The frontal transition zone and microphysical properties of associated clouds

A study of the cloud structure and precipitation in the transition zone of cold fronts over western Victoria shows that the important precipitation parameters, such as cloud top temperature and depth, are determined synoptically. Pre‐frontal clouds experience slow uplift over substantial depths; this results in cold deep clouds with lifetimes of the order of tens of hours. In the post‐frontal clouds the uplift is mesoscale and shallow and causes short‐lived cloud elements. These dynamic and thermodynamic featurea determine the cloud microphysics and largely the associated rainfall. Precipitation in pre‐frontal clouds forms mostly through lightly rimed ice crystals produced at low temperatures; the process can be very efficient in glaciating the cloud but not necessarily efficient in producing ice crystals large enough to survive the fall to the surface. In the post‐frontal systems the ice crystals are produced at much higher temperatures in the presence of higher liquid water contents and the precipitation depends very strongly on cloud depth and duration.