Idealized Model Simulations Examining the Mesoscale Structure of Winter Lake-Effect Circulations

An array of 35 idealized mesoscale model simulations was used to examine environmental and surface forcing factors controlling the meso- beta -scale circulation structure resulting from cold flow over an isolated axisymmetric body of water at the midlatitudes. Wind speed, lake-air temperature difference, ambient atmospheric stability, and fetch distance were varied across previously observed ranges. Simulated meso- beta -scale lake-effect circulations occurred within three basic regimes (e.g., vortices, shoreline bands, widespread coverage), similar to observed morphological regimes. The current study found that the morphological regimes of lake-effect circulations can be predicted using the ratio of wind speed to maximum fetch distance (U /L ). Lake-effect environmental conditions producing low values of U /L (i.e., approximately < 0.02 m s-1 km-1) resulted in a mesoscale vortex circulation. Conditions leading to U /L values between about 0.02 and 0.09 m s-1 km-1 resulted in the development of a shoreline band, and U /L values greater than approximately 0.09 m s-1 km-1 produced a widespread coverage event. It was found that transitions from one morphological regime to another are continuous and within transitional zones the structure of a circulation may contain structural features characteristic of more than one regime. Results show that 1) the U /L criterion effectively classifies the morphology independently of the lake-air temperature difference for the parameter value combinations examined and 2) the Froude number, suggested as a potential lake-effect forecasting tool in previous studies, does not permit the unique classification of lake-effect morphology.