MODIFICATION OF THE THERMODYNAMIC STRUCTURE OF THE LOWER TROPOSPHERE BY THE EVAPORATION OF PRECIPITATION - A GEWEX CLOUD SYSTEM STUDY

The importance of subcloud evaporation to the thermodynamics and movem ent of cold fronts is investigated through inclusion of an explicit cl oud scheme within a 30-km resolution limited-area model. Two cases are examined: 18 November 1984 and 26 February 1995. The effect of the su bcloud evaporation is deduced by comparing simulations with and withou t the evaporation for these two cases. The implications of these resul ts for weather prediction and climate models are discussed. The first case occurred during the Australian Cold Fronts Research Programme wit h mesoscale data available to verify the model simulation. The results indicate that the movement of the synoptic cold front was retarded wh en a prefrontal cool change developed in response to the evaporation o f the grid-resolved precipitation. However, the cooling ahead of the f ront effectively accelerated the cool change, more in line with observ ations. The second case involved the prognoses of a cool change crossi ng Victoria in southeast Australia during a potential bushfire day. In this case, most of the precipitation occurred along and behind the co ld front. With precipitation evaporation, the cool change accelerated several hundred kilometers farther in 36 h than in the simulation with out precipitation evaporation.