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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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• 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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