6.4
A study of heavy precipitation occurring in continental and marine environments
John R. Gyakum, McGill University, Montreal, QC, Canada; and R. McTaggart-Cowan, P. A. Sisson, G. Toth, P. Lewis, and J. K. Parker
This research focuses on the physical processes relating to two recent events of precipitation in which at least 25 mm of liquid equivalent precipitation occurred within a 12Ðh period. The first event was that of the heavy snow in Albany, New York of 7 January 2002, and the second event was that of a heavy snow in Halifax, Nova Scotia of 4-5 February 2002. Each event was evidently unrelated to topographic forcing. Each case was also mostly unforecasted by the operational models with horizontal resolutions ranging to 6 km. Although each case occurred in a large-scale flow that was favorable for cyclogenesis, several crucial details relating to the extreme mesoscale development are identified and studied. Additionally, cyclogenetic mechanisms are studied.
The mesoscale environment is studied in the context of both upright and slantwise convection. The processes that were responsible for this destabilization include upper-level cooling associated with the advance of an upper-level short-wave trough, and the development of conditional symmetric instability.
Frontogenetic forcing was especially large in the middle troposphere in each case. The fact that ascent maxima of Ð20 to Ð50 microbars per second occurred in the vicinity of these zones of frontogenesis suggest the importance of this forcing contributing to these events' strength.
Considering that the upper-level frontogenesis was associated with a sloping dynamic tropopause, we investigate the role of the diabatically-induced effects of the observed moist convection in each case. To do this, we use a high-resolution version of the Mesoscale Compressible Community Model (MC2) with an accurate microphysical parameterization and explicit representation of cumulus effects. We investigate the hypothesis that moist upright convection produces a positive feedback on surface cyclogenesis through the mechanism of diabatically-induced ridging. This ridging shortens the wavelength, and increases the cyclonic vorticity advection over the developing cyclone.
Session 6, Winter Weather and Climatological Studies
Wednesday, 14 August 2002, 8:00 AM-10:00 AM
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