Tropical Meteorology

Jeff GawrychMet. 10

Introduction

• Tropical weather is very different than our mid-latitude weather. – No fronts, little day-to-day variation

• Tropical weather systems form due to warm air rising (convection).

• Thermodynamics says– Heat added to system will increase the temperature

and increase ability to perform work• Warm oceans are fuel• Latent heat release when warm air rises and condenses• Tropical storms and hurricanes act like heat engines

Background• Tropical cyclones/Hurricanes are perhaps the most Tropical cyclones/Hurricanes are perhaps the most

severe of the weather phenomena. They can cause severe of the weather phenomena. They can cause both massive property damage and loss of life.both massive property damage and loss of life.– in 1972 a TC killed an estimated 300,000-500,000 people in in 1972 a TC killed an estimated 300,000-500,000 people in

Bangladesh.Bangladesh.

– in 1992 Hurricane Andrew hit the southern US.in 1992 Hurricane Andrew hit the southern US.

– in 1999 created massive flooding & mud slides in Honduras and in 1999 created massive flooding & mud slides in Honduras and GuatemalaGuatemala

– in 2004, 4 hurricanes hot the Florida coast. Charley, Frances, Ivan, in 2004, 4 hurricanes hot the Florida coast. Charley, Frances, Ivan, Jeanne.Jeanne.

– http://www.cnn.com/SPECIALS/2004/hurricanes/interactive/hurricane.paths/index.html

ITCZ

• Inter-Tropical Convergence Zone– defined as the result of the equatorial

Hadley cell where air converges near the equator, rises and diverges near the tropopause

• Creates large thunderstorms called hot towers.

• Usually between 5 and 10 degrees latitude.

Definition• These phenomena are commonly known as These phenomena are commonly known as • ______________________________ (Indian Ocean & South Pacific), (Indian Ocean & South Pacific), • ______________________________ (Atlantic and Eastern Pacific) (Atlantic and Eastern Pacific) • ______________________________ (North Pacific). (North Pacific).

• Tropical storms differ considerably from mid-latitude cyclones: For example, tropical Tropical storms differ considerably from mid-latitude cyclones: For example, tropical storms havestorms have– – –

Tropical CyclonesTropical Cyclones

Hurricanes*Hurricanes*

TyphoonsTyphoons

Warm coreWarm coreNo frontsNo frontsStrongest wind Strongest wind near the surfacenear the surface

* Named for Carib god of evil: * Named for Carib god of evil: HuracanHuracan

Stages of development•Tropical Disturbance:Tropical Disturbance:

•Tropical depression:Tropical depression:

•Tropical storm: Tropical storm:

•Hurricane:Hurricane:

some closed isobars and winds 20-34 knots (more organization)some closed isobars and winds 20-34 knots (more organization)

mass of thunderstorms with slight wind circulationmass of thunderstorms with slight wind circulationSome organizationSome organization

packed isobars, winds 35-64 knotspacked isobars, winds 35-64 knots

when winds above 64 knots (74 mph)when winds above 64 knots (74 mph)

Atlantic Hurricanes…

• Begin with easterly waves– Form as the Sahara Desert intensely

warms up in the summertime– Wind blows to the west at ~ 15 deg. N– Triggers thunderstorm formation

Hurricane Classification

Recipe for Tropical Cyclone Formation

1.1. Warm ocean temperatures (T> 80F):Warm ocean temperatures (T> 80F):– Typical in subtropical Atlantic/Pacific in Typical in subtropical Atlantic/Pacific in

summer/early fall summer/early fall – N. H. hurricane season: N. H. hurricane season:

2.2. Rotation:Rotation:– Thunderstorms must organize to produce Thunderstorms must organize to produce

converging airconverging air– Cannot form at equator Cannot form at equator

3.3. Weak winds: Weak winds:

June-NovemberJune-November

Coriolis force=0Coriolis force=0

Strong wind can disrupt organizationStrong wind can disrupt organization

Hurricane movement• Steered by prevailing windsSteered by prevailing winds

– Easterly winds of tropical latitudesEasterly winds of tropical latitudes– Westerly winds of middle latitudesWesterly winds of middle latitudes

• Middle latitude high/low pressure systems Middle latitude high/low pressure systems can cause quick changes in hurricane can cause quick changes in hurricane position.position.

• Often challenging to predict!! Often challenging to predict!!

Hurricane GeorgeHurricane George

Evolution• Tropical Tropical

cyclones are cyclones are driven by the driven by the warm ocean warm ocean temperaturetemperature

• Thus, they will Thus, they will weaken and weaken and eventually die eventually die when they move when they move over over – –

Evolution• Tropical Tropical

cyclones are cyclones are driven by the driven by the warm ocean warm ocean temperaturetemperature

• Thus, they will Thus, they will weaken and weaken and eventually die eventually die when they move when they move over over – land or land or – colder waters.colder waters.

Formation30%12% 15% 12%

12% 7% 12%

Hurricane Structure• Eye of hurricaneEye of hurricane

– Extremely low pressure, Extremely low pressure, – weak winds and generally clear skiesweak winds and generally clear skies

• Eye wallEye wall– Intense thunderstormsIntense thunderstorms– Strongest winds, heaviest precipStrongest winds, heaviest precip

• Spiral rain bandsSpiral rain bands– Areas corresponding to upward motion and heavy rainAreas corresponding to upward motion and heavy rain– Alternating with rain free areasAlternating with rain free areas

Why is eyewall the most intense part of the Hurricane?• Law of Conservation of angular

momentum– Wind speed (v) X distance from axis of

rotation (r) must remain constant • Close to eye --> r is small --> v must be large• Far from eye --> r is large --> v is small

Hurricane Destruction• Winds can have speeds over 100mph.Winds can have speeds over 100mph.

– Extensive damage to buildings and vegetation.Extensive damage to buildings and vegetation.

• Storm surge:Storm surge:

• Hurricane may spawn tornadoes.Hurricane may spawn tornadoes.

abnormal rise in ocean level responsible for most abnormal rise in ocean level responsible for most damage via floodingdamage via flooding