Lake-effect snow storms can produce some of the most severe weather observed in communities along coastal regions of the Great Lakes during the fall and winter seasons. These storms develop within convective boundary layers that are generated as cold air flows over the relatively warm lake water. Snow generated by lake-effect processes has been shown to increase the average total wintertime snowfall accumulation in coastal regions by 50-100% or more. While numerous modeling studies of lake-effect snow include cloud and snow processes, there are few detailed observations to which the model results can be compared. The goal of this work is to quantify and understand the evolution of clouds and snow across Lake Michigan during a lake-effect snow event.

The Lake-Induced Convection Experiment (Lake-ICE), which was conducted over Lake Michigan during December 1997 and January 1998, provides a unique dataset with which the development and evolution of clouds and snow within a lake-effect boundary layer can be examined. On 10 January 1998, the NCAR Electra aircraft conducted thirty flight legs approximately parallel to the wind about 170-270 m above the lake surface. At the same time, the University of Wyoming King Air flew four cross-wind flight stacks, with individual flight legs ranging in height from about 150 m to about 1200 m above the lake. Aircraft, satellite, and rawinsonde observations show that this lake-effect snow event remained in a quasi-steady state throughout the approximately four and a half hour observational period, allowing microphysical data from the two research aircraft to be combined to construct an overall view of the clouds and snow that developed within the lake-effect boundary layer.

On 10 January 1998, the lake-effect boundary layer grew from a height of about 675 m to a height of about 910 m over the 80 km distance between the westernmost and easternmost King Air flight stacks. Maximum cloud particle concentrations were found just below the top of the boundary layer, while maximum snow particle concentrations were typically found in the lowest in-cloud flight segments. In addition to these general characteristics, two surprising features related to the evolution of clouds and snow across Lake Michigan were noted. First, although cloud development was estimated to begin 14-18 km from the Wisconsin shoreline, snow particles were observed at least 3 km upwind of the clouds up to a height of about 650 m. These snow particles are believed to be the result of transient clouds located near the upwind shore. Second, snow particle concentrations were observed to peak near the middle of the lake before decreasing toward the downwind shore. Based on analyses of snow size spectra, this midlake peak in snow particle concentrations was found to identify the location after which the dominant snow growth mechanism transitioned from deposition to aggregation. Developing a better understanding of the structure and evolution of lake-effect clouds and snow may ultimately lead to improved forecasts of these significant winter weather events along the downwind shores of the Great Lakes.