Numerical simulation of two heavy rainfall events over coastal southeastern Australia

Abstract

Predicting rainfall along the New South Wales (NSW) coast is a major forecasting problem because of sharp gradients in rainfall amounts with the heaviest falls on the coastal fringe decreasing rapidly inland. On some occasions the rainfall pattern is less spatially coherent and consists of isolated maxima. Both rainfall patterns are associated with mesoscale coastal ridging. The first rainfall pattern arises from coastal ridging occurring in combination with an offshore trough. In the first case study presented here, a typical ridge-trough system was aligned parallel to the coast, and located just offshore, with the observed rainfall heaviest at the coast, decreasing rapidly from over 60 mm to near zero 30 km inland. The model captured well the southward temporal evolution of the maximum relative humidity values and rainfall. The second rainfall pattern occurs when shallow coastal ridging interacts with downdrafts from thunderstorm activity over the ranges to the west. The second case study was one in which convergence and condensation generated a quasi-stationary line of thunderstorms, resulting in flash flooding. The model precipitation rates and accumulations matched very closely those of the squall line, as revealed by radar precipitation intensities and the observed rainfall.