train.


FIGURE 11.41 The fastest winds of a hurricane and the highest storm surge lie in the right-front quadrant. In

this region, winds push water towards shore (point R will be in this quadrant). At point L, offshore winds can actually lower water levels as the hurricane goes by.

Not surprisingly, the strong winds in the right-front quadrant of a hurricane produce the storm surge. We'll get into the details of storm surge in just a moment.

Meteorologists sometimes liken a hurricane to a "heat engine," which is a device that converts heat to some type of mechanical work. Let's see if such a comparison

. makes any sense. Great amounts of water evaporate from warm seas. In turn, the great release of latent heat of condensation in rising air allows parcels to stay positively buoyant to great altitudes. When parcels sink into the eye of the hurricane, they warm dramatically by compression. This "warm core" in the eye of a hurricane paves the way for surface pressure to decrease and for the hurricane to intensify. Given all of these thermodynamic references, you can understand why likening hurricanes to heat engines is a pretty good analogy.

With a hurricane's engine humming on all cylinders , let's look under the hood and inspect the structure of a mature hurricane.

FEATURES OF A HURRICANE: WITH AN EYE TO THE SKY Figure 11.42a is a visible satellite image of Hurricane Katrina on August 28, 2005. We show you this image to impress upon you that the eye of a hurricane is not necessarily clear. Nor is it entirely calm. Indeed, momentum from fast-moving air in the eye wall, where the hurricane's strongest winds are found, can sometimes mix


(a)

FIGURE 11.42

inward into the eye, creating swirls of turbulent clouds and gusty winds on the periphery of the eye. If you look closely at the inset in Figure 11.42a, you can observe a mesovortex inside Katrina's eye. For the re~ord, a mesovortex ( in the context of a hurricane's eye) is a re_latively small, cyclonic swirl of low cloud~ ( cyclomc means counterclockwise in the Northern Hermsphere). In light of this small but awesome fe~ture, we want you to keep your mind open to some new ideas as we delve further into the greatest storms on earth.

Just so you don't dismiss everything you've hea~d before about hurricanes, the central region of the ey~ is, for the most part , an island of relatively light wmdsthough , again, winds can be stronger inside the eye, particularly near the eye wall. It is truly a wonder of nature that light winds characterize the c~ntral part _of the eye of a hurricane, while ten or so miles away m the eye wall winds could be blowing over 130 kt (150 mph).

H~w does the eye of a hurricane form? You've already observed that air parcels spiraling inward toward the lowest pre~sure at the center accelerate while trying to co~~ serve their angular momentum. As a result of the1r curving paths , air parcels are subject to the outward-acti11g-'C:ntrifugal force , whose magnitu~e increases ~s the

/s peed of the inward-spiraling parcels mcreases (t~mk of your bottom sliding outward as your car negotiates a curve-that's the centrifugal force at work, and the faster you negotiate the curve, the greate~ the o~tward fo~ce on your bottom). At some point , the mcrea~mg cent~1fugal force nearly offsets the hurricane 's large, mward-drrected

(b)

pressure-gradient force, and air parcels stop crossing is~bars (here we have assumed that the _Coriolis fo~ce is small, and therefore a minor player). With t?e centnfugal force essentially offsetting the pressure gradient force ( see Figure 11.43), inward-spiraling parcels stop short of the center, converging and rising from the stormy eye wall .

FIGURE 11.43

Velocity

Centrifugal

Above the level where the effects of friction with the sea surface vanish, there is a balance

. I f close to the between the pressure gradient and the centnfuga orces . center of a hurricane (assuming that the Coriolis force is relat,velye small). So, as air parcels spiral inward toward the center of the ey , they stop their inward spiral in the eye wall, where the two :orces e

· ' f test winds ar essentially balance each other. Thus, a hurricanes as . found in the eye wall, rendering the eye an island of relative calm.


In reality, this near-balance of forces occurs above the boundary layer, which is the layer in which friction with the surface operates. This layer typically has a depth of 5 00--1000 m ( about 1600-3200 ft). Near the ocean surface, friction between the air and very turbulent seas substantially reduces the speed of inward-spiraling parcels. As a result, the centrifugal force is not as large and air parcels penetrate closer to the center in response to the very strong pressure gradient force, whose magnitude across the eye wall is greatest near the ocean surface.

Figure 11.44 shows a typical vertical profile of wind speed in the eye wall of a hurricane. Note that the maximum speeds typically occur at an altitude near 500 meters (1640 ft). By way of background, conventional thinking has held that the strongest winds in a hurricane occurred 2-3 km above the surface, but new evidence based on wind data collected by dropsondes now indicate the level of maximum wind speeds lies at lower altitudes. Before we discuss the underlying science for Figure 11.44, we first point out that a dropsonde (see Figure 11 .45) is a dispensable canister of electronic weather in-

3000

2500 - - - - ... - - - - - - - - - - - - - - - ·-- - - - - - - - - - - - - - - - - - - - - - - -

2000 -------- - -------------- --- - ----- -- ------- --- -

r -'5, 1500 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ·-- - - - - - - -·; ::r

1000 ·- - ----- ----- ------ ------ --- - -- - ---- ----- - ----

Wind Speed --~

A plot of wind speed with altitude in the eye wall, using data gathered by dropsondes in

more than 200 hurricanes, indicates that the maximum wind speeds in the eye wall occur about 500 meters (1640 ft) above the surface. Above the level of maximum wind speeds, winds in the eye wall gradually weaken with increasing altitude (adapted from Franklin, et. al., 2003, Volume 18, Weather and Forecasting).

struments released from Hurricane Hunter aircraft, which often fly through storms at an altitude of about three kilometers. Tethered to a parachute , a dropsonde measures temperature, pressure and relative humidity as 1t descends toward the surface . By tracking the movement of a dropsonde using the Global Positioning System, scientists can also retrieve wind speed and direction.

Now that you know the source for the data plotted in Figure 11.44, we '11 explain why the fastest winds in a hurricane tend to occur near an altitude of about 500 meters. The "slowest" winds occur right at the ocean surface, where friction is strongest, and winds increase with increasing height above the ocean surface as the effect of surface friction decreases. Ordinarily, you would expect winds to continue to increase above the boundary layer, 'but downdrafts in eye-wall thunderstorms transport momentum from fast winds downward into the boundary layer. This momentum doesn't really have any impact on winds closest to the surface, where friction is strongest. But this momentum does get transported down far enough to affect high-rise buildings . In

FIGURE 11.45 A dropsonde is a pispensable canister of electronic weather instruments released from

Hurricane Hunter aircraft. Tethered to a parachute, a dropsonde measures temperature, pressure and relative humidity as it descends toward the ocean surface. By tracking the motion of a dropsonde using the Global Positioning System, scientists can also retrieve wind speed and direction (courtesy of the U.S. Air Force).


fact, research suggests that people taking refuge on the upper floors of a high-rise building along the coast could experience winds a full Saffir-Simpson category stronger than the winds at the ground! So residents forced to seek refuge in high-rise buildings as a hurricane makes landfall would be safest on lower floors that are high enough to escape the storm surge.

The diameter of a hurricane's eye is, on average, about 40 km (25 mi). For rapidly intensifying hurricanes, the eye wall and its radius of maximum winds usually contract inward in response to the increasing pressure gradient (the radius of maximum winds is the distance from the center of the eye to the ring of strongest winds in the eye wall). In the process, this contraction reduces the diameter of the eye. Sometimes the eye becomes so small that it looks like a pinhole from space. For example, in the 24 hours ending at 06Z on October 19, 2005, Tropical Storm Wilma (see Wilma's track in Figure 11.46a) rapidly intensified from 60 kt (69 mph) into a Category-5 hurricane with maximum sustained winds of 150 kt (173 mph). Such a rapid intensification is unprecedented for an Atlantic hurricane. Six hours later (12Z on October 19), Wilma's maximum sustained winds reached 160 kt (184 mph) and the central pressure dropped to 882 mb, an all-time record low barometric reading in the Atlantic Basin. At the time, aircraft reconnaissance measured the diameter of the eye at just two nautical miles (2.3 mi), the smallest eye known to the entire staff at the National Hurricane Center ( see Figure 11.46b ).

Cirrus clouds sometimes blow over a hurricane's eye, hiding it from full view on standard visible and infrared satellite imagery. But a special instrument mounted on some satellites can get around this issue. This instrument detects microwave radiation from hail and graupel ( snowflakes coated with ice) in the upper portions of tall thunderstorms. The small ice crystals in cirrus clouds essentially are not detectable at this wavelength, so, for all practical purposes, this instrument can "see through" cirrus clouds. Figure 11.47a is a standard infrared image of Hurricane Katrina over the Gulf of Mexico at 0430Z on August 27, 2005 . You can see that clouds mas~ed the eye from full view at this time. Figure 11.4 l b: which is a microwave satellite image at about the sa~ ~1me, s~ows the presence of hail a~d gra~pel in

/,, ,Jh~ upper P?rt1?ns of thunderstorms associated with Ka/ tnna (red md1cates the deepest convection). At this

wavelength, forecasters were able to "see through" the cirrus clouds to get a better idea of the size of the hurricane's eye and the structure of the eye wall.

To understand how cirrus clouds can blow over the eye, let's start with the observation that many parcels of

(a)

(b)

FIGURE 11.46 (a) The track of Hurricane Wilma, October 15-25, 2005. Positions are shown every six hours. The

color of the dots corresponds to the tropical cyclone's intensity, from tropical depression to tropical storm to the five Categories on the Saffir-Simpson scale (courtesy of Wikipedia); (b) A visible satellite image at 1245Z on October 19, 2005, shows the pinhole eye of Hurricane Wilma. At the time, the diameter of the eye was a mere two nautical miles (2.3 mi), the central pressure was a record low (for the Atlantic Basin) 882 mb, and maximum sustained winds were 160 kt (184 mph) (courtesy of NOAA and the Naval Research Laboratory).

air rising to high altitudes in the eye wall spread outward from the center of the storm. This divergenc e of high-altitude air is orchestrated by a high-pressure system that forms in the upper troposphere over the eye (remember that pressure decreases very slowly with al-


(a) (b)

FIGURE 11.41 (a) An infrared satellite image of Hurricane Katrina at 0430Z on August 27, 2005, shows cirrus clouds obscuring the eye of the storm; (b) A colorful microwave satellite image at approximately the same time shows the size of the eye and the structure of deep convection around Katrina (courtesy of NASA and the Naval Research Laboratory).

titude in a column of warm air, translating to high pressure at high altitudes over the oppressively warm eye). The circulation around such a lofty high is no different tban around other highs. Indeed, on animations of satellite images of Northern Hemisphere hurricanes, cirrus clouds, acting as a tracer to the pattern of high-altitude divergence over a hurricane, often circulate clockwise and outward away from the storm. The combing of cirrus over a storm's bald eye signals that the hurricane has moved into an environment where there's vertical wind shear. If the shear is too strong, surface pressures will increase as the sputtering exhaust system allows some air to pile up aloft.

Regardless of whether there's a cirrus "comb-over," the vertical structure of the eye is often reminiscent of a stadium, with eye-wall thunderstorms tilting outward with increasing altitude ( see Figure 11. 48). To get a sense

for the science behind this so-called "stadium effect," revisit Figure 11.44 and recall that the screaming message from this graph is that wind speeds in the eye wa11 decrease with increasing altitude above about 500 meters. That's because the magnitude of the horizontal pressure gradient across the eye wall also decreases with increasing altitude. To understand this assertion, check out Figure 11.49, which shows various constant pressure surfaces in the eye and eye wall. Because pressure decreases more slowly with height in the eye's warm air column, the constant pressure surfaces become "flatter" with increasing height, indicating that the magnitude of the horizontal pressure gradient decreases with increasing altitude (which explains why wind speeds decrease with increasing altitude above 500 meters). With the inward spiral of air parcels limited by a horizontal pressure gradient that relaxes with increasing height ( and reverses at very high

FIGURE 11.48 The clouds and thunderstorms

in the eye wall lean outward with increa~ing altitude. Meteorologists sometimes refer to this as the "stadium effect."

494 CHAPTER 11 Tropical Weather, Part 11: Hurricanes

FIGURE 11.49 A schematic of various constant pressure surfaces in the eye and eye wall of a hurricane.

The horizontal pressure gradient between the eye and the outer edge of the eye wall is greatest at the surface. Because pressure decreases relatively slowly in the eye's warm air column, the horizontal pressure gradient decreases with increasing altitude . (indicated by the flattening of the constant press~re surfaces with

height).

altitudes , where high pressure forms over the eye)'. the vertical structure of the eye takes on the look of a stadmm.

Having thoroughly examined the eye and eye wall, w_e will end this discussion about the features of a humcane with spiral bands . These tentacles of thu~derstorms that pinwheel around the center of a hurncane correspond to areas of enhanced surface convergence and strong updrafts (see Figure 11.50) The weather associated with spiral bands is best described as "squally," with fitful but temporary rains, very gusty winds , and sometimes tornadoes. Indeed, the Storm Prediction Center in Norman , OK, routinely issues tornado watches whenever a hurricane is expected to make landfall along the Southeast Coast or the coast of the Gulf of Mexico.

Speaking of making landfall , what controls a hurri

cane 's movement? Read on.

Hurricane MoJement: Subtropical Highs at the S~g _Wheel _ . . Hurnc a-ne Ike with maximum sustamed wmds of 95 z: ' . kt ( 109 mph) , slammed into the upper Texas ~oast Just after 07Z on September 13, 2008, devastatmg Galveston Island. A week earlier, landfall anywhere on the Gulf Coast seemed like a huge longshot. Check out Figure 11.51, which shows the track of Hurricane Ike. Foc~s your attention on 23.7°N, 61 °W, the northernmost posi-

FIGURE 11.50 A radar image at 08Z on September 16, 2004, shows an intense spiral band to the east of

landfalling Hurricane Ivan. The spiral band spawned m?re than two dozen tornadoes in southwestern Georgia and the Florida panhandle, several of them causing fatalities (courtesy of WSI Corporation).

tion that Ike reached when it was in the open Atlantic Ocean on September 5. We chose this point because I~e turned southwestward around this time , a fateful shift in track that ultimately sealed the fate of Galveston

Island. . Why was Ike's eventual entry into the Gulf ofMexic~

such a longshot? As it turns out, only a handful of~umcanes and tropical storms on record that developed m t?-e Atlantic, moved westward, and passed near the pomt

FIGURE 11.51 The track of Hurricane Ike in September 2008· Positions are shown every six hours. The color

of the dots corresponds to the tropical cyclone's intensity, from tropical depression to tropical storm to the five Catego_ries on th: N Saffir-Simpson scale. The southwestward turn near latitude ~3-; ' longitude 61 °w paved the way for the storm to enter the Gui 0

Mexico (courtesy of Wikipedia).


where Ike made its turn to the southwest, ever made it into the Gulf of Mexico. In fact , had Ike behaved like nearly all previous storms that had followed a similar track in the open Atlantic, it would have either made landfall somewhere along the Eastern Seaboard or made a clockwise tum and completely missed the United States.

The track of Hurricane Floyd in 1999, shown in Figure 11.52, is representative of the typical path that tropical cyclones take when they pass longitude 61 °W longitude heading west, north of 20°N latitude. The truth be told, Floyd passed this longitude line south of Ike's position. Yet it still made a pronounced northward tum (just off the coast of Florida) and, as a result , missed the Gulf of Mexico. The track of Hurricane Floyd is superimposed on the average 500-mb heights during the period September 8-17, 1999. The most striking feature on this 500-mb chart is the closed center of high heights sprawled across the Atlantic Ocean (you can't miss it). This high is the 500-mb reflection of the surface Bermuda high, so we '11 also refer to this feature as a "subtropical high." If you imagine the clockwise circulation of air around the periphery of this 500-mb high, you '11 see that the flow of air pretty much mirrors the track of Hurricane Floyd. If you're now getting the idea that subtropical high-pressure systems provide the primary steering currents for hurricanes, then you're "on the right track."

To get a sense of the subtropical high's role in the movement of tropical cyclones, we first note that the steering current for hurricanes is the average wind in a layer of air that spans from the lower to the upper troposphere. As a result, the flow at mid-tropospheric levels

Average 500-mb Heights (m) September 8-17, 1999

5280 5400 5520 5640 5760 5880

The track of Hurricane Floyd in September 1999 superimposed on a chart of 500-mb heights

averaged over the period September 8-17, 1999 (courtesy of NOAA).

such as 500 mb, where winds are close to geostrophic, can serve as a proxy for the mean steering current.

Given that the Bermuda high is a semi-permanent feature during the warm season, it stands to reason that hurricanes moving westward over the central Atlantic as far north as 20°N latitude are likely to get caught up its steering flow. As a result , an eventual turn to the north is likely, removing all but the remotest chances of storms getting into the Gulf of Mexico.

So why did Ike make that fateful southwestward jog? Carefully study Figure 11.53, which shows the 500-mb height pattern at 1200Z on September 5, 2008, three hours after Ike reached its northernmost latitude in the open Atlantic Ocean. Note the lobe of high 500-mb heights extendi~g southwestward from the subtropical high. With the 500-mb flow nearly geostrophic and clockwise around the Atlantic subtropical high, steering winds for Ike blew from the northeast at this time. As a result, Ike swerved to the southwest, greatly increasing its chances of getting into the Gulf of Mexico.

When gradients in 500-mb heights are weak (and thus steering winds are weak), tropical cyclones tend to move slowly and sometimes erratically. Or they may simply stall. Either way, weak steering currents set the stage for lethargic tropical cyclones to produce protracted heavy rain and flooding. A good example is Tropical Storm Fay in 2008. After mak ing landfall on the Florida Keys on August 18, Fay took aim at the Florida Peninsula (see Fay's track in Figure 11.54; note that the storm made landfall in Florida a record four

500-mb Heights (m) 122 September 5, 2008

5280 5400

FIGURE 11.53

5520 5640 5760 5880

The 500-mb height analysis at 12Z on September 5, 2008, three hours after Ike reached 23.7°N,

61 °w. Steering currents associated with the Altantic subtropical high blew from the northeast in the vicinity of Ike, helping to send the storm on its southwestward jog. Here we assume 500 mb is a proxy for the steering level of Ike (courtesy of NOAA).


FIGURE 11.54 The track of Tropical Storm Fay in August 2008. Positions are shown every six hours. The color

of the dots corresponds to the tropical cyclone's intensity, from tropical depression to tropical storm to the five Categories on the Saffir-Simpson scale (courtesy of Wikipedia).

times) . On August 20, as Fay neared the east coast of Florida, it dramatically slowed down as it encountered weak steering currents. For all practical purposes, the tropical storm essentially stalled for a short time (note the bunching of blue circles near Florida's east coast in Figure 11.54). The following is a verbatim excerpt from the late morning discussion issued by the National Hurricane Center on August 21 , 2008:

TROPICAL STORM FAY DISCUSSION NUMBER 24 NATIONAL HURRICANE CENTER MIAMI FL 1100 AM EDT THU AUG 21 2008

THERE IS BASICALLY NOTHING NEW TO REPORT. FAY HAS BEEN MEANDERING FOR THE PAST 12 HOURS OR SO WITH LITTLE CHANGE IN

I J

INTENSITY . .. STEERING CURRENTS HAVE REMAINED VERY LIGHT ... CONSEQUENTLY FAY HAS BARELY MOVED SINCE YESTERDAY.

Given Fay's slow advance across Florida during the period August 20-23, rainfalls were prodigious (see Figure 11.55). During Fay's siege, a whopping 27.65 inches (70 cm) fell about eight miles northwest of Melbourne, ?n the east-c~ntral coast of Florida. According to prelimmary data f~~m the National Weather Service, Fay was the third w~ test tropical cyclone on record in the state ofF~o~?d the wettest on record for east-central Florida. 07 :he_past 30 years, inland flooding has accounted

for nearly 60 percent of the fatalities caused by tropical cyclones . The threat posed by hurricanes ( or tropical storms) doesn't end with heavy rain, of course. Storm surges and tornadoes are also part of a hurricane's formidable arsenal. Let 's investigate.

- 3 - 5 - 1 - 10 - 15 - 20 - 25

\

I \

\

FIGURE 11.55 Total rainfall (in inches) produced by Tropical Storm Fay in August 2008. The black line marks the track of Fay (courtesy of NOAA).

Hour of Danger: Landfall and Storm Surge

As Hurricane Katrina bore down on Louisiana on August 28, 2005, it intensified into a Category-5 storm, with maximum sustained winds reaching 150 kt (173 mph) at 1800Z (see Figure 11.56a). Later that night, as a spiral band of thunderstorms encircled the eye wall (see Figure 11.56b), Katrina weakened before making landfall at 1 lZ on August 29 as a strong Category-3, with maximum sustained winds of 110 kt (127 mph). Essentially, the band of thunderstorms that encircled Katrina 's core robbed moisture from the eye-wall thunderstorms, paving the way for Katrina's maximum sustained winds to weaken.

At face value, Katrina's weakening seems like it would have mitigated damage. But not so fast. With the spiral band of thunderstorms around the storm's eye wa ll intercepting moisture destined for Katrina's core, the radiu~ of hurricane-force winds expanded outward, extending at least 130 km (about 80 mi) to the east of Katrina 's center . The radius of tropical-storm-force winds extended nearly 370 km (230 mi) east of the storm's center. In terms of size, Katrina was massive.

The large swath of strong onshore winds pushed the sea toward land, causing water to pile up in coastal shallows and leading to a devastating inland surge of water. Observations of high-water marks indicated that the storm surge was 24-28 ft (7.3- 8.5 m) in a 32-km (20 mi) swath along the Mississippi Coast, 10-15 ft (3.0-4.6 m) along the coast of western Alabama, and 6 ft (1.8 m) along the western panhandle of Florida.

Large waves contributed to these high-water marks. Within the 24 hours prior to landfall, Katrina generate d


(b)

(a) A satellite image from the a~ernoon of August 28, 2005, just after Hurricane Katrina had intensified into a category-5 st?r~ (courtesy of MODIS Rapid Response Project at NASA/GSFC); (b) A TRMM-satellite ima e showin a ·

FIGURE 11.56

of thunderstorms enc_1rchng the ~ye wall ?f Katrina around 02Z on August 29, 2005. The spiral band (arrows pain; to it) rob~ed :;~~~~nd tNhAuSnAdersdtorhmsNof moisture, causing Katrina to weaken to a strong Category-3 hurricane before its first landfall in Louisiana (courtesy of

an t e aval Research Laboratory)

large ocean swells while it was a Category-5 hurricane. According to the National Hurricane Center , a buoy located about 110 km (70 mi) south of Dauphin Island, AL, reported a significant wave height of30 ft (9.1 m) at 00Z on August 29 (the significant wave height is the

average height of the highest one-third of the waves) . Eleven hours later , the same buoy measured a significant wave height of 55 ft (16.8 m), the largest ever measured by a buoy operated by the National Data Buoy Center. Figure 11.57 shows the impact of the

FIGURE 11.51 Storm surge pushed a large ship onshore in

south Plaquemines Parish near the point where Hurricane Katrina first made landfall in Louisiana (courtesy of NOAA).


FIGURE 11.58 Northerly winds to the west of the track of Hurricane Katrina produced a large storm surge

along the southern shores of Lake Pontchartrain, causing levees that protected New Orleans to fail.

storm surge in south Plaquemines Parish near Katrina's first landfall.

There was another very important storm surge to the west of the track of Katrina (see Figure 11.58). Along the southern shores of Lake Pontchartrain, strong northerly winds produced a storm surge of 10-14 ft (3.0-4.3 m).

This surge caused some of the levees protecting New Orleans to fail, flooding many parts of the city which lie below sea level.

In terms of the threat posed by storm surge, New Orleans is arguably the most vulnerable city in the United States. Worldwide, the low lands of Bangladesh, located along the northern shores of the Bay of Bengal, are probably the most vulnerable of all. In November 2007, Very Severe Cyclon ic Storm Sidr, with maximum sustained winds of 135 kt (155 mph), slammed into Bangladesh, causing a massive storm surge that killed approx imately 10,000 people (see Figure 11.59). In 1991, a 20-ft (6.1 m) storm surge produced by a strong tropical cyclone overwhelmed southeastern Bangladesh, killing 138,000 people and leaving an estimated ten million people homeless.

Obviously, a tropical cyclone's strong winds can inflict plenty of damage. Figure 11.60 shows some of the wind damage in southern Florida inflicted by Hurricane Andrew as it made landfall around 5 a.m. (local time) on August 24, 1992 . We note that Andrew briefly intensified around landfall as low-level convergence increased in response to greater surface friction over land. When most hurricanes make landfall , sustained winds typically decrease, but wind gusts near the ground usually increase as mechanical eddies energized by friction mix momentum from faster winds higher up toward the

FIGURE 11.59 A color-enhanced infrared satellite

image of Very Severe Cyclonic Storm Sidr at 110oz on November 15, 2007. At the time, maximum sustained winds were 135 kt (155 mph). Sidr's storm surge killed an estimated 10,000 people (courtesy of the Naval Research Laboratory).


ground. In the case of Andrew , enhanced frictional convergence boosted updrafts in eye-wall thunderstorms . In the path of this energized eye wall, some 2000 streaks of damage were attributed to this temporarily intensified convection. After landfall , Andrew weakened over the Florida peninsula as its lifeline of moisture from warm seas was cut off.

Near and after landfall of a tropical cyclone, tornadoes can also pose a danger, particularly along the Gulf and Southeast Coasts. Routinely, the Storm Prediction Center issues tornadoes watches for areas in the right-front quadrant of the storm (see Figure 11.61). Here , surface winds are slowed by friction , while speedy winds aloft are, for the most part, unencumbered. In effect , friction ~reates a layer in the lower troposphere where wind speeds mcrease rapidly with altitude . In turn, a "horizontal roll" ~an form, much like a pencil held in one hand (representmg slower winds) rolls when your other hand rubs over it (representing faster winds aloft). In tum, thunderstorm updrafts can tilt the roll into an upright position, possibly generating a tornado . The 2004 hurricane season was a reco~d-setting one for producing tornadoes, as landfalling tropical systems spawned around 300 twisters in the United States. The remnant circulation from Hurricane Ivan alone produced 123 tornadoes during the period September 15-17, 2004 , from Florida to Pennsylvania , the most from any U.S. tropical cyclone on record.

We ciose with some final thoughts about forecasting tropical cyclones.

FIGURE 11.60 Hurricane Andrew (1992),

the last hurricane to make landfall in the United States at Category-5 intensity, inflicted serious wind damage in southern Florida (courtesy of NOAA).

FIGURE 11.61 The radar reflectivity shortly before Hurricane fke made landfall on September 13, 2008. At the

time, the Storm Prediction Center in Norman, OK, had issued a tornado watch for the counties outlined in red, most of which lay in the right-front quadrant of the hurricane. Preliminary data indicate that Ike spawned approximately 25 tornadoes (courtesy of Storm Prediction Center).


THE ROLE OF THE NATIONAL HURRICANE CENTER (NHC): ITS MISSION AND VISION NHC's Mission: To save lives, mitigate property loss, and improve economic efficiency by issuing the best watches, warnings.forecasts and analyses ofhaza:dous tropical weather, and by increasing understandzng of these hazards.

NH C's Vision: To be Americas calm, clear and trusted . . the eye oifthe storm and, with our partners, en-vo~e m '

able communities to be safe from tropical weather threats.

Over the past few decades, a majority of the fatalities caused by hurricanes and tropical storms ':ere a consequence of inland flooding from heavy_ rams. _Unfortunately, Hurricane Katrina served as a gnm remmder that storm surge still poses the greatest threat from lan~falling tropical cyclones. Although the final de~th_ toll is not certain, approximately 1500 people lost ~heir lives to direct impacts from Hurricane Katrina. W1thou~ reservation, storm surge likely caused most of the _estui:iat~d 1300 deaths in Louisiana and the 200 deaths m M1ss1~sippi (the fatalit ~es in Mississippi occurred largely m three coastal communities). , . .

The National Hurricane Center in Miami, FL, has worked closely with coastal communities to de'-j~lop evacuation plans based on the predicted heights of storm surges. Of course, the height of the storm surg~ depends on the wind strength ( category) of the hurncane and the location where the storm makes landfall. It also ~epends on the topography of the coastline and the adJacent ocean floor . Indeed, if the slope of the ocean flo~r adjacent to the coast is relatively gentle,_ wate~ thats pushed toward land by hurricane- for~e wmds piles up more readily, setting the stage for a higher storm surge compared to an ocean floor which has a steeper slope away from land. .

To help communities formulate evacuat10n plans, the National Hurricane C , ter provides local emergency management officials "th output from a computer model called SLOSH (whi h stands for Sea, Lake and Overland Surges from Hu ricanes). Figure 11.62 shows the SLOSH model's pre iction for the storm surge of Hurr icane Katrina, gen¢rated by the 03Z model r~n on August 29 , 2005. The 1black line is NH C's _predicted path of the eye. To get your bearings, the height of the storm surge is color-coded in feet. Note that the SLOSH model predicted a storm surge along the southern sho~e of Lake Pontchartrain (which is where New Orleans 1s located) , and a monstrous 30-foot surge into Hancock County, MS .

FIGURE 11.62 The SLOSH model's prediction for the storm surge of Hurricane Katrina (generated by the

03Z model run on August 29, 2005). The height of the storm surge is color-coded in feet (red indicates 25 feet and higher). The black line indicates the National Hurricane Center's predicted path of the eye of the hurricane (courtesy of NOAA).

The National Hurricane Center alerts commu~itie~ to the threat of hurricanes with two levels of adv1sones . NHC forecasters issue a hurricane watch for ~ c~astal area when hurricane conditions are possible :v1thm _the next 36 hours. The second, more urgent ad:-7isory, is a hurricane warning, which NHC forecasters iss_ue _when they expect hurricane conditions to ar:ive_ w1thm the next 24 hours. When issuing these adv1s?n_es, the _National Hurricane Center must weigh confl1ctmg societal impacts . NHC must allow enough lead tim~ to_ evacuate densely populated areas, especially on barner islands or other locations where evacuation routes can be cut o~ by rising water. On the flip side, evacuations are expensive. It costs time and money ~o board wi~dows and_ sandb:~ homes and businesses m preparat10n for wmds a waves. Businesses lose money when people ev~c~ate shore areas. And it costs a lot of money just to f~c1ht~~ the evacuation. The estimated costs for pr~paratwn a d

·11· le ofw arne evacuation average about $1 mi wn per mi . · achy ear as coastline. These preparat10n costs mcrease e .

1 rty 1 es mcrease. coastal populations and coasta prope va u . h mote sensmg, Advances in technology, researc ' re . kill

. · · ove the s and forecasting techmques contmue to impr_ 1 Hurri -and accuracy of forecasts issued by the Natwna . es cane Center. Indeed, forecasts of the tracks ofhum caF~g-

. . 1 ore accurate. I and tropical storms are mcreasmg y m . e 24- 36-ure 11.63 shows the average annual err~r m th and ~opand 48-hour forecasts for Atlantic hurncanes

CHAPTER 11 Tropical Weather , Part II: Hurricanes 50 1

NHC Official Annua.1 Av~rage Track ' Errors Atlantic Basin Tropical Storms and Hurricanes

600

500

300

200

1975 1980 1985 1990 1995 2000 2005

Year

FIGURE 11.63 Annual average errors in 24-hour (red), 48-hour (green) and 72-hour (yellow) track forecasts for

·Atlantic Basin tropical storms and hurricanes, for the period 1970 to 2007, in nautical miles (1 nautical mile equals 1.15 miles). Errors in 96-hour and 120-hour forecasts are shown from 2001 to 2007 (courtesy of the National Hurricane Center).

ical storms during the period 1970-2007 (it also includes the average annual error in the 96- and 120-hour forecasts from 200 I to 2007). For one, two, and threeday track forecasts, the improvement averages between one and two percent per year . Two-day hurricane track forecasts today are as accurate as one-day forecasts were in 1970. Still, the average track error at 48 hours is about 100 miles (160 km). Because of this uncertainty, NHC track forecasts always include a "cone of uncertainty" that widens with time (see Figure 11.64).

In contrast to track forecasts, NH C's forecasts of the intensity of hurricanes and tropical storms have improved only slightly since 1970. One of the primary reasons that intensity forecast improvements have lagged those of track forecasts is that complex processes such as eye-wall replacement cycles are the primary controllers of intensity changes, and these cycles are very difficult to accurately model. For the record, an eye-wall replacement cycle occurs in major hurricanes when a spiral band completely encircles the existing eye wall. This spiral band robs the inner eye wall of moisture, so the inner eye wall collapses and the coiling spiral band forms a new outer eye wall. During the replacement cycle, the hurricane weakens, but, after the cycle is complete, the hurricane may become as strong ( or stronger) as it was before the replacement cycle began .

Figure 11.65a is a satellite image that shows deep convection (yellow and red shades) associated with Hurri-

ropical Storm Katrina ugust 24, 2005 PM EDT Wednesday

NWS TPC/Nationol Hurricane Center Advisorys

Current Center Location 25.6 N n 2 w Max Sustained Wind 4S mph Current Movement NW at 9 mph

@ Current Center location • Forecast Center Positions

H Sustained wind • 73 mph ,...___ S Sustained 1vind 39-73 mph ........:::,,.. Potential Day 1-3 Track Area cr Potentiol Da.y45 Track Area

Hurricane Watch - Tropicol Storm Warning

Troplcol Stonn Watch

FIGURE 11.64 NHC's five-day track forecast for then Tropical Storm Katrina, issued at 5 p.m. (EDT) on

August 24, 2005. The official forecast is the black line, and the white swath represents the uncertainty in the track forecast. Note that this "cone of uncertainty" widens with time, indicating the growing uncertainty in the track forecast. Indeed, Katrina's track could have fallen anywhere within this cone of uncertainty. Focus your attention on the possible landfalls along the northern Gulf Coast. Although the official forecast took Katrina into the Florida panhandle on Monday, August 29, the possible landfalls spanned from eastern Louisiana-including New Orleans-to South Carolina. Of course, the cone of uncertainty related to Katrina's first landfall in southern Florida was much narrower, indicating less uncertainty, given that the eventual landfall was less than 48 hours away (courtesy of NOAA).

cane Wilma at 07Z on October 19, 2005. For the record, the satellite sensor passively detected microwaves emitted by precipitation in the eye wall and spiral bands of Wilma. The eye was a "pinhole" at this time, consistent with Wilma's Category-5 status, with maximum winds of 150 kt ( 173 mph) . Note the spiral band starting to wrap around the eye wall, signaling the beginning of an eye-wall replacement cycle. Figure 11.65b shows the end result of the eye-wall replacement cycle, with the spiral band having formed a new (outer) eye wall. At the time , Wilma was too close to land (the Yucatan Peninsula) to regain ariy strength-maximum sustained winds were 125 kt (144 mph).

So what's our point here? Given that such comple x processes cannot be accurately modeled, it's no wonder why NH C's advance in intensity forecasting has lagged behind improvements in track forecasting .


10/.19/05 0600Z 241 WILMA 10/.19/.05 0709Z ASUA-1 89H 101/191/05 0545Z G ES-12 IR

(a) (b)

FIGURE 11.65 (a) A satellite image showing the deep convection (in yellow and red) associated with Hurricane Wilma around 07Z on October 19, 2005. At the time, Wilma was a Category-5 hurricane, with maximum sustained winds of 150 kt (173 mph).

Note the spiral band starting to wrap around the eye wall, signaling the start of an eye-wall replacement cycle. The eye of Wilma was only about 2.3 mi (3.7 km) in diameter, and the satellite could not resolve (detect) the "pinhole eye"; (b) A satellite image at 1845Z on October 20 shows the deep convection associated with a new (outer) eye wall. As the spiral band encircled the inner eye wall, it intercepted moisture destined for the inner eye wall. As a result, the inner eye wall collapsed and the now circular spiral band took over as the new (outer) eye wall. Wilma's maximum sustained speed at this time was 125 kt (144 mph) (courtesy of NASA and the Naval Research Laboratory).

We end this section with some sobering thoughts . More than 70 million people currently reside in the more than 400 shore and near-coastal c~ ies abutting the Atlantic Ocean and the Gulf of Mexico. T hese numbers swell dramatically during peak holiday dr vacations periods when millions more visit the/ ation 's shores.

Coastal populations continue to increase at a rate of four to five percent per year, on average. Indeed, because the lure of the ocean is so strong , people will always be attracted to live in harm 's way. Education and preparatio n are the keys to preventing another disastrous loss ofli fe from a future tropical cyclone.

What a Difference a Day Makes It has been said that the most beautiful day to be seen is the one that precedes a hurricane. This tidbit of tropical folklore is confirmed by satellite imagery of most hurricanes-just check a few of the images in this chapter.

The old adage "What goes up, must come down" helps to explain why skies surrounding a hurricane are relatively clear. Within a hurricane, rising currents of air sustain the thunderstorms that drive the storm's heat engine. Once updrafts reach the top of the storm, air spreads out and diverges, eventually sinking on the outer


edges of the hurricane (of course, some air also sinks into the eye). This compensating subsidence often creates a rather sharp transition from cloudy to clear conditions, with cumulus cloud development suppressed hundreds of kilometers ahead of the storm. But, as careful observers dating back to Christopher Columbus have noted, the app~arance of cirrus clouds (fanning off the tops of thunderstorms within the hurricane), coupled with rising swells over the ocean surface and a steadily falling barometric pressure, can belie the blue skies above foretelling the tempestuous approach of a hurricane. '

learning objectives€¦ · hierarchy of tropical cyclones in each of the seven basins (tropical...

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