Download Midlatitude Cyclones

AT 350 – Intro to Weather and Climate
The Midlatitude Cyclone
Ahrens, Chapter 12
Midlatitude Cyclones
Waves on
the polar
vortex
Hemispheric
westerlies typically
organized into 4-6
“long waves”
Wind blows through
them, but the waves
themselves
propagate slowly
(east to west!) or
not at all
Fronts
A “Family”
Family” of Cyclones
A Front -
is the boundary between air masses; normally
refers to where this interface intersects the
ground (in all cases except stationary fronts, the
symbols are placed pointing to the direction of
movement of the interface (front)
Warm Front
Cold Front
Stationary Front
Occluded Front
Scott Denning CSU ATS – Spring 2008
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AT 350 – Intro to Weather and Climate
Characteristics of Fronts
• Across the front - look for one or more
of the following:
– Change of Temperature
– Change of Moisture characteristic
Midlatitude Cyclones
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 -- shallower
clouds may form if warm air is stable
• RH, Td
– Change of Wind Direction
– Change in direction of Pressure Gradient
– Characteristic Precipitation Patterns
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
Scott Denning CSU ATS – Spring 2008
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AT 350 – Intro to Weather and Climate
Midlatitude Cyclones
The Wave Cyclone Model
(Norwegian model)
•
•
•
•
•
•
Lifecycle of a Midlatitude Cyclone
•
Stationary Front
Nascent Stage
Mature Stage
Partially Occluded Stage
Occluded Stage
Dissipated Stage
•
•
•
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Lifecycle of a Midlatitude Cyclone
Stationary front
Incipient cyclone
Open wave
Convergence and Divergence
Green
shading
indicates
precipitation
Mature stage
occlusion
dissipating
What initiates “cyclogenesis?”
Low
Takes
several days
to a week,
and moves
1000’s of km
during
lifecycle
High
500 mb height
Scott Denning CSU ATS – Spring 2008
Pressure surfaces tilt
because of N-S
temperature contrast
Passing wave initiates
divergence and cyclonic
vorticity
Cold air undercuts warm,
and flows south
Cold air advection
undermines upper trough,
deepening it
N-S mixing in cyclone
eventually consumes the
available potential energy,
and cyclone dies
When upper-level
divergence is
stronger than lowerlevel 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.”
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AT 350 – Intro to Weather and Climate
Midlatitude Cyclones
Upper Air/Surface Relationship
Cyclone Development
• baroclinic instability (baroclinic means temperature varies
on an isobaric surface) causes initial ‘perturbation’ to grow.
• occurs in the presence of strong temperature gradients.
Imagine a short wave trough passes overhead (looking
North):
Where will surface low develop?
Low
DIV
High
east
Scott Denning CSU ATS – Spring 2008
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AT 350 – Intro to Weather and Climate
Midlatitude Cyclones
(looking North):
Near the surface, where will we have cold and warm
advection?
Will this amplify or weaken the upper level low?
How about the upper level divergence?
Will a more intense upper level low strengthen or weaken the
surface low?
Low
cool
DIV
Cyclone initiation
Passage of a shortwave often initiates
the formation of a surface low.
High
warm
Low
east
Cyclone development:
Strong north south gradient+passage of a shortwave trough
Can lead to rapid cyclogenisis via baroclinic instability
(baroclinic means temperature varies on an isobaric surface)
Scott Denning CSU ATS – Spring 2008
Cyclone Development
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AT 350 – Intro to Weather and Climate
Midlatitude Cyclones
What maintains the surface low?
Imagine a surface low forming directly below upper level low
Actual
vertical
structure
Upper level low is
tilted westward with
height with respect
to the surface.
Surface convergence
“fills in” the low
Surface divergence
“undermines” the high
Baroclinic Instability
• Upper level
shortwave
passes
• Upper level
divergence
-> sfc low
• Cold advection
throughout lower
troposphere
• Cold advection
intensifies upper
low
• Leads to more
upper level
divergence
UPPER LEVEL
DIVERGENCE
INITIATES AND
MAINTAINS A
SURFACE LOW.
Summary of Cyclone Weather
Roles of
convergence
and divergence
aloft
Pattern of
clouds,
precipitation,
and
temperatures
on the ground
Temperature advection is key!
Scott Denning CSU ATS – Spring 2008
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AT 350 – Intro to Weather and Climate
Conveyor Belt Model
This model describes
rising and sinking air
along three
“conveyor belts”
belts”
A warm conveyor belt
rises with water vapor
above the cold
conveyor belt which
also rises and turns.
Finally the dry
conveyor belt
descends bringing
clearer weather behind
the storm.
What is the source of energy for
Midlatitude cyclones?
• Potential energy arising from the
temperature differences found in the
different air masses.
• Cold, dense air pushes warmer, less dense
air up and out of the way.
• “Up warm, down cold”
Scott Denning CSU ATS – Spring 2008
Midlatitude Cyclones
Questions to Think About
• What is the vertical structure of a developing storm?
• Where is the strongest upper/lower level
divergence/convergence occurring?
• Why aren’t the ‘lows’ vertically stacked?
• What is required for a storm to develop?
• Where is rising motion occurring?
• Where is precipitation occurring?
• What is the ultimate source of energy for a
midlatitude storm?
• Why does a storm “die”?
• During what time of year would you expect more
midlatitude cyclones?
• Why doesn’t baroclinic instability occur in the tropics?
Development of surface cyclones
is related to upper air wind patterns
• Linked through the convergence /divergence
patterns at the surface and aloft;
• Convergence at the surface produces upward
motion - divergence at the surface produces
downward motion;
• Convergence at upper levels produces
downward motion - divergence produces upward
motion
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AT 350 – Intro to Weather and Climate
Midlatitude Cyclones
Poleward Energy Transport
• MMC carries
energy out of
tropics
(as gz)
• Eddy motions carry
it the rest of the
way (mostly as
transients)
• Consider d/dϕ of
energy flux
(divergence)
• Thermally indirect
MMC in midlats is
response to strong
eddy flux
divergences
Global Energy Cascade
•
•
•
•
•
Heterogeneous heating
creates available potential
energy (APE)
Thermally-direct MMC
converts APE to zonal
mean KE
Baroclinic instability
converts zonal mean KE to
eddy KE
Eddies shed eddies “and
so on to viscosity”
Dissipation of KE converts
energy back to heat
The “Big Picture”
Picture”
• We’ve emphasized horizontal transport of energy to
balance the planetary energy budget:
– Hadley Cell
– Subtropical divergence
– Midlatitude cyclones and conveyor belts
• What about vertical motion?
– “Up-warm, down cold”
– “Up moist, down-dry”
• Severe weather is all about vertical motion, and
represents local release of energy that contributes to
planetary energy balance
Scott Denning CSU ATS – Spring 2008
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