Rapid cyclogenesis (bombs)

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Rapid cyclogenesis (bombs)
textbook section 8.5
Contents
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definition:
∆
∆
sin(60) 24 
sin() 24 ℎ
–
SLP deepening rate
–
24 mb / 24 hours is known as 1 “bergeron” (Sanders and Gyakum 1980)
>
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example: 18 Feb 2004, off the US East Coast
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climatology
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physical processes
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–
–
–
–
horizontal temperature gradient
surface heat fluxes
diabatic heating (air-sea interaction instability)
jet streak interactions
tropopause folds
H
1018
1008
992
970
rapid cyclogenesis
climatology
most common
- in winter
- off east coasts
Gulfstream axis
Atlantic
Pacific
Labrador current
Gulf
Stream
Rapid cyclogenesis mechanisms: near-surface processes
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1. Baroclinicity
– 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
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2. Surface sensible heat flux reduces the low-level static stability
3. Surface latent heat flux fuels the storm:
Rapid cyclogenesis mechanisms, cont’d
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Bombs are primarily baroclinic destabilizations, yet some intensification
may occur through a barotropic process, air-sea interaction instability
(Emanuel 1986)
more BL water
vapor
(a) low pressure implies surface wind
 larger sfc LH flux
(b) low pressure implies z >0 in friction layer
 Ekman pumping and LL convergence
more LH release
in updraft
stronger updraft
cyclogenesis
spin-up (z)
Rapid cyclogenesis mechanisms aloft: QG argument
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Large low-level water vapor content implies diabatic heating (typically
peaking between 850-700 mb)
2 
fo 
1
k
[abs _ vort _ adv]  2 [thermal_ adv]  2 J
s p
s
sp
– 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
s   o
d ln  o
dp
Horizontal temperature
gradient
3 different values of
stability parameter
dash – high s
solid – medium s
dot – low s
surface
deepening rate
(mb/hr)
wavelength
From: Sanders 1971
rapid cyclogenesis (anywhere) may also be
driven by upper-tropospheric processes
(a) Strong coupled jet-front circulation systems
– superposition of two upper-level jet streak ascent
regions. The interaction is between
• a thermally-direct circulation located within the
entrance region of a downstream jet streak
• and a thermally-indirect circulation in the exit region of
an upstream jet streak
• This interaction not only enhances omega, but also
contributes to differential moisture and temperature
advections, and establish an environment within which BL
processes specific to the East Coast region (e.g., cold-air
damming, coastal frontogenesis, the development of a
low-level jet) can further contribute to cyclogenesis and
snowstorms.
(Uccellini and Kocin 1987)
rapid cyclogenesis (anywhere) may also be
driven by upper-tropospheric processes
(b) Strong WAA aloft due to tropopause depression (or ‘fold’) may cause rapid
cyclogenesis in some cases (hydrostatic lowering of SLP)
Tropopause folds and ‘occlusions’
p
  g
z
RT
dz  
d ln p
g
z1000
R
 ztop 
g
1000
 Td ln p
ptop
note: d ln p ~ dz
z1000
R
 ztop 
t
gH
developing
(H: scale height = RT/g)
H
top
T
 t dz
sfc
 Surface height falls (cyclogenesis) relates to
warming in the column aloft, with all layers of
equal depth weighted equally.
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 (1991)
Example of a “normal” tropopause depression
color fill: potential vorticity (0.1 PV units, i.e. 10-7 m2 s-1 K kg-1)
12 March 1993:
storm of the 20th century:
impressive tropopause fold
@ dynamic tropopause (1.5 PVU)
 and wind
pressure
SLP, 850 PV, and 850 e
450
00 Z 12 March
150
1009
150
400
00 Z 13 March
998
150
Rapid cyclogenesis
00 Z 14 March
750
(from Bosart in Shapiro and Gronas 1999)
150
972
References
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Emanuel, K.A., 1986: An Air-Sea Interaction Theory for Tropical Cyclones. Part I: Steady-State
Maintenance. J. Atmos. Sci., 43, 585–605.
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.
Sanders, F., 1971: analytic solutions of the nonlinear omega and vorticity equations for a
structurally simple model of disturbances in the baroclinic westerlies. Mon. Wea. Rev., 99, 393–
407.
Sanders, F., and J.R. Gyakum, 1980: Synoptic-Dynamic Climatology of the “Bomb”. Mon. Wea.
Rev., 108, 1589–1606.
Uccellini, L.W., D. Keyser, K. F. Brill and C. H. Wash, 1985: The Presidents' Day Cyclone of 18–19
February 1979: Influence of upstream trough amplification and associated tropopause folding on
rapid cyclogenesis. Mon. Wea. Rev., 113, 962–988.
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. 289–309.

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