Influences of upwind lakes on the wintertime lake-effect boundary layer

Lake-to-lake snow bands are important factors in forecasting in the Great Lakes region. However, little quantified information is available on how the modified boundary layer from an upwind lake (i.e., thermodynamic properties, mesoscale circulations) influences boundary layer convective development over a downwind lake. An incomplete understanding of this preconditioned environment poses forecasting difficulties during lake-effect events affecting the lower peninsula of Michigan, northern Indiana and northeastern Illinois, and the eastern Great Lakes.

A example case study is presented which describes the impact of Lake Superior on boundary layer growth and lake-effect snow over Lake Michigan on 16 January 2000. Well-defined wind-parallel cloud streets oriented north to south over Lake Superior, crossed the upper peninsula of Michigan and fed into cloud bands over Lake Michigan. Simulations with and without Lake Superior indicate that the boundary layer air over Lake Superior crossed northern Michigan and arrived at the northern shore of Lake Michigan warmer and moister than without Lake Superior. This resulted in rapid initial boundary layer development and intensification of clouds and precipitation over Lake Michigan. However, by the time the air had passed over Lake Michigan for a distance greater than about 50 km, there were only small differences in boundary layer height, vertical velocities and cloud amounts. Despite the small difference in boundary layer characteristics, precipitation and simulated reflectivities were still higher. (such precipitation enhancement can continue to the Michigan shore as was described by Mann et al, 2002)

Initial results of an observational study to examine the frequency of occurrence of, and environmental conditions favorable for, lake-to-lake band formation over the Great Lakes will also be presented. These results indicate that lake-to-lake bands can extend from any of the northern or western Great Lakes to the southern and eastern lakes. Bands between the eastern lakes (Ontario and Erie) are rare.