Based on a one-dimensional eddy diffusion model, the model for studying the heat and mass exchange between the atmosphere and water body is developed, in which the phase change of water in winter and summer is included and the importance of the convective mixing process is emphasized. The model uses enthalpy instead of temperature as the forecast variable, which helps us to easily deal with phase change process. The model ie verified by observed data of lake ICinneret, which emphasizes adding convective mixing ptocesa is in accord with the practicalphysical mechanism of heat and mass exchange between the atmosphere and water body. The sensibility tests of wind velocity and surface friction velocity of lake verify their great influence on lake surface temperature and latent heat.