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