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Rapid cyclogenesis (bombs)textbook section 8.5
• definition: – SLP deepening rate
– 24 mb / 24 hours is known as 1 “bergeron” (Sanders and Gyakum 1980)
• 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
• 2. Surface sensible heat flux reduces the low-level static stability• 3. Surface latent heat flux fuels the storm:
Rapid cyclogenesis mechanisms, cont’d
• Bombs are primarily baroclinic destabilizations, yet some intensification may occur through a barotropic process, air-sea interaction instability (Emanuel 1986)
(b) low pressure implies z >0 in friction layer Ekman pumping and LL convergence
more LH release in updraft
stronger updraft
spin-up (z)cyclogenesis
more BL water vapor
Rapid cyclogenesis mechanisms aloft: QG argument
• Large low-level water vapor content implies diabatic heating (typically peaking between 850-700 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
Jp
kadvthermaladvvortabs
p
fo 222 ]_[1
]__[
dp
d oo
ln
Hori
zonta
l te
mpera
ture
gra
die
nt
wavelength
surface deepening rate (mb/hr)
From: Sanders 1971
3 different values of stability parameterdash – high ssolid – medium sdot – low s
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’
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.
developing
mature Hirshberg and Fritsch (1991)
top
sfc
top
p
top
dzt
T
gH
Rz
t
z
Hdzpdnote
pTdg
Rzz
pdg
RTdz
gz
p
top
1000
1000
1000
~ln:
ln
ln
(H: scale height = RT/g)
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
00 Z 12 March
00 Z 13 March
00 Z 14 March
q and wind
@ dynamic tropopause (1.5 PVU)
pressure SLP, 850 PV, and 850 qe
Rapid cyclogenesis
(from Bosart in Shapiro and Gronas 1999)
1009
998
972
150
450
400
750
150
150
150
References
• 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.