Three modifications to the Australian Bureau of Meteorology's Australian Region Primitive Equations (ARPE) model are described. These include the introduction of high resolution topography, a three-layer soil moisture scheme for the prediction of surface moisture and a second-moment turbulent closure scheme for the vertical transport of momentum, heat and moisture. A case study of an east coast cyclone which inundated Sydney with extreme rainfall on 5 August 1986 is presented to demonstrate the impact of these modifications. This case has been previously studied by the second author and the results here are an extension of that work. We demonstrate that the ARPE initialisation procedure of simply interpolating the coarse mesh topographical heights and model fields to fine mesh grid-points is conditionally unstable and can produce excessive rainfall at isolated grid-points. Computing new surface pressures and sigma surfaces based on the topographical heights and adjusting the model fields to these surfaces provides a robust initialisation. Sensitivity of the results to model resolution and physics is illustrated. The location, value of maximum precipitation and other objective measures of skill are improved with the new model. It also produces a rain-band over the sea at the location of the cloudband shown in the GMS satellite picture, which the existing model does not. The skill of the new model, however, deteriorates for 24-hour precipitation amounts greater than 50 mm.
McInnes et al. (Sun,) studied this question.