Urban scale modelling: The Sydney hailstorm of 14 April 1999

Summary The aim of this study is to point to the very large improvements that are taking place in a range of modelling applications in the urban areas. The particular phenomenon chosen in this study is a supercell, but it could well have been any other aspect of urban modelling. The Sydney hailstorm of 14 April 1999 was a long-lived, high precipitation supercell that produced a massive damage bill of over 2 billion Australian dollars from its hail swath. The Sydney hailstorm was poorly forecast for a number of reasons including: the severe weather season had officially ended so there were no specialist staff on duty when the hailstorm struck Sydney; the storm proved very difficult to predict and it was expected to continue heading out to sea; and the forecast guidance from all available operational numerical models was inadequate at the resolution required for a supercell simulation. Here, our interest is on the last of the problems, namely, the quality of the operational numerical model guidance, especially given the impact it had on a densely populated urban region.In this study, we compare the numerical guidance available at the time with current modeling capability which, although in research mode at present, will soon be available in real-time mode. The operational models were hydrostatic models run at horizontal resolutions of 25km at best, compared with 1km horizontal resolution for the non-hydrostatic research model. The research model also had a high-order differencing scheme and a sophisticated six phase cloud physics scheme compared with the much simpler parameterized convection in the operational models. The operational model produced very little convective precipitation and it was displaced well to the north of Sydney. The research model generated a supercell with a track and a hail size distribution that was encouragingly close to the observed.