Chapter 13, Part 2
Upper-Air Support
Overview
• Last time: how do middle latitude cyclones form?
• Today: when (why) do they form?
Longwaves
• Shown upper level
(500mb) pressure chart.
• There are long waves
disturbances over
distances of 1000’s of
kilometers.
• Typically 3 to 6 low
pressure troughs.
• Also called planetary
waves or Rossby waves.
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Shortwaves
• Imbedded in the longwaves are shortwaves.
Motion of Shortwaves
• Shortwave move faster than long waves.
• Shortwaves deepen around a low pressure trough.
Barotropic and Baroclinic
• Barotropic – contour lines (&wind) parallel to isotherms
• Baroclinic – contour lines (&wind) cross isotherms (1&3)
2
Warm and Cold Advection
• Cold advection – transport of cold air by the
wind from colder to warmer regions.
• Warm advection – transport of warm air by
the wind from warmer to colder regions.
• On the previous slide,
– cold advection to the west of pressure troughs
– warm advection to the east of pressure troughs.
• Adding cold air will raise the surface pressure,
while adding warm air will lower it.
Baroclinic Instability
• If a shortwave moves into a region near the polar front
just west of a frontal wave, it will enhance the low at the
center of the frontal wave and the high in the cold region.
Upper-Air Support
• Cold air convergence
strengthens the surface
cold high.
• Warm air divergence
strengthens the surface
low.
• Both act to intensify
the cyclone.
3
Upper Level Winds after Occulusion
• Warm surface air is cut-off
and surface low fills in.
• Shown: vertically stacked
surface and upper level lows.
• Wind flows around upper
level lows, which may remain
stationary for many days.
Polar Front Jet Stream
• Remember, the polar front jet forms at the polar front
where there are sharp temperature and pressure
differences.
• The wind speeds in the jet stream are high.
Jet Streak
• A region of maximum wind speeds in the jet stream is
called a jet streak.
• There is strong convergence before (to the west) of the
jet streak and strong divergence after (to the east) of it.
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Role of Jet Stream
• Convergence (C) and divergence (D) near the jet
streak intensify surface highs (H) and lows (L) and
help drive the storm.
Jet Stream Air Support
• Without upper-level support the storm gradually dies out.
• In winter polar front and jet stream are in US, while in
summer they are in Canada (Alberta and NWT).
Summary Wave Cyclone Development
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Conveyor Belt Model
• Snowstorms along east
coast of US.
• Warm conveyor starts in
warm sector.
• Cold conveyor starts
from the ocean and is
forced upwards near L.
• Dry conveyor belt
descends from behind
cold front. A branch can
sweep in creating a
“comma” cloud.
The March Storm of 1993
• Shown: color enhanced infrared satellite image from
March 13, 1993. Note comma clouds.
Surface Weather Map for March 13
Pressure at low
is 975 mb, which
is very low.
• Open wave cyclone centered in north Florida.
• Ahead of cold front, head thunderstorms (squall
line) pound Florida with heavy rain, high winds,
and tornadoes.
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500 mb Weather Map for March 13
• A low pressure trough exists west of the surface low.
• Wind and contour lines cross isotherms near the low
pressure trough – baroclinic atmosphere.
Development of March 13 Storm
Winds of 125 mph
measured on Mt.
Washington, NH.
Note low is 960 mb,
pressure similar to
category 3 hurricane.
• Frontal wave off of Texas coast at 7AM, 3/12.
• Open wave centered in N. Florida at 4AM, 3/13.
• Occluded front over Virginia at 4PM, 3/13.
Summary
• Upper level air support is needed for a
middle latitude cyclone to develop.
• Upper level winds and pressure are
determined by both long and short waves.
• A low pressure trough above and to the
west of a surface low causes
– converging cold air to strengthen the high
– diverging warm air to strengthen the low.
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