Extended Abstract (348K)8.2
The effects of a precipitating wintertime synoptic system on a lake-induced convective boundary layer
Joshua J. Schroeder, Univ. of Illinois, Urbana, IL; and D. A. R. Kristovich and M. R. Hjelmfelt
It is well established that lake-effect snowstorms are the result of convective boundary layer growth over large bodies of relatively warm water. Isolated lake-induced snows have been extensively studied over the Great Lakes and elsewhere. However, comparatively little is known regarding lake-effect boundary-layer processes occurring in conjunction with synoptic-scale cyclones. Such a "lake-enhanced" situation developed on the first operational day of the Lake-Induced Convection Experiment (Lake-ICE) on 5 December 1997.
Rapid convective boundary layer growth across Lake Michigan was observed, as well as heavy snow (heaviest of the Lake-ICE operations), despite the presence of relatively marginal conditions for lake-effect snows (i.e., lake surface to 850-mb temperature differences of only ~ 16-18ºC). It is believed that the interaction of larger-scale features with the lake-induced forcing acted to produce these effects. Such features included the presence of a surface trough with associated snowbands and a closed low at midlevels of the troposphere. In particular, seeding of the boundary-layer clouds with ice particles from above is found to be an important component of this interaction.
Evidence for the existence of cloud decks above the lake-induced convective boundary layer and associated seeding is apparent in in-situ aircraft measurements, airborne and land-based (operational) radar data, special sounding data taken during Lake-ICE, and satellite imagery. This presentation explores the effects of seeding for the 5 December 1997 case, including: (1) the alteration of microphysical processes in boundary-layer clouds, (2) the degree of precipitation intensity enhancement, and (3) boundary layer growth and the convective structure of the entrainment zone. Differences in these three areas from "classical" lake-effect situations are examined. National Weather Service radar data are used to discern regions where seeding was taking place. Preliminary results indicate that snow size spectra are considerably "broadened" and that boundary-layer depth is increased in regions of seeding. Results of numerical model simulations will be shown to give physical insight into the effects of seeding on lake-effect processes.
Session 8, Precipitation Processes
Thursday, 6 June 2002, 10:30 AM-12:00 PM
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