AOSC400-2009November 12, Lecture # 21

Chapter # 12

Mid-Latitude Cyclones

�Polar Front Theory

�Where do mid-latitude cyclones tend to form

�Vertical structure of deep dynamic lows

�Upper level waves and mid-latitude cyclones

�Conditions for developing mid-latitude cyclones

Mid-latitude cyclones

• Mid-latitude cyclones are the cause of most of the stormy weather in the United States, especially during the winter season.

• A mid-latitude cyclone is an area of low pressure located between 30 degrees and 60 degrees latitude. Since the continental United States is located in this latitude belt, these cyclones impact the weather in the U.S.

• Understanding the structure and evolution of mid-latitude cyclones is crucial for predicting significant weather phenomena such as blizzards, flooding rains, and severe weather.

(a)Typical paths of winter

mid-latitude cyclones.

The lows are named after the region where they form.

b) Typical paths of winter anticyclones.

Mid-latitude cyclone and the Polar Front Theory

• Polar front is a semi-continuous boundary separating cold, polar air from more moderate mid-latitude air

• Mid-latitude cyclone (wave cyclone) forms and moves along polar front in wavelike manner

• Related Concepts: Frontal wave, warm sector, mature cyclone, triple point, secondary low, family of cyclones

Mid-latitude cyclone (Extra tropical storms)

• Norwegian scientists in Bergen,developed a model that explains the life cycle of an extra tropical storm-a storm that forms in mid-latitudes, outside the tropics.

• Vilhelm Bjerknes, his son Jacob, HalvorSolberg and Tor Bergeron published after World War II the Polar Front Theory of a Developing Wave Cyclone.

Fig. 12-1, p. 314

The idealized life cycle of a mid-latitude cyclone (a through f) in the Northern

Hemisphere based on the polar front theory. As the life cycle progresses, the

system moves eastward in a dynamic fashion. The small arrow next to each L

shows the direction of storm movement.

Development of a wave cyclone begins

at the Polar Front:

• Stationary front, a trough flow in opposite direction

sets a cyclonic wind shear

• Under right conditions, a kink forms-a frontal wave

• Overturning ahead of warm front, narrow band of

precipitation

• Within 24 hours becomes fully developed-open wave

• Triple point-the warm sector is removed from the

center of storm-no rising air, dissipation

• Developing of cyclone-cyclogenesis, a region with

propensity for cyclogenesis

Fig. 12-2, p. 315

A series of wave cyclones (a “family” of cyclones) forming along the polar front.

Fig. 12-3, p. 316

Visible satellite image of the north Pacific with two mid-latitude cyclones in different stages of development during Feb. 2000.

Where do mid-latitude cyclones tend to form?

• Lee-side lows (cyclogenesis)

• Nor’easters

• Hatteras low

• Alberta Clipper

• Explosive cyclogenesis

As westerly winds blow over a mountain range, the airflow is deflected in such a way that a trough forms on the downwind

(leeward) side of the mountain. Troughs and developing cyclonic

storms that form in this manner are called lee-side lows.

Where do mid-latitude cyclones tend to form?

Northeasters

–Mid-latitude cyclones that

develop or intensify off the

eastern seaboard of North

America then move NE along

coast

Fig. 1, p. 318

The surface weather map for 7:00 A.M.

(EST) December 11,

1992, shows an intense low-pressure area

(central pressure 988 mb) which is generating

strong northeasterly winds and heavy

precipitation from the

mid-Atlantic states into

New England. This

northeaster devastated a wide area of the

eastern seaboard, causing damage in the

hundreds of millions

of dollars.

• Nor’easters, move NE along Atlantic coast, bring big winds and heavy snow or rain to coastal areas

• When extra tropical cyclone deepen rapidly more than 24 mb in 24 hrs - explosive cyclogenesis.

• Key to the development of a wave cyclone -upper air wind flow in the region of high-level westerlies.

• Surface storm centers travel across the US at about 16 knots in summer and 27 knots in winter.

•

Fig. 12-5a, p. 317

Typical paths of winter mid-latitude cyclones. The lows are named after the region where they form.

Fig. 12-5b, p. 317

Typical paths of winter anticyclones

The Necessary Ingredients for Development

of Mid-latitude Storm

• Key to the development of a wave cyclone is found in upper wind flow in the region of high level westerlies

• Energy for storm-warm air rises and cold air sinks transforming potential energy into kinetic energy; condensation-latent heat reliese. As surface air converges towards the low center, wind speed increases, more kinetic energy.

Fig. 12-6, p. 319

If lows and highs aloft were always directly above lows

and highs at the surface,

he surface systems would quickly dissipate.

Fig. 12-7, p. 319

An idealized

vertical

structure

of a middle-

latitude cyclone

and

anticyclone.

The formation of convergence (CON) and divergence (DIV) of air with a constant wind speed in the upper troposphere. Circles

represent air parcels that are moving parallel to the contour lines

on a constant pressure chart. Below the area of convergence the air is sinking, and we find the surface high (H). Below the area of

divergence the air is rising, and we find the surface low (L).

Fig. 3, p. 320

As the faster-flowing air in the ridge moves toward the slower-flowing air in the trough, the air piles up and converges. As the slower-moving air in the trough

moves toward the faster-flowing air in the ridge, the air spreads apart and diverges.

Upper Level Waves and Mid-latitude Cyclones

• Longwaves and shortwaves

• Barotropic vs. baroclinic

• Cold and warm air advection

Upper level waves and surface storms

• An upper level chart shows shows a series of ridges and troughs known as long waves, about 4-6, often found to the east of topographic barriers- RossbyWaves.

• Embedded in them are the shortwaves that move eastward fast at a speed proportional to average wind at 700 mb.

• If isoterms are parallel to isobars, there is no temperature advection-atmosphere is known to be barotropic.

• When isoterms cross isobars, winds produce temperature advection-the atmosphere is baroclinic.

Fig. 12-8, p. 321

A 500-mb map of the

Northern Hemisphere

from a polar perspective

shows five longwaves

encircling the globe.

Note that the wavelength

of wave number 1 is as great as the width of the United States.

Solid

lines are contours.

Dashed lines show

the position of

longwave troughs.

(a)Upper-air chart showing a longwave with three

shortwaves (heavy dashed lines) embedded in the flow.

Twenty-four hours later the shortwaves have moved rapidly around the longwave. Notice that the shortwaves labeled 1 and 3 tend to deepen the

longwave trough, while shortwave 2 has weakened as it moves into a ridge. Notice also that as the longwave deepens in diagram (b), its length

actually shortens. Dashed lines are isotherms in °C. Solid lines are contours. Blue arrows indicate cold advection and

Fig. 12-9, p. 321

Fig. 12-10, p. 323

An idealized view of the formation of a mid-latitude cyclone during baroclinic instability. (a) A longwave trough at 500 mb lies parallel to and directly

above the surface stationary front. (b) A shortwave (not shown) disturbs the flow aloft, initiating temperature advection (blue arrow, cold advection; red

arrow, warm advection). The upper trough intensifies and provides the necessary vertical motions (as shown by vertical arrows) for the

development of the surface cyclone. (c) The surface storm occludes, and without upper level divergence to compensate for surface converging air,

the storm system dissipates.

A portion of a 300-mb chart (about 33,000 ft above sea level) that shows the core of the jet—the region of maximum winds

(MAX)—called a jet streak.

Fig. 5, p. 324

Changing air motions within a straight jet streak (shaded area) cause strong convergence of air at point 1 (left entrance region)

and strong divergence at point 3 (left exit region).

An area of strong divergence (DIV) can form with a curving jet

streak. Below the area of divergence are rising air, clouds, and

the developing mid-latitude cyclonic storm.

Fig. 12-11, p. 325

(a)As the polar jet stream and its area of maximum winds (the jet streak, or core) swings over a developing mid-latitude cyclone, an area of divergence (D) draws warm surface air upward, and an area of convergence (C) allows cold air to sink. The jet stream removes air above the surface storm, which causes surface pressures to drop and the storm to intensify. (b) When the

surface storm moves northeastward and occludes, it no longer has the upper-level support of diverging air, and the surface storm gradually dies

out.

Summary of clouds, weather, vertical motions, and upper-air support associated with a developing midlatitude cyclone.

A color-enhanced infrared satellite image that shows a developing mid-latitude

cyclone at 2 a.m. (EST) on March 13, 1993. The darkest shades represent clouds with the coldest and highest tops. The dark cloud band moving through Florida represents a line of severe thunderstorms. Notice that the cloud pattern is in the

shape of a comma.

Storm of the Century

The Storm of the Century, also known as the ’93 Super-storm, was a large cyclonic storm that occurred on March 12-13, 1993, on the East Coast of North America. It was unique for its intensity, massive size and wide-reaching effect. The storm stretched from Canada to Central America; the main impact was on the Eastern United States and Cuba.

An ara of low pressure that formed in the Gulf of Mexico joined an arctic high pressure system in the Midwestern Great Plains, brought into the mid-latitudes by an unusually steep southward jet stream. These factors combined to produce unusually low temperatures across the eastern half of the United States.

Forecasting the Storm

The 1993 Storm of the Century marked a milestone in U.S. weather forecasting. By March 8 several numerical weather prediction models at the US National Weather service recognized the possibility of a significant snowstorm. This marked the first time that National Weather Service meteorologists were able to accurately predict a system's severity five days in advance. Official warnings were issued twodays before the storm arrived. Forecasters were sufficiently confident to support decisions by several Northeastern U.S. states to declare a State of Emergency before the snow even started to fall In the South. Temperatures in the days prior to the storm were typical for March. Many residents doubted that freezing temperatures could return so rapidly; nor that snow waslikely due to the rarity of significant snowfall later than February. Many TV stations were reluctant to broadcast the forecast models, but the models turned out to be right.

Surface weather map for 4 a.m (EST) on March 13, 1993. Lines on the map are isobars. A reading of 96 is 996 mb and a reading of 00 is 1000 mb. Green shaded areas are receiving precipitation. The orange arrow represents warm, humid air; the light blue arrow, cold, moist air; and the dark blue arrow, cold,

arctic air.

Fig. 12-17, p. 328

The 500-mb chart for 7 a.m. (EST) March 13, 1993. Solid lines are contours where 564 equals 5640 meters. Dashed lines are isotherms in °C. Wind entries in red show warm advection. Those in blue show cold advection.

Those in black indicate no appreciable temperature advection is occurring.

The development of the ferocious mid-latitude cyclonic storm of March, 1993. A

small wave in the western Gulf of Mexico intensifies into a deep open-wave

cyclone over Florida. It moves northeastward and becomes occluded over Virginia where its central pressure drops to 960 mb. As the occluded storm continues its northeastward movement, it gradually fills and dissipates. Arrows show direction

of movement.