The spatial evolution of clouds and snow in a lake-effect boundary layer

When clouds and precipitation develop, their interactions with boundary layer circulations are believed to play an important role in the growth and development of convective boundary layers. Snow from lake-effect storm systems, a common occurrence downwind of the Great Lakes during the winter season, is ultimately the result of both microphysical and boundary layer processes. Lake-effect snow storms develop within convective boundary layers that are generated as cold air flows over the relatively warm lake water. While the total snowfall produced in lake-effect events can be significant, relatively little is known about how clouds and snow develop and evolve within a lake-effect boundary layer. Furthermore, the interactions that occur between the convective motions found within the boundary layer and the clouds and precipitation are not well understood. Numerous modeling studies of lake-effect snow include cloud and snow processes, but there are few detailed observations to which the model results can be compared. The goal of this work is to document the evolution of clouds and snow and their relationship to boundary layer circulations and characteristics across Lake Michigan during a lake-effect snow event.

The Lake-Induced Convection Experiment (Lake-ICE) provides a unique dataset with which the development and evolution of clouds and snow within a lake-effect boundary layer can be examined. Lake-ICE was conducted over Lake Michigan during December 1997 and January 1998. During most Lake-ICE intensive operation periods, the NCAR Electra aircraft flew approximately parallel to the wind at low levels over the lake while the University of Wyoming King Air flew cross-wind flight stacks at several locations across the lake. These coordinated aircraft flights provide microphysical and air motion data from throughout the boundary layer. Satellite and rawinsonde observations from Lake-ICE show that the lake-effect event on 10 January 1998 remained in a quasi-steady state throughout the observational period. The lack of significant temporal changes and the presence of widespread boundary layer rolls allow 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 boundary layer grew from about 900m near the upwind shore to about 1250m near the downwind shore, a fetch of about 90km. While significant cloud development was not observed until about 25km from the upwind shore, snow particles were observed up to a height of about 900m at a fetch of 11km from the Wisconsin shoreline. Possible explanations for this observation include wind-blown snow from Wisconsin, residual snow particles from transient clouds within the first 11km of the shore, and snow particles remaining from clouds that were present earlier in the day.

Aircraft microphysical, motion, and radiation observations reveal intriguing relationships between vertical motions and microphysical properties. Initial analyses suggest that clouds tended to be located downwind of regions of low-level updrafts, and low-level snow particle concentrations tended to peak in downdraft regions. Additionally, there is a local maximum in low-level snow particle concentrations just west of the middle of the lake, possibly indicating the location at which aggregation began to dominate the snow growth process. These results show that the development and evolution of clouds and snow within a lake-effect boundary layer may not occur in the uniform manner often depicted in conceptual models. The localized variability in the evolution of clouds and snow that is observed in this case may have important implications on the overall lake-effect snow process.