Mid-Latitude Cyclones: Vertical Structure

Lecture 9

November 4, 2009

The figure to the right represents a typical midlatitude cyclone

• Cold, dry air is advected eastward behind the cold front

• Warm, moist air is advected north behind the warm front

• The fronts move in the direction the “teeth” point

Review

Review Continued

• Extratropical cyclones generally first develop along an intersection of two airmasses (like a stationary front)

• As the cyclone develops, warm and cold fronts form, and the cold front slowly approaches the warm front

• Once an occluded from forms, the cyclone is normally at its most intense, and will begin to weaken afterward

• This is because it is no longer near a region of a horizontal temperature gradient (which is why it developed in the first place)

Review Continued

• Finding fronts on weather maps is very useful• It is often useful to first find the area of lowest

pressure, since fronts typically originate from it• In the case of most fronts (except occluded

fronts), there should be a large temperature change across them

• All fronts should also have a fairly sharp wind shift from one side to another

• Other factors, like precipitation, cloud cover, and moisture gradients can indicate a front

Review Continued• Last week, we discussed the surface structure of

mid-latitude cyclones which are crucial in maintaining a temperature equilibrium on our planet.

• We know that the winds move counter-clockwise and converge around a surface low low pressure center (this is because of the frictional forcefrictional force at the surface)

• This Convergence/Divergence suggests that there must be movement of air in the vertical (continuity of mass)

• Also, the flow in the upper troposphere is generally in geostrophic balance, geostrophic balance, so there is no friction forcing convergence/divergence.

So let’s look at Upper Levels…

Similarly to lower levels, at upper levels of the atmosphere, there is often a series of high pressures (high heights) and

low pressures (low heights)

Typical 500 mb height pattern

Upper Levels

Ridge Trough Ridge

Why Do These Patterns Occur?

• These patterns of convergence and divergence have to do with vorticity advection

• If there is positive vorticity advection, divergence occurs

• If there is negative vorticity advection, convergence occurs

• Let’s explain vorticity …

VorticityVorticity is simply a measure of how much the air rotates on a horizontal surface

Positive vorticity is a counterclockwise (i.e. cyclonic) rotation

Negative vorticity is a clockwise (i.e. anticyclonic) rotation

Therefore, troughs contain positive vorticity, and ridges contain negative vorticity

Trough Ridge

Let’s Revisit …

Vorticity < 0 Vorticity < 0

Vorticity > 0

Diagnosing Vorticity Advection

• To determine vorticity advection, first find the locations of maximum (positive) vorticity and minimum (negative) vorticity

• Then, determine what direction the wind is moving

• Areas of negative vorticity advection (NVA) will be just downstream of vorticity minima, and areas of positive vorticity advection (PVA) will be just downstream of vorticity maxima

Positive vorticity

advection

Negative vorticity

advection

Vorticity Advection and Vertical MotionVorticity Advection and Vertical Motion

* Positive vorticity advection (PVAPVA) results in divergence at the level of advection

* Negative vorticity advection (NVANVA) results in convergence at the level of

advection

Vorticity Advection and Vertical MotionVorticity Advection and Vertical Motion

Remember that convergence at upper levels is associated with downward vertical motion (subsidence), and divergence at upper levels is associated with upward vertical motion (ascent).

Then, we can make the important argument that . . .

Upper Tropospheric Flow and Convergence/DivergenceUpper Tropospheric Flow and Convergence/Divergence

• Downstream of an upper tropospheric ridge, there is convergence, resulting in subsidence (downward motion).

• Likewise, downstream of an upper tropospheric trough, there is divergence, resulting in ascent (upward motion).

Upper Tropospheric Flow and Convergence/DivergenceUpper Tropospheric Flow and Convergence/Divergence

• Downstream of an upper tropospheric ridge axis is a favored location for a surface high pressuresurface high pressure, and of course, downstream of an upper tropospheric trough axis is a favored location for a surface surface low pressurelow pressure center.

Upper Tropospheric Flow and Convergence/DivergenceUpper Tropospheric Flow and Convergence/Divergence

• Surface cyclones also move in the direction of the upper tropospheric flow!

• The surface low pressure center in the diagram above will track to the northeast along the upper level flow

Vertical Structure of CyclonesVertical Structure of Cyclones

What else do these diagrams tell us?

• Because the surface cyclone is downstream from the upper tropospheric (~500 mb) trough axis, mid-latitude cyclones generally tilt westward with height!

Vertical Structure of CyclonesVertical Structure of Cyclones

To the right is another depiction illustrating the same point:

500 mb positive vorticity advection results in divergence and ascent, inducing a surface cyclone.

Cyclone Growth And Decay

• Based on what we’ve learned, the position of the surface cyclone in relation to the upper level structure is key to development

• A cyclone will grow if it is below an area of PVA, and weaken if below an area of NVA

• Commonly, a cyclone will intensify until it becomes situated in an unfavorable location in relation to the upper levels

An Example:Time 1

Above: Upper Level Height and Wind Speed

Right: Surface Pressure

An Example:Time 1

Above: Upper Level Height and Wind Speed

Right: Surface Pressure

Time 2

Above: Upper Level Height and Wind Speed

Right: Surface Pressure

Time 2

Above: Upper Level Height and Wind Speed

Right: Surface Pressure

Time 3

Above: Upper Level Height and Wind Speed

Right: Surface Pressure

Time 3

Above: Upper Level Height and Wind Speed

Right: Surface Pressure

Summary of Event

• At time 1, the upper levels and lower levels are perfectly set up for the surface cyclone to intensify

• At time 2, the upper trough is almost above the surface cyclone, so the intensification slows

• By time 3, the upper trough is exactly over the surface cyclone, so the intensification has halted

Cyclone Decay

• Recall that due to friction, air blows across isobars near the surface

• This means that the air is always converging at the center of low pressure areas

• Therefore, unless there is at least enough divergence at upper levels to counteract the convergence at low levels, the surface cyclone will weaken because more mass will be added to the air column

• This will force the surface pressure to rise

Cyclone Intensification/WeakeningCyclone Intensification/Weakening

How do we know if the surface cyclone will intensify or weaken?

• If upper tropospheric divergenceupper tropospheric divergence > surface surface convergenceconvergence, the cyclone will intensify (the low pressure will become lower)

• If surface convergencesurface convergence > upper tropospheric upper tropospheric divergencedivergence, the cyclone will weaken, or “fill.”

• Think of an intensifying cyclone as exporting mass, and a weakening cyclone as importing mass.