Transcript How do the blizzards form?

How do the blizzards from?
Review of last lecture
1. Definition of airmasses. Bergeron classification of air
masses (3 letters)
2. Surface weather analysis: Station model, wind speed
code, present weather
3. Fronts: 6 types.
4. What is a cold front? Steep, narrow, fast
5. What is a warm front? Less steep, wide, slow
6. What is an occluded front? Two types
Winter Storms and Blizzards (Mid-latitude cyclones)
U.S. Annual mean fatalities 57, annual mean loss $329 million
What is a mid-latitude cyclone?
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The mid-latitude cyclone
is a synoptic scale low
pressure system that has
cyclonic (counterclockwise in northern
hemisphere) flow that is
found in the middle
latitudes (30N-55N, 30S55S).
It has a larger size than a
tropical cyclone
bring abrupt changes in
wind, temperature, and
sky conditions
Midlatitude cyclones often form near the
fronts and jet streams
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Jet streams are caused by
steep temperature gradients
between cold and warm air
masses
Polar front - marks area of
contact, steep pressure
gradient  polar jet stream
Low latitudes 
subtropical jet stream
Stronger in winter, affect
daily weather patterns
Conceptual models
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Norwegian cyclone model. Developed by Bjerknes in
1919, this model has formed the basis of our
understanding of the life cycle of extratropical cyclones,
an amazing achievement given that this conceptual
model was developed during a time period of highly
limited upper air data.
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Conveyer belt model. Developed by Browning (1973)
and Carlson (1980), this model emphasizes the sources
of the incoming airstreams for an extratropical cyclone.
How does a mid-latitude cyclone form?
In mid-latitude there is a boundary
between northern cold air and
southern warm air
In the boundary a initial cyclone
can advect warm air northward
and cold air southward
Mature stage. Cold air begins to
catch up with warm air (occluded).
If the upper level low is to the west
of surface low, the cyclone will
amplify and precipitation will form.
Cold air cools down the cyclone.
Dissipation.
Structure of Mature Cyclones
• Well-developed fronts circulating about a deep
low pressure center describe a mature cyclone.
• The system is capable of creating snow, sleet,
freezing rain, and/or hail. Chance of precipitation
increases toward the storm center.
• Heavy precipitation (thunderstorms, squall
lines) stems from cumulus development in
association with the highly unstable conditions in
the warm sector ahead of the cold front.
• Lighter precipitation is associated with stratus
clouds of the warm front.
• Winds, spiraling counterclockwise toward the
low, change accordingly as the system, and its
associated fronts, moves over particular regions.
Conveyer belt model
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Emphasizes the
sources of the
incoming
airstreams
Example of Mature Cyclones
Video: The worst storm yet 2/1/2011
http://www.youtube.com/watch?v=Vnfw
r1y016Q
Why do some frontal waves develop
into huge cyclonic storms, but others
don’t?
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Complex challenge to forecasting;
Atmospheric conditions at the surface and aloft affect
cyclogenesis.
Key is to look at the upper level winds (longwaves,
shortwaves).
Longwave disturbances (Rossby waves)
Earth's poles are
encircled by 3 to 6
longwaves, or Rosby
waves, directing
upper level winds
around lows at the
500 mb surface.
Small disturbances in
these waves can
trigger storms.
Shortwave Disturbances
Shortwave ripples
within the Rossby
waves move faster,
and propagate
downwind into the
Rossby troughs and
cause them to deepen.
Barotropic conditions,
where isobars and
isotherms are parallel,
then degenerate into a
baroclinic state where
the lines cross and cold
or warm air is advected
downwind.
Regions of cyclogenesis and typical tracks
• Gulf of Mexico, east coast
• Alberta Clipper from eastern side of Canadian Rockies
• Colorado Low from eastern slope of American Rockies
 Lee-side lows, lee cyclogenesis
Climate Change and Midlatitude Cyclones
• Midlatitude cyclones are associated with the polar front
and polar jet stream.
• It’s important to consider if climate change will cause a
shift in the average tracks of midlatitude cyclones.
• Recent studies have shown that the average tracks for
midlatitude cyclones has shifted towards the poles.
• The IPCC believes that continued warming and climate
change will continue to move the tracks of the
midlatitude cyclones towards the poles. This will have
serious climate consequences to the Earth.
• Moreover, global warming weakens the polar vortex,
causing more cold air intrusion into midlatitudes
Anticyclones, heat waves and droughts
• Anticyclones are associated
with clear skies and calm
conditions.
• If anticyclones linger over a
region, it can lead to heat
waves and/or droughts.
• Anticyclones are responsible
for the Santa Ana wind
conditions over the west
coast, US.
Summary
1. The developmental stages and vertical structure of
middle latitude cyclones (boundary between
northern cold air and southern warm air, upper level
low to the west of surface low)
2. How upper level longwaves and shortwaves may
enhance cyclonic development at the surface (upper
level low to the west of surface low)
3. The three regions of cyclogenesis and typical tracks
4. Climate change: move the tracks of the midlatitude
cyclones towards the poles
5. Anticyclones, heat waves and droughts