Two-dimensional simulations of flow in ice-covered lakes with horizontal variations in surface albedo

We present numerical simulations of radiatively driven convection at temperatures below the temperature of maximum density, as observed in ice-covered lakes in early spring. The purpose of these simulations is to isolate the phenomenon of lateral circulation driven by horizontal variations in surface albedo (e.g., due to partial snow cover) in an idealized and simplified system. The system we consider is one with uniform solar radiation except in a small "shadowed" region at the center of the domain which has damped radiation intensity. By comparing cases with and without a shadowed region, we identify gravity currents at the surface flowing away from the shadowed region. Not only do these gravity currents represent a mechanism for lateral transport at the surface below ice cover, but they also act as a catalyst for inducing earlier vertical mixing that develops at a faster rate than the Rayleigh-Taylor-like instabilities which drive vertical convection away from the shadow. To the authors' knowledge, only bathymetry and wind forcing at the surface have been presented as major mechanisms for lateral circulation in ice-covered lakes, and hence these simulations may provide a hitherto unreported mechanism for inducing lateral circulation.