Severe Thunderstorm Environments: What Does the Future Hold for Australia?

Severe thunderstorms present a relatively infrequent but significant threat to property and life in Australia during the spring and summer. These thunderstorms can produce hailstones over 2cm in diameter, winds in excess of 90kmh^-1 and less frequently tornadoes. Any of these phenomena can result in high impact severe events. Recent examples of this potential are illustrated by damage caused by the 1999 Sydney Hailstorm, 2008 Gap Microburst, 2010 Perth and Melbourne Hailstorms and the 2011 Melbourne Hailstorms on Christmas Day. This risk makes the implications of a changing and variable climate on severe thunderstorms important to understand. Recent studies into the impacts of anthropogenic climate change on severe weather events, including thunderstorms, have suggested a potential increasing trend in both frequency and intensity for Australia. While current convective parameterizations in both global and regional climate models limit direct assessments of future convection, the use of environmental parameters to estimate changes in severe thunderstorm environments has been successful in developing a climatology of Australian environments. It is therefore reasonable to wonder “Is the frequency and distribution of severe thunderstorm environments in Australia likely to change in the future?”

Key to identifying these environments is developing a relationship between an observed severe thunderstorm and its environment. The relationships between severe reports and associated environments from reanalysis proximity pseudo-soundings have previously been used to determine covariate discriminants. These covariates use variables describing the potential for organization of a thunderstorm (deep-layer wind shear), and the potential of instability over the depth of the atmosphere (convective available potential energy). The discriminants are then used to identify the potential of an environment to produce severe thunderstorms for Australia, and have been demonstrated to be applicable to a variety of datasets. We refer to environments that exceed the covariate discriminant threshold as severe thunderstorm environments; environments favorable to the occurrence of severe thunderstorms non-conditional on initiation.

Utilizing these discriminants, three ‘historical' climatologies of severe thunderstorms have been developed for the period 1980-2000. The first was produced using the ERA-Interim reanalysis and acts as an effective ‘observational' dataset of convective parameters in the current climate against which to evaluate simulations from two climate models. Analyses for convective variables were derived from the output of global CSIRO Mk3.6 model using T63 spectral resolution and 18 vertical levels and a downscaled regional climate model, CCAM with higher 0.5° horizontal resolution. These two model climatologies are then compared to the reanalysis to test the ability of the respective models to resolve seasonal variability, the seasonal pattern of convective parameters and climatological patterns of severe thunderstorm environments.

Recognizing the limitations of climate model simulations, convective variable discriminants were then analyzed for future simulations. These assessments were based on simulations using a high emissions scenario for the period 2080-2100. Using these 21st century simulations of severe thunderstorm environments, comparison was made to 20th century environments to determine whether changes in both convective parameters and environments exceeding discriminant thresholds will be seen. The simulations are used to present a picture of potential future severe thunderstorm environments for Australia under the specter of a changing climate.