Mid-latitude-cyclones.pdf

Lecture 21

Midlatitude Cyclones

Observation Homework Due 11/24

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Midlatitude Cyclones

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Previous Lecture

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

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

Identifying Fronts

Warm Front

Across the front - look for one or more of the following:

Cold Front

1. Change of Temperature 2. Change of Moisture characteristic (RH, Td)

Stationary Front

3. Change of Wind Direction 4. Change in pressure readings (falling vs rising pressure

Occluded Front

5. Characteristic Precipitation Patterns 6. Characteristic Cloud Patterns

Frontal symbols are placed pointing in the direction of movement of the front (except in the case of the stationary front). 5

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Typical Warm Front Structure

Typical Cold Front Structure •

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Cold air replaces warm; leading edge is steep in fastmoving front shown below due to friction at the ground

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– Strong vertical motion and unstable air forms cumuliform clouds – Upper level winds blow ice crystals downwind creating cirrus and cirrostratus •

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Slower moving fronts have less steep boundaries and less vertically developed clouds may form if warm air is stable

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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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Cyclogenesis: the Formation of a Cyclone

Midlatitude Cyclone

More commonly known as a Winter Storm

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

Cold, heavy air sinks, displacing warm air, which rises, thus converting potential energy into kinetic energy in the form of a cyclonic wind circulation. 9

Energy Source for Winter Storms

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Life Cycle of Midlatitude Cyclone Incipient Stage 2. Mature Stage 3. Occluded Stage 4. Dissipating Stage 1.

Temperature Gradients Fuel Cyclogenesis

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Stationary Front

Stationary Front cP

cP

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

mT Isotherms

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

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Mature Stage

Incipient Stage

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A kink forms on the front and cold air starts to move southward. Warm air starts to move northward. 15

Cold air continues to move south, and warm air north. Low pressure develops in the center and converging air strengthens the fronts. 16

Mature Stage

Weather Map of a Mature Storm 1016

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Temperature dashed lines

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1024

1 0 2 1 1 7 1023

2 0 1022

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1023 25

19 1 0 2 4

1 8 1025

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1 9 1025 27 22 1 0 2 4 2 1 1 0 2 6

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32 1 0 2 1

1023

2 9 1023

3 2 1023 3 2 1024

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2 2 1022

1 9 1023

Fonts - heavy lines with barbs

2 9 1023

2 4 1021

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1022

2 4 1019 29

4 2 1025 1016

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3 3 1011

38 1 0 2 1 33

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1006

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2 4 1013

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1005 42

3 8 1020

1009

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2 3 1022 3 5 1012 4 9 1005

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1019

4 1 1014

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1025

1022 4 5 1016 4 8 5 3 1022

1004

4 1 1002

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1016

1014

2 2 1020

16 1 0 2 1

1020

3 7 1024 3 8 1023 3 5 1024 3 5 1026

1021

1021

1 4 1019

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3 9 1021

1023

1 0 1023

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1020

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2 0 1023

Pressure - solid lines

1020

1 2 1024

72 7 6 1008 7 2 1005

4 8 1016

6 3 1013

1011

6 4 1020 7 0 1017

Mature Wave Cyclone

64 74

5 5 1013 75 66 5 5 1013

1010 7 7 1017

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Occluded Stage

Occluded Stage •

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Cyclone is mature, precipitation and winds are most intense.

Cyclone matures, precipitation and winds become more intense. 19

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Dissipating Stage

Occluded Stage •

Cyclone is mature, precipitation and winds are most intense.

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

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Formation of Occluded Fronts

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Formation of Occluded Fronts

East of the Rockies - Cold Occluded Fronts

West of the Rockies Warm Occluded Fronts

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Air Mass Modification and the Dry Line

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Air Steams in Midlatitude Cyclones are Three Dimensional

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

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

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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

Cold Air Steam

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Dry Air Stream

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All Three Air Streams

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Warm, Cold, Dry 32

All Three Air Streams

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

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Another view of air streams in cyclones

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Warm -red

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Cold - blue

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Dry - yellow

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.

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What Causes the Surface Low? convergence and divergence aloft Low

High Surface convergence “fills in” the low

Surface divergence “undermines” the high

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.”

Upper level pressure contours 35

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Vertical Structure of Midlatitude Cyclones

Cyclogenesis • •

Upper-level divergence initiates and maintains a surface low.

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

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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! 37

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Deepening Lows Tilt Westward with Height

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.

Lows at surface are located east of the corresponding upper-level troughs. 39

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Deepening Lows Tilt Westward with Height

Deepening Lows Tilt Westward with Height

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

Lows at surface are located east of the corresponding upper-level troughs. 41

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Storm Track

Vertical Structure of Cyclone cold cold

cold

warm

warm

warm 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.

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. 43

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Questions?

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