This study examines the frontal passages of 21-22 January 2004 and 24-25 April 2002 over Lake Michigan. The 21-22 January 2004 case is an example of a cold frontal passage over relatively warm lake waters with associated pre-frontal precipitation and represents a typical lake effected snow scenario. The 24-25 April 2002 case is an example of a cold frontal passage over a relatively cool lake and represents a typical spring convective precipitation event. Observational analysis reveals that the presence of Lake Michigan resulted in an apparent enhancement of the pre-frontal precipitation associated with the 21-22 January 2004 cold front, and an apparent suppression of convective development associated with the 24-25 April 2002 cold front.
Numerical Simulations with the Weather Research and Forecast Advanced Research WRF (WRF-ARW) model are compared with observations of the two cases to understand the impacts of the lake surface on the frontal characteristics of each case. Methods include with-lake, and no-lake simulations to better understand the importance of the lake on the frontal boundary. Two model sensitivity tests are conducted for each case: one involving the increasing the surface roughness lengths of the Great Lakes to values similar to the surrounding land surface to further understand the effects of surface roughness on the frontal boundary. The second test involves modifying the surface skin temperatures of the lake surface to closely match the surface skin temperatures of the surrounding land. This provides a better understanding of the impacts of lake-land temperature difference on a cold frontal passage.
Examination of observations and the model simulations reveals that Lake Michigan had a substantial impact on the synoptic-scale fronts. Most notably, the lake slowed the progress of the 21-22 January 2004 cold front, weakened the temperature gradient across the front, and enhanced the pre-frontal precipitation associated with the cold front. The effects of increased lake-land temperature difference and decreased surface roughness appeared to compete, with the lake-land temperature difference being the dominating factor in the slowing of the frontal boundary. Thus, it can be expected that a cold front progressing over a relatively warm lake surface will propagate slower than the same front progressing over land surfaces. Passage over the lake surface appeared to accelerate the 24-25 April 2002 cold front, develop a near-surface stable layer, and strengthen the temperature gradient across the frontal boundary. The effects of increased lake-land temperature difference and decreased surface roughness appeared to be working together in accelerating the frontal boundary. Therefore, it can be expected that a cold front progressing over a relatively cool lake surface will propagate much more quickly than the same front progressing over land surfaces.