Instability

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Instability
Baroclinic instability (needs vertical shear,
i.e., horizontal
T gradient)
through the lifting of relatively warm, light air
and the sinking of relatively cold, dense air
Barotropic instability (needs horizontal shear)
What kind of instability is required for
mid-latitude cyclone to develop?
Baroclinic instability!
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Cyclone
Cyclone (mid-latitude cyclone or extratropical
cyclone) A region of low pressure, which has a
scale of 1000 km and is surrounded by strong
wind rotating in the counterclockwise direction
in NH and clockwise in SH.
cold air mass
direction of propagation
accompanying with cold front and warm front (and
maybe with occluded front as well)
L
cold front
warm air mass
warm front
Low pressure center at surface is around 1000
mb Life cycle is about several days and moves
following steering wind.
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Cyclogenesis
Cyclogenesis A fall in pressure of greater
than a certain magnitude
One potential formation of a cyclogenesis - Lee
cyclogenesis
Another potential formation of a cyclogenesis
C
e.g., continental polar air maass
W
e.g., marine warm tropical air mass
Cyclogenesis
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Cyclone evolution
Norwegian model (developed during and shortly
after World War I) By T. Bergeron and H. Solberg,
based on surface observations
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Cyclone and Comma-shaped Cloud
Comma cloud
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Cyclogenesis
preferred locations of cyclone formation in US
(i) east of the Rockies (ii) off the east coast
of N America (iii) Gulf of Mexico (iv) mid-west
initiated in continental polar mass from Canada
(Lee cyclogenesis)
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Symmetrical thermal structure
low
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Asymmetrical thermal structure
Cyclones slant back into colder air masses and
strengthen with height, sometimes exceeding 9 km
in depth
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Cyclone Development
Favorable conditions for cyclone to develop

• Low level warm temperature advection (WTA)
• Low level moisture advection (often come with
  WTA)
• Positive vorticity advection at 500 mb or upper
  level diffluent flow (so downstream of a
  diffluent trough is really good for surface
  cyclone to develop!)
• Note The wave should not be too long (beta
  effect is too strong) or too short. The static
  stability should not be too large (suppress w)
• 4. The right side of the entrance or the left
  side of the exit of the 200/300 mb Jet streak

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Cyclone Development
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Cyclone Development

• Most cyclones deepen underneath downstream of a
  diffluent trough
• The rest deepen underneath downstream of a
  regular trough
• Very few deepen underneath downstream of a
  confluent trough
• - This may have other mechanisms at different
  level to help.

Divergence diffluence
500 mb surface
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Cyclone Development
Divergence diffluence
1002.7 mb
X
X
Surface map
300 mb map
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Cyclone Development
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Cyclone life cycle 500 mb height field
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Latent Heat Sensible Heat
Sensible heat temperature gradient in vertical
Latent heat moisture gradient in vertical
Height tendency
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Bomb
Bomb Explosive cyclogenesis
(slightly smaller in size than regular cyclones)
Pressure deepening rate 1 mb/hr for at least 24
h!
Most bombs occur over the ocean during the
season, downstream from
near the strongest
.
cold
diffluent upper level troughs
sea surface temperature gradient
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Bombs