Vertical Structure of Cyclones

1
Vertical Structure of Cyclones

• April 28, 2009
• AOS 101-302

2
Cyclone Intensification/Weakening

• Winds converge at a surface low pressure center
  diverge from a surface high pressure center
  (because of friction)
• This suggests that there must be movement of air
  in the vertical (continuity of mass)

3
Cyclone Intensification/Weakening

• How do we know if the surface cyclone will
  intensify or weaken?
• If upper tropospheric divergence gt surface
  convergence, the cyclone will intensify (the low
  pressure will become lower)
• If surface convergence gt upper tropospheric
  divergence, the cyclone will weaken, or fill
• Think of an intensifying cyclone as exporting
  mass and a weakening cyclone as importing mass

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Cyclone Intensification/Weakening

• So to deepen a surface cyclone you must have
  ascent (upward vertical motion)
• What drives these vertical motions???
• 1. Positive vorticity advection (PVA) at upper
  levels
• Look at upper-level (e.g. 500 hPa) winds and
  trough/ridge axes.
• 2. Warm air advection (WAA) near the ground
• Look at surface temperature and winds

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Upper-level Waves

• Typical 500 mb height pattern
• Troughs and ridges
• In the northern hemisphere mid-latitudes, lower
  pressure is usually to the north of higher
  pressure

L
H
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1. Vorticity Advection
-


-

• If the wind has counterclockwise spin, it has
  positive vorticity
• If the wind has clockwise spin, it has negative
  vorticity
• Vorticity can be directional (top), or speed
  shear vorticity (bottom)

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1. Vorticity Advection

• Vorticity Advection
• First find vorticity minima maxima
• Negative vorticity advection (NVA) occurs just
  downwind from a ridge axis
• Positive vorticity advection (PVA) occurs just
  downwind from a trough axis

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1. Vorticity Advection
Divergence
Convergence
Convergence
Divergence

• PVA results in divergence (at the level of
  advection)
• NVA results in convergence (at the level of
  advection)
• Remember
• Convergence at upper levels Downward vertical
  motion (sinking)
• Divergence at upper levels Upward vertical
  motion (ascent)

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1. Vorticity Advection

• Downwind of an upper tropospheric ridge, there is
  convergence
• Subsidence (downward motion)
• Downwind of an upper tropospheric trough, there
  is divergence
• Ascent (upward motion)

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1. Vorticity Advection Example

• At 700 mb, we can see that downwind of troughs
  there is ascent (red/orange), and upwind there is
  subsidence (purple/blue)

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1. Vorticity Advection

• Downwind of an upper tropospheric ridge axis is a
  favored location for a surface high pressure
• Downwind of an upper tropospheric trough axis is
  a favored location for a surface low pressure
  center.

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1. Vorticity Advection

• 500 mb PVA results in divergence and ascent,
  inducing a surface cyclone

PVA
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2. Warm Air Advection

• The 2nd mechanism is surface warm air advection
• We already know what this looks like!
• Look at the temperatures and the winds...

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Ex Surface Cyclone Forced by Upper Flow

• 300 mb flow which resulted in a massive cyclone
  development over the Midwest
• PVA downwind of the trough

1.
PVA
TROUGH AXIS
Cyclone from 11/10-12/1998
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Ex Surface Cyclone Forced by Upper Flow
1.

• Weak cyclone downwind of the trough
• Weak fronts
• Some warm air advection associated with weak warm
  front

WAA
L
TROUGH AXIS
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Ex Surface Cyclone Forced by Upper Flow

• 12 hours later
• Trough moves west, strong jet rotates around
  trough
• There are two trough axes

2.
PVA
TROUGH AXIS
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Ex Surface Cyclone Forced by Upper Flow

• The surface cyclone has deepened to a very low
  977 mb
• The trough axis appears to be catching up to the
  surface cyclone
• Strong Warm Air advection north of the cyclone

2.
WAA
L
TROUGH AXIS
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Ex Surface Cyclone Forced by Upper Flow
3.

• 12 hours later
• Trough is now cut off

PVA
TROUGH AXIS
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Ex Surface Cyclone Forced by Upper Flow

• Surface cyclone is under upper-level
  trough--vertically stacked cyclone
• No PVA on top of surface cyclone
• No WAA
• No Intensification!

3.
L
TROUGH AXIS
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Summary

• Cyclones will tend to form downwind of an
  upper-level trough in an area of PVA
• As the cyclone develops, WAA will occur on the
  warm front further deepening the cyclone
• Whole system will tilt westward with height
• While developing, cyclone will move closer to
  trough axis
• While occluding, cyclone will move nearly under
  trough axis in an area void of PVA/WAA

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Summary
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Longwaves vs. Shortwaves

• Longwaves
• Typically 4-6 of these around the planet
• A pattern can last for 2-3 weeks on occasion, and
  can result in long periods of anomalous weather
• Shortwaves
• Embedded within the longwave pattern
• Smaller scale
• Move quickly eastward
• Their effects intensity when they hit a longwave
  trough
• Often result in huge cyclogenesis events such
  as noreasters or midwest snowstorms.

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Longwaves

• To the left is a polar projection (the North Pole
  is at the center, and the equator is at the
  edges)
• 300 mb heights (contoured) and wind speed
  (colored)
• Note the prominent longwave troughs and
  ridges---especially over North America

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Shortwaves

• 500 mb height (contour) and vorticity (colored)
• 2 longwave troughs (one over New England and one
  over eastern Canada)
• Shortwave over Montana/Wyoming