Simulating the Sydney hailstorm of 9 December 2007 using WRF

This study first extended Sydney’s most comprehensive hybrid hailstorm database until the end of the 2014/2015 June-May hail season to discern whether an apparent decline in hailstorm frequency over the 1991-2010 period (8.45 ± 1.57 hailstorms per annum) has continued in recent years. Hailstorm numbers were found to average 10.89 ± 1.20 per annum from 1950-2014. However, the average became 12.00 ± 4.68 hailstorms per annum during 2011-2014. This suggests that the apparent decline may be attributed to inter-annual and inter-decadal variability, rather than a long-term reduction in frequency. As a case study, a high-resolution numerical simulation of the 9th of December, 2007 hailstorm was undertaken in order to assess advancements in Numerical Weather Prediction (NWP) models for such local-scale hazardous weather. The storm was simulated using version 3.6.1 of the Weather Research and Forecasting (WRF) Model. Sensitivity tests on cloud microphysics and radiation schemes were performed to determine their impacts. Microphysics options that included hail as one of their hydrometeors reproduced the hailstorm significantly better than those that did not. The Goddard microphysics scheme using the hail option coupled with the Goddard shortwave radiation scheme simulated the event most accurately, although the storm arrived approximately 1.5 hours earlier than the actual event.