Parameterization of Lakes in Numerical Models for Environmental Applications

Lakes significantly affect the structure of the atmospheric surface layer and therefore the surface fluxes of heat, water vapour and momentum. This effect has not been systematically studied so far and is poorly understood. In most numerical weather prediction (NWP), climate modelling and other numerical prediction systems for environmental applications, the effect of lakes is either entirely ignored or is parameterized very crudely. A physically sound model is required to predict the lake surface temperature and the effect of lakes on the structure and transport properties of the atmospheric surface layer. Apart from being physically sound, a lake model must meet stringent requirements of computational economy. The problem is twofold. For one thing, the interaction of the atmosphere with the underlying surface is strongly dependent on the surface temperature and its time-rate-of-change. It is com-mon for NWP systems to assume that the water surface temperature can be kept constant over the forecast period. The assumption is to some extent justified for seas and deep lakes. It is doubtful for small-to-medium size relatively shallow lakes, where the short-term variations of the surface temperature (with a period of several hours to one day) reach several degrees. At present, a large number of such lakes are indistinguishable sub-grid scale features. These lakes will become re