Download Lecture 21 Midlatitude Cyclones Observation Homework Due 11/24

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).
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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.
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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
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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.
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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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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
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19 1 0 2 4
1 8 1025
30
1 9 1025
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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
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4 2 1025
1016
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3 3 1011
38 1 0 2 1
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1006
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2 4 1013
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1005
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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
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7 6 1008
7 2 1005
4 8 1016
6 3 1013
1011
6 4 1020
7 0 1017
Mature Wave Cyclone
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5 5 1013
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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.
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Dissipating Stage
Occluded Stage
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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
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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
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Vertical Structure of Midlatitude Cyclones
Cyclogenesis
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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!
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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.
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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.
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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.
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Questions?
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