Precipitation induced by the GSL frequently occurs in concert with orographic precipitation over northern Utah. Such lake-effect precipitation is of scientific interest because of the need to understand and predict how mesoscale precipitation features are created and evolve in regions of complex orography. Recent field studies on lake-effect snow in the Great Lakes region (e.g., LOWS; Reinking et al. 1993) did not afford a rigorous evaluation of such orographic influences.

Steenburgh et al. (1999) have examined lake-effect snowstorms of the GSL using WSR-88D radar imagery, Rapid Update Cycle (RUC) analyses, and conventional surface and upper-air observations. During the four-year study period, 16 well-defined lake-effect events were identified. Two major types of lake-effect structures were found: wind-parallel bands that were aligned along the major axis of the lake and broad-area precipitation shields that were located over and downstream of the southeastern (lee) lake shoreline. Many of the large-scale characteristics of these events were found to be similar to those of lake-effect snowstorms of the Great Lakes (e.g., Niziol 1987; Niziol et al. 1995).

It is likely that lake-effect bands are initiated by low-level convergence produced by local mesoscale circulations. Over the Great Lakes, thermally-driven convergence is critical for the development of mid-lake and shoreline snowbands (e.g., Peace and Sykes 1966; Lavoie 1972; Passarelli and Braham 1981; Ballentine 1982; Hjelmfelt 1990), while studies over northern Europe show the importance of the geometrical configuration of coastlines in triggering convective snowbands over the Baltic Sea (Andersson and Gustafsson 1994). Radar-derived statistics presented by Steenburgh et al. (1999) show that lake enhancement by the GSL is greatest (1) during periods of large lake-land temperature differences and (2) during the late night and early morning hours. Both of these effects occur when convergence due to land-breeze circulations would be strongest. Numerical simulations provide further support for this hypothesis, but also show that flow channeling by the topography may contribute to snowband initiation (Onton and Steenburgh 1998).

Detailed observations from IPEX will also be used to test the hypothesis that lake-effect snowbands associated with the GSL are initiated by thermally- and topographically-driven circulations. We will also test the hypothesis, based on the radar climatology of Steenburgh et al. (1999), that the diurnal evolution of thermally-driven circulations, such as land breezes, greatly modulate the intensity of over-lake convergence and subsequent lake-effect precipitation. Other important issues that will be examined include (1) the role of frictional convergence, which may initiate or intensify lake-effect precipitation events, (2) the degree of boundary-layer modification over the GSL and the importance of ambient upstream moisture due to the limited impact of latent heat fluxes by the small size and hypersaline composition of the lake, and (3) the cloud structure and dominant microphysical processes responsible for precipitation production.