Modification of the Thermodynamic Structure of the Lower Troposphere by the Evaporation of Precipitation: A GEWEX Cloud System Study

J.J. Katzfey and B.F. Ryan

Monthly Weather Review
(in press)

ABSTRACT

The importance of subcloud evaporation to the thermodynamics and movement of cold fronts is investigated through inclusion of an explicit cloud scheme within a 30~km resolution limited-area model. Two cases are examined: 18 November 1984 and 26 February 1995. The effect of the subcloud evaporation is deduced by comparing simulations with and without the evaporation for these two cases. The implications of these results for weather prediction and climate models are discussed.

The first case occurred during the Australian Cold Fronts Research Programme (CFRP) with mesoscale data available to verify the model simulation. The results indicate that the movement of the synoptic cold front was retarded when a prefrontal cool change developed in response to the evaporation of the grid-resolved precipitation. However, the cooling ahead of the front effectively accelerated the cool change, more in line with observations.

The second case involved the prognoses of a cool change crossing Victoria in southeast Australia during a potential bushfire day. In this case, most of the precipitation occurred along and behind the cold front. With precipitation evaporation, the cool change accelerated several hundred kilometers further in 36 hours than in the simulation without precipitation evaporation.