AUSTRALIAN WINTER MOUNTAIN STORM CLOUDS - PRECIPITATION AUGMENTATION POTENTIAL

Two Australian winter mountain storm field research projects were cond ucted by the Commonwealth Scientific and Industrial Research Organisat ion Division of Atmospheric Research and the Desert Research Institute Atmospheric Sciences Center in the austral winters of 1988 and 1990. These projects gained information about winter storms in support of th e ongoing Melbourne Water randomized cloud seeding experiment aimed at increasing runoff into Melbourne's main water supply, the Thomson Res ervoir. This paper discusses some of the 1988 instrumentation data. On e variable of interest is the precipitation augmentation potential eta . It is the difference between (a) the horizontal supercooled liquid w ater flux in the clouds crossing the mountains and (b) the vertical pr ecipitation flux at the surface from the clouds. These fluxes are base d on calculations of supercooled liquid water depth in clouds with a m icrowave radiometer, Omegasonde wind velocity, and rates of precipitat ion from gauges. It was found that a varies systematically during a wi nter storm. The greatest potential occurs in the post-cold-frontal sta ge of a storm when the cloud-top temperature is warm and about -12 deg rees C and the wind direction of 240 degrees is approximately orthogon al to the main southwest face of the predominant orographic feature, B aw Baw Plateau, of the study area. The potential is significantly less during the prefrontal and frontal stages, with cloud-top temperatures of about -35 degrees C and a wind direction of about 300 degrees para llel to the Baw Baw Plateau. The results show that cloud seeding would have the greatest benefit in the postfrontal stage.