Download Mid-Latitude Cyclones: Vertical Structure

Mid-Latitude Cyclones:
Vertical Structure
Lecture 9
November 4, 2009
Review
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 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 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, so there is no
friction forcing convergence/divergence.
So let’s look at Upper Levels…
Typical 500 mb height pattern
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)
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 …
Vorticity
Vorticity 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
Negative
vorticity
advection
Positive
vorticity
advection
Vorticity Advection and Vertical Motion
* Positive vorticity advection (PVA) results
in divergence at the level of advection
* Negative vorticity advection (NVA)
results in convergence at the level of
advection
Vorticity 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/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/Divergence
• Downstream of an upper tropospheric ridge axis is a favored
location for a surface high pressure, and of course, downstream of
an upper tropospheric trough axis is a favored location for a surface
low pressure center.
Upper 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 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 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/Weakening
How do we know if the surface cyclone will intensify or
weaken?
• If upper tropospheric divergence > surface
convergence, the cyclone will intensify (the low pressure
will become lower)
• If surface convergence > upper tropospheric
divergence, the cyclone will weaken, or “fill.”
• Think of an intensifying cyclone as exporting mass, and
a weakening cyclone as importing mass.