Rapid cyclogenesis bombs

1
Rapid cyclogenesis (bombs)

• definition
• Deepening rate gt 24 mb d-1 ( 1 bergeron)
• Example 18 Feb 2004, off the US East Coast
• climatology
• Most common over warm ocean in winter
• factors
• horizontal temperature gradient
• surface heat fluxes
• diabatic heating (air-sea interaction
  instability)
• jet streak interactions
• tropopause folds

2
H
3
1018
4
1008
5
992
6
970
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rapid cyclogenesis climatology
Atlantic
Pacific
Gulfstream axis
8
Atlantic
Pacific
Labrador current
Oya Shio
Kuro Shio
Gulf Stream
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mechanisms

• 1. Horizontal temperature gradient
• Large SST gradient and land-sea contrast
• strong thermal wind
• strong omega forcing
• Even weak cross-isotherm winds produce large LL
  temperature advection
• LL cyclonic flow readily alters thickness field
  and amplifies UL trof/ridge ? large PVA
• 2. Surface sensible and mainly latent heat fluxes
  yield fuel the storm

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mechanisms contd

• (1) large DT and (2) large sfc heat fluxes
  suggest that cyclogenesis is often triggered by
  low-level processes
• 3. Large low-level water vapor content implies
  diabatic heating (typically peaking between
  85-700 mb)
• local max in J (diabatic heating) makes the last
  term positive ? stronger updraft
• also, the static stability parameter s tends to
  be small in the warm sector over warm water

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mechanisms, contd

• Bombs are primarily baroclinic destabilizations,
  yet some intensification may occur through
  air-sea interaction instability (barotropic)
• Cycl vort in friction layer
• ? Ekman pumping
• ? LL convergence
• ? larger sfc LH flux (2)
• ? LH release in ascent (3)
• ? stronger updraft
• ? spin-up (z?, Z? )
• (see Bluestein section 1.1.7)

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Horizontal temperature gradient
3 different values of stability parameter dash
high s solid medium s dot low s
surface deepening rate (mb/hr)
From Sanders 1971
wavelength
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rapid cyclogenesis (anywhere) may also be driven
by upper-tropspheric processes

• (a) Strong coupled jet-front circulation systems
• superposition of two jet streak ascent regions
  (left exit right entrance) (Uccellini and Kocin
  1987). The interaction is between
• 1) a direct circulation located within the
  confluent entrance region of an upper-level jet
  streak over the northeastern US with
• 2) an indirect circulation in the diffluent exit
  region of a jet streak associated with a trough
  nearing the East Coast
• U K (1987) believe that this interaction not
  only enhances omega, but also contributes to
  differential moisture and temperature advections,
  and establish an environment within which
  boundary layer processes (e.g., cold-air damming,
  coastal frontogenesis, the development of a
  low-level jet) can further contribute to
  cyclogenesis and east coast snowstorms.
• (b) Strong WAA aloft due to tropopause depression
  (or fold) may cause rapid cyclogenesis in some
  cases (hydrostatic lowering of SLP)

• Uccellini, Louis W., Paul J. Kocin, 1987 The
  Interaction of Jet Streak Circulations during
  Heavy Snow Events along the East Coast of the
  United States. Weather and Forecasting Vol. 2,
  No. 4, pp. 289309.

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Tropopause folds and occlusions
d(lnp) dz/H (H scale height RT/g) ? Surface
height falls (cyclogenesis) relates to warming in
the column aloft, with all layers of equal depth
weighted equally.
developing
? Tropopause depressions always occur in the
mature stages of cyclogenesis in the UL trof,
causing the surface L to move into the cold
air. ? Tropopause folds below 500 mb are rare
and may contribute to rapid cyclogenesis.
mature
Hirshberg and Fritsch, MWR, 91, 496-555.
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Example of a normal tropopause depression
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(No Transcript)
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12 March 1993 storm of the century impressive
tropopause fold
_at_ dynamic tropopause (1.5 PVU)
SLP, 850 PV, and 850 qe
q and wind
pressure
00 Z 12 March
1009
00 Z 13 March
998
Rapid cyclogenesis
00 Z 14 March
972
(from Bosart in Shapiro and Gronas 1999)
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Reference (tropopause folds and occlusions)

• Uccellini, L.W., D. Keyser, K. F. Brill and C.
  H. Wash, 1985 The Presidents' Day Cyclone of
  1819 February 1979 Influence of upstream trough
  amplification and associated tropopause folding
  on rapid cyclogenesis. Mon. Wea. Rev., 113,
  962988.
• Hirshberg, P.A., and M.J. Fritsch, 1991a
  Tropopause undulations and the development of
  extratropical cyclones. Part I Overview and
  observations from a cyclone event. Mon. Wea.
  Rev., 119, 496-517.
• Hirshberg, P.A., and M.J. Fritsch, 1991b
  Tropopause undulations and the development of
  extratropical cyclones. Part II Diagnostic
  analysis and conceptual model. Mon. Wea. Rev.,
  119, 518-550.