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The Midlatitude CycloneAhrens, Chapter 13
The Wave Cyclone Model(Norwegian model)
• Stationary Front
• Nascent Stage
• Mature Stage
• Partially Occluded Stage
• Occluded Stage
• Dissipated Stage
Green shading indicates precipitation
Takes several days to a week, and moves 1000’s of km during lifecycle
Stationary frontIncipient cyclone Open wave
Mature stage occlusion dissipating
Lifecycle of a Midlatitude Cyclone
Large-Scale Setting
Hemispheric westerlies typically organized into 4-6 “long waves”
Where is upperlevel 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
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.
Figure 13.18
Figure 13.19
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.
What enhances“cyclogenesis?”
500 mb height
Low
High
Convergence and Divergence
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.”
Upper Air/Surface Relationship
Cyclone Development
Baroclinic Instability
• Upper level shortwavepasses
• Upper level divergence-> sfc low
• Cold advectionthroughout 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