Download Lecture 21 Midlatitude Cyclones Observation Homework Due 11/21

Lecture 21
Midlatitude Cyclones
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Observation Homework Due 11/21
Quiz #3 on Thursday
November 2nd
Topics to be Covered
• Atmospheric optics
• Hurricanes, formation, structure
• Hurricanes near Hawaii/Hurricane impacts
• Air masses/air mass modification
• Fronts
Sunset on Sunday Oct. 22 2006
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Midlatitude Cyclones
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
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Identifying Fronts
Across the front - look for one or more of the
following:
Warm Front
Cold Front
1. Change of Temperature
Stationary Front
3. Change of Wind Direction
2. Change of Moisture characteristic (RH, Td)
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
• 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).
• Cold air replaces warm; leading edge is steep in fastmoving 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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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
Life Cycle of Midlatitude Cyclone
1.
2.
3.
4.
Incipient Stage
Mature Stage
Occluded Stage
Dissipating Stage
Temperature Gradients Fuel Cyclogenesis
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Stationary Front
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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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Incipient Stage
Mature Stage
Warm wind from south increases ahead of
low, and cold winds from north increase
behind low causing the wave to grow.
Small disturbance, or wave, forms on the
polar front. Cold air moves southward and
sinks. Warm air moves northward and rises.
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Map of Midlatitude Cyclone
Temperature dashed lines
Pressure - solid
lines
Mature Stage
1016
2 0 1022
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24 1 0 2 1
1022
25
2 4 1019
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1016
3 3 1011
1006
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2 4 1013
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3 5 1012
4 9 1005
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1019
4 1 1014
4 8 1016
4 2 1025
1016
3 8 1020
1005
4 2 1009
4 5 1016 4 8
4 5 1025
1022
5 3 1022
1004
4 1 1002
2 3 1022
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3 9 1021
3 7 1024
38 1 0 2 1
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1014
33
16 1 0 2 1
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2 9 1023
1020
3 8 1023
3 5 1024
3 5 1026
3 0 1021
2 2 1020
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2 9 1023
1023
1021
1020
13
32 1 0 2 1
3 2 1024
2 2 1022
1 4 1019
1 2 1024
1 9 1025
2 7 1023
3 2 1023
22 1 0 2 4 2 1 1 0 2 6
1 0 1023
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19 1 0 2 4
1 8 1025
2 0 1023
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1 4 1020
2 5 1023
1 0 1024
1 0 2 1 1 7 1023
1 9 1023
Fonts - heavy
lines with barbs
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6 3 1013
7 2 1011
7 6 1008
7 2 1005
6 4 1020
7 0 1017
Mature Wave Cyclone
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5 5 1013
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5 5 1013
1010
7 7 1017
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Occluded Stage
Dissipating Stage
The dissipating stage is characterized by warm air
being pushed aloft and the size of the warm air
wedge at the surface decreases significantly.
The Occluded Stage begins when the cold
front starts to overrun the warm front
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Life Cycle of Midlatitude Cyclone
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Life Cycle of Midlatitude Cyclone
Mature wave
cyclone
Partially occluded
cyclone
Partially occluded
wave cyclone
Occluded
cyclone
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Air Steams in Midlatitude Cyclones
are Three Dimensional
Warm Air Stream
• Warm air stream brings warm moist (mT) air in
the warm sector and lifts it over the warm front.
• Cold air stream bringsTextcold 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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Cold Air Steam
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Cold Air Steam
Dry Air Stream
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All Three Air Streams
Three Airstreams
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•
•
•
Air streams in cyclones
Warm -red
Cold - blue
Dry - yellow
Warm, Cold, Dry
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What maintains the surface low?
What Causes the Surface Low to Form?
Imagine a surface low forming directly below an upper level low.
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.
Surface convergence
“fills in” the low
Surface divergence
“undermines” the high
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Surface Pressure Changes
Lows Tilt Westward with Height
• Lows and highs at surface
are located east of the
corresponding upper-level
troughs and ridges.
• this is because the air tends
to be colder on west side of
lows than on the east side.
• And the air tends to be
warmer on west side of highs
than on east side of highs.
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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Vertical Structure of Midlatitude Cyclones
What Causes the Surface Low?
convergence and
divergence aloft
Low
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.”
High
Upper-level divergence
initiates and maintains a
surface low.
Upper-level low is tilted
westward with height
with respect to the
surface.
500 mb height
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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.
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Vertical Structure of Occluded Low
Temperature advection is key!
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Storm Track
Weather Hazards in Winter Storms
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.
Hazards associated with Midlatitude Cyclones
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Weather Hazards
Winter Storm - Hazards
• High Winds
– Nor’easter, Blizzard Conditions, Turbulence
• Frozen Precipitation
Blizzard
High winds
– Blizzard Conditions, Snow, Sleet and Freezing
Rain, Aircraft Icing, Riming, Snow Avalanches
Sleet and ZR
Severe T-storms
Tornados
High winds
• Heavy Rain
– Flooding, Flash Floods, Mud Slides
• Large Ocean Swell and Waves
– High Surf, Storm Surge
• Severe Thunderstorms
– Tornados, Large Hail, High Winds, Lightning
Where are the hazards?
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Hazards – High Winds
High Winds
Where do we find the strongest winds in a midlatitude cyclone?
• Surface
– Where the pressure gradient is largest, often on NW side of
storm.
– Near the fronts
– Near convective clouds (including tornados)
Upper
Air
•
– Where the pressure gradient is largest
– In pulses superimposed on the jet stream
– these pulses are called jet streaks
– turbulence is found near jets in regions of large wind shear
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Where do the Hazards Occur?
Animation of the “Perfect Storm”
Blizzard conditions
occur N and NW of
surface-low center.
Nor’easters
• Strong low pressure
systems moving up
Atlantic seaboard
• Strong winds and heavy
precipitation (blizzards).
The storm—created from a collision between a high pressure
system, a low pressure system and the remnants from a
dying hurricane—sent high winds and Atlantic Ocean waves
crashing into the East Coast, from New England to North
Carolina. The Andea Gail sank in this storm on 10/28/91.
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Hazards – Large Waves
Hazards – Large Swell
Oahu, North Shore
January 1998.
Surf's up!
Heavy surf on the Columbia River
bar tests a Coast Guard vessel
approaching the mouth of the
Columbia River.
Peggotty Beach, Massachusetts
February 9, 1978
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Precipitation Patterns
Hazards – Heavy Rain and Flooding
Radar View of
Precipitation
Flash floods often cause
fatalities by drowning when
people try to drive across
flood waters.
Where does air rise
• Along the fronts
• In areas of convection
• Around surface the low
center
Oahu flood, November 1995
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Hazards – Heavy Rain and Flooding
Hazard – Freezing Rain & Sleet
An ice storm in New York
also caused flooding of
water front properties in the
region, January 1998. .
Kansas City, MO, January 31, 2002.
Freezing rain and sleet cause
significant disruption of
transportation and resulting
economic impact.
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Hazards – Sleet and Freezing Rain
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Hazards – Sleet and Freezing Rain
The Rain – Snow Line
• North of the warm
front there is a
transition from rain to
snow called the rainsnow line.
• Freezing rain and
sleet fall on the warm
side of this line.
As the depth of the cold air near the ground increases north of the warm
front, precipitation changes from rain to freezing rain to sleet to snow.
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Freezing
Rain
Freezing
Rain
Television Tower
Raleigh, NC
January 1990
Television Tower
Raleigh, NC
January 1990
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Freezing
Rain
Freezing
Rain
Television Tower
Raleigh, NC
January 1990
Television Tower
Raleigh, NC
January 1990
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Hazards – Blizzard
Superstorm of March 1993
Blizzard conditions include winds !35 mph
and visibility less than 1/4 mi.
This enormous winter storm killed over 250 people and
cancelled 25% of the United States' flights for two days.
It spawned blizzard conditions, hurricane force winds, 25
tornados, and a storm surge.
North Dakota: A March 1966 blizzard
nearly buried utility poles.
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Hazards – Heavy Snow
‘93 Blizzard
Snowstorms leave a band of
heavy snow in their wake.
Blizzard conditions include heavy snowfall and blowing snow with
strong winds. Note the tight spacing of isobars at left.
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Hazards – Heavy Snow
Hazard – Snow Avalanche
Death in blizzards
comes from exposure
to cold.
Western U.S. mountains generally record over 100,000 avalanches
every winter. Some avalanche slides can reach speeds of over 100
miles per hour.
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Hazard – Snow Avalanche
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Hazards – Severe Thunderstorms
In US roughly 25 deaths
per year due to snow
avalanches.
Large Hail
Tornados
High Straight-line winds
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Where do the Hazards Occur?
Weather Hazards in
Winter Storms
Severe Thunderstorms
Produce:
• Tornados
• Large Hail
• High Straight-line Winds
(and Lightning)
Severe Thunderstorms
associated with a cold
front.
Occur where jet stream
crosses the cold front.
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Questions?
Questions?
Blizzard
High winds
Sleet and ZR
Severe T-storms
Tornados
High winds
Three areas with distinct hazards in winter storms.
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