The Winchelsea Convergence - using radar and mesoscale NWP to diagnose cool change structure

A particular feature of dry cool changes in southern Victoria that is seen in clear-air radar reflectivity is described in this paper. The feature manifests itself as a trailing line echo oriented approximately normal to the leading edge of a dry cool change and on the cool-air side of the change. Using clear-air radar echoes, mesoscale NWP data, and observations, the evolving morphology of these convergence lines is described. It is shown that these convergence lines result from the confluence of diabatically modified post-frontal air that has had some hours of movement over land, and that of the cool air behind a coastal surging part of the same cool change. As the wind shifts associated with these dry cool changes are of critical importance to fire fighting operations, the concepts presented in this paper have potential for aiding very short-range forecasts of these wind changes and the consequent forecasting implications are discussed.