Download Mid-latitude Cyclones

The Midlatitude Cyclone
Ahrens, Chapter 13
The Wave Cyclone Model
(Norwegian model)
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Stationary Front
Nascent Stage
Mature Stage
Partially Occluded Stage
Occluded Stage
Dissipated Stage
Lifecycle of a Midlatitude Cyclone
Stationary front
Incipient cyclone
Mature stage
occlusion
Open wave
Green
shading
indicates
precipitation
dissipating
Takes several
days to a
week, and
moves 1000’s
of km during
lifecycle
Large-Scale
Setting
Hemispheric
westerlies typically
organized into 4-6
“long waves”
Where is upper
level divergence
most likely to
occur?
A “Family” of Cyclones
Cyclone initiation
Passage of a shortwave often initiates
the formation of a surface low.
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)
What maintains the surface low?
Imagine a surface low forming directly below upper level low
Surface convergence
“fills in” the low
Surface divergence
“undermines” the high
Vorticity & Cyclone Spin
Figure 13.18
Figure 13.19
Vorticity describes the spin of an air parcel, which is positive in
counterclockwise cyclonic flow.
Due to the conservation of angular momentum, vorticity increases
with a decrease in parcel radius (e.g. stretching due to divergence
aloft) and increase in earth's latitude.
Actual
vertical
structure
Upper level low is
tilted westward with
height with respect
to the surface.
UPPER LEVEL
DIVERGENCE
INITIATES AND
MAINTAINS A
SURFACE LOW.
Convergence and Divergence
What enhances“cyclogenesis?”
Low
High
500 mb height
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.”
Upper Air/Surface Relationship
Cyclone Development
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
Temperature advection is key!
1. Divergence aloft
2. Leads to lower
SLP underneath
divergence
3. Flow associated
with low SLP leads
to cold/warm
advection in lower
troposphere.
4. Temp advection
increases upper
level trough, which
leads to even more
divergence.
Intensification occurs
typically just ahead
of upper air trough
axis.
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”
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
Where do we find steady,
wide spread precipitation?
• Upwind of the warm front
• Wrapping around the Low Pressure
Center
Summary of Cyclone Weather
Roles of
convergence
and divergence
aloft
Pattern of
clouds,
precipitation,
and
temperatures
on the ground
Rapid Cyclone Intensification
An Example from
November 9, 10 and 11, 1998
The “Big 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