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

Abstract

An array of 35 idealized mesoscale model simulations was used to examine environmental and surface forcing factors controlling the meso-β-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-β-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.