Lecture 21 Midlatitude Cyclones

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Observation Homework Due 11/24

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Midlatitude Cyclone or Winter Storm• Cyclogenesis• Energy Source• Life Cycle• Air Streams• Vertical Structure• Storm Hazards

Midlatitude Cyclones

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Previous Lecture

Air masses and Fronts• Air mass formation• Types of air masses• Types of Fronts• Identifying Fronts• Formation of Fronts

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Four Types of Fronts

Warm Front

Cold Front

Stationary Front

Occluded Front

Frontal symbols are placed pointing in the direction of movement of the front (except in the case of the stationary front).

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Identifying Fronts

Across the front - look for one or more of the following:1. Change of Temperature2. Change of Moisture characteristic (RH, Td)3. Change of Wind Direction4. Change in pressure readings (falling vs rising

pressure5. Characteristic Precipitation Patterns6. Characteristic Cloud Patterns

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Typical Cold Front Structure• Cold air replaces warm; leading edge is steep in fast-

moving front shown below due to friction at the ground– Strong vertical motion and unstable air forms cumuliform clouds– Upper level winds blow ice crystals downwind creating cirrus and

cirrostratus• Slower moving fronts have less steep boundaries and less

vertically developed clouds may form if warm air is stable

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Typical Warm Front Structure• In an advancing warm front, warm air rides up over colder

air at the surface; slope is not usually very steep.• Lifting of the warm air produces clouds and precipitation well

in advance of boundary.• At different points along the warm/cold air interface, the

precipitation will experience different temperature histories as it falls to the ground (snow, sleet, fr.rain,& rain).

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Midlatitude CycloneMore commonly known as a Winter Storm

Cold, heavy air sinks, displacing warm air, which rises, thus converting potential energy into kinetic energy in the form of a cyclonic wind circulation.

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Cyclogenesis: the Formation of a Cyclone

Cyclones develop along frontal zones because denser, cold air is located at the same height as nearby, less-dense, warm air.

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Energy Source for Winter Storms

Temperature Gradients Fuel Cyclogenesis

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Life Cycle of Midlatitude Cyclone

1. Incipient Stage2. Mature Stage3. Occluded Stage4. Dissipating Stage

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Stationary Front

Cyclone begins with a stationary polar front that separates cold easterlies and warm westerlies.

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Isobars Isotherms

Stationary Front

Note the two air masses, cP and mT, that are involved in the early formation of this front.

mT

cP

mT

cP

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Incipient Stage

• A kink forms on the front and cold air starts to move southward. Warm air starts to move northward.

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Mature Stage

• Cold air continues to move south, and warm air north. Low pressure develops in the center and converging air strengthens the fronts.

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Weather Map of a Mature Storm

4 5

1 0

2 5

3 1

3 9 1 0 2 1

1 0 2 0

3 8 1 0 2 3

3 8 1 0 2 0

38 1 0 2 1

1025

4 2 1 0 2 5

3 5 1 0 2 63 5 1 0 2 4

3 7 1 0 2 4

2 9 1023

5 3 1 0 2 24 8 1 0 2 2

6 4 1020

2 0

30 1021

2 9

2 9

1 0 2 3

3 2 1 0 2 327 1023

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42 10091 0 1 6

4 9 1 0 0 4

72 1011

6 3 1 0 1 3 7 0 1 0 1 7

4 9

7 7 1 0 1 7

7 5

7 4

6 4

2 1 1 0 2 622 1 0 2 4

2 2

1 9 1 0 2 5

2 2 1 0 2 0

1 8 1 0 2 2

3 5

3 3 1 0 1 1

2 4 1 0 1 9

2 1 1 0 2 3

1 0 2 32 4

2 5

1 0 2 11 9

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4 1 1 0 0 2

3 5

4 8

3 4 1019

1022

1023

1016

1006

1014

1012

1005

10161016

7 2 1 0 0 5

4 1

6 6 1 0 1 0

5 5 1 0 1 3

1 7

2 3 1 0 2 2

7 6 1 0 0 8

4 9 1 0 0 5

2 4 1 0 1 3

2 0 1 0 2 2

1 0 2 1

1 0 2 3

1 4 1 0 1 9

1 0 2 1

1 0 2 116

2 8

14 1020

32 1 0 2 1

1 2 1024

3 2 1 0 2 4

3019 1 0 2 4

25 1023

1 8 1 0 2 517 1023

10 1024

1 0 1 6

5 5 1 0 1 3

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1 3

1 0 2 0

1 0 1 425

Temperature - dashed lines

Pressure - solid lines

Fonts - heavy lines with barbs

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Mature Wave Cyclone

Mature Stage

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Occluded Stage

• Cyclone matures, precipitation and winds become more intense.

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Occluded Stage

• Cyclone is mature, precipitation and winds are most intense.

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Occluded Stage

• Cyclone is mature, precipitation and winds are most intense.

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Dissipating Stage

• Cyclone continues to occlude (end of life cycle) and begins to dissipate or weaken.

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East of the Rockies - Cold Occluded Fronts

Formation of Occluded Fronts

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Formation of Occluded Fronts

West of the Rockies - Warm Occluded Fronts

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Air Mass Modification and the Dry Line

• Dry air entering eastern Texas from the west encounters warm moist air moving north from the Gulf of Mexico, resulting the formation of a dry line.

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Air Steams in Midlatitude Cyclones are Three Dimensional

• Warm air stream brings warm moist (mT) air in the warm sector and lifts it over the warm front.

• Cold air stream brings cold moist (mP) air westward to the north and beneath the warm front to the low pressure center.

• Dry air stream brings cold dry (cP) air from the north west and descends behind the cold front.

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Warm Air Stream

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Warm Air Stream

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Cold Air Steam

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Cold Air Steam

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Dry Air Stream

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All Three Air Streams

• Warm, Cold, Dry32

• Another view of air streams in cyclones

• Warm -red• Cold - blue• Dry - yellow

All Three Air Streams

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What Causes the Surface Low to Form? Relationship of Surface and Upper-level Lows

When upper-level divergence is greater than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low deepens.

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What maintains the surface low?Imagine a surface low forming directly below an upper level low.

Surface convergence “fills in” the low

Surface divergence “undermines” the high

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Low

High

convergence and divergence aloft

Upper level pressure contours

When upper-level divergence is stronger than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low intensifies, or “deepens.”

What Causes the Surface Low?

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Vertical Structure of Midlatitude Cyclones

Upper-level divergence initiates and maintains a surface low.

Upper-level low is tilted westward with height with respect to the surface.

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Cyclogenesis• Upper level

shortwave passes.• Upper level

divergence leads to sfc low.

• Cold advection throughout lower troposphere.

• Cold advection intensifies upper low.

• Leads to more upper level divergence.

Temperature advection is key!38

Surface Pressure Changes

Cold air moving in behind the cold front causes the pressure to rise. Warm air moving over the warm front causes pressure to fall.

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Deepening Lows Tilt Westward with Height

Lows at surface are located east of the corresponding upper-level troughs.

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Deepening Lows Tilt Westward with Height

Lows at surface are located east of the corresponding upper-level troughs.

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Deepening Lows Tilt Westward with Height

Lows at surface are located east of the corresponding upper-level troughs.

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Vertical Structure of Cyclone

a) Incipient stage, b) mature stage, c) occluded stage. Thin contours are sea level pressure, thick arrow show jet-stream level flow. Dashed lines show temperature, with cold air to the NW and warm to the SE.

cold

warm

cold

warm

cold

warm

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Storms are steered by flow in the upper troposphere. The location and strength of the jet-stream flow is governed in part by the distribution of sea surface temperature. Thus, el niño influences the storm track.

Storm Track

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Questions?

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