Estimating future trends in severe hailstorms over the Sydney Basin: A climate modelling study

This study estimates future trends in the characteristics of severe hailstorms that affect the most heavily populated region of Australia, the Sydney Basin, using coupled climate model simulations under both fixed (no change) greenhouse gas concentrations and the IPCC SRES A1B future climate scenario. The “future climate”, as defined here, is the 50-year period January 1, 2001 to December 31, 2050. First, an assessment is made of the ability of the climate model, in very high-resolution mesoscale model mode, to simulate three of the most severe hailstorms recorded in the Sydney Basin during the “present climate” period, defined here as the years 1990 to 2002. These simulations, nested down to 1 km grid spacing, are compared with the archived hail observations of the storms. The climate model then is used to provide estimates of projected changes in hailstorm frequency, tracks, intensity, duration, and hail size over the Sydney Basin for the “future climate” period. The model employed in all simulations is the University of Oklahoma Coupled General Circulation Model, known as OU-CGCM, which also can be run as a high-resolution NWP model. The high-resolution version of the OU-CGCM used for the case studies employs a hierarchy of graded mesh and nested model domains, with a sophisticated 10-ice phase cloud microphysics scheme used in the highest resolution domain (1 km horizontal grid spacing) of the model. This work builds upon preliminary hail modelling case studies over eastern New South Wales carried out by the present authors with an earlier version of the model. The model results under the SRES A1B future climate scenario show significant trends out to 2050 in the key characteristics of severe hailstorms over the Sydney Basin, relative to both the 1990–2002 present climate and the 2001–2050 no-change future climate.