Rapid cyclogenesis (bombs) textbook section 8.5 Contents

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rapid cyclogenesis climatology Pacific Atlantic Gulfstream axis most common - in winter - off east coasts

Pacific Atlantic Gulf Stream Kuro Shio Labrador current Oya Shio

Rapid cyclogenesis mechanisms: near-surface processes 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 ) (a) low pressure implies surface wind  larger sfc LH flux (b) low pressure implies  >0 in friction layer  Ekman pumping and LL convergence more LH release in updraft stronger updraft spin-up (  ) cyclogenesis more BL water vapor

Rapid cyclogenesis mechanisms aloft: QG argument Large low-level water vapor content implies diabatic heating (typically peaking between mb) –local max in J (diabatic heating) makes the last term positive  stronger updraft –also, the static stability parameter  tends to be small in the warm sector over warm water

Horizontal temperature gradient wavelength surface deepening rate (mb/hr) From: Sanders different values of stability parameter dash – high  solid – medium  dot – low 

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) (H: scale height = RT/g)

Example of a “normal” tropopause depression

color fill: potential vorticity (0.1 PV units, i.e m 2 s -1 K kg -1 )

12 March 1993: storm of the 20 th century: impressive tropopause fold 00 Z 12 March 00 Z 13 March 00 Z 14 March  and dynamic tropopause (1.5 PVU) pressure SLP, 850 PV, and 850  e Rapid cyclogenesis (from Bosart in Shapiro and Gronas 1999)

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, 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, 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.