Spring convection in ice-covered freshwater lakes

The regime of penetrative convection driven by a vertically inhomogeneous r adiation heating is considered. Similar convection is observed in ice-cover ed freshwater lakes, where the water temperature is below the maximum-densi ty temperature during late spring. The convective length, velocity, and tem perature scales are introduced, which are suitable for the description of c onvection driven by a vertically inhomogeneous heating. On the basis of the se scales and the similarity hypotheses for the vertical temperature profil es and turbulence characteristics in the convective mixed layer, a simple m odel of the convection regime under consideration is proposed. The entrainm ent regimes characteristic of the convection in ice-covered lakes are studi ed. It is shown that such convection is satisfactorily described by the equ ation used for convection in both the atmospheric and oceanic layers if the Deardorff convective scales based on the buoyancy flow through a fluid sur face are replaced by the convective scales, which take into account the thr ee-dimensional character of radiation absorption. The results of model calc ulations are in good agreement with observational data.