Historic Rainfall and Temperature Relationships and Implications for Climate Chanhe Assessment

  • Mr Rhys Hardwick Jones, WMAwater, Water and Environmental Engineers, Sydney, New South Wales, Australia, Australia
  • Dr Seth Westra, School of Civil and Environmental Engineering, University of New South Wales, Sydney, New South Wales, Australia, Australia
  • Dr Ashish Sharma, School of Civil and Environmental Engineering, University of New South Wales, Sydney, New South Wales, Australia, Australia

Projected increases to the intensity of extreme rainfall are likely to represent one of the most important impacts of climate change, with flooding regularly resulting in significant economic losses as well as the potential for loss of life. There remains a high level of uncertainty about how much extreme rainfall will change. A frequently used approximation is that the short-duration extreme rainfall intensity is proportional to the moisture-holding capacity of the atmosphere, which according to the Clausius-Clapeyron relation increases by approximately 7% per degree, under the assumption of unchanged relative humidity. There has been evidence supporting ‘super Clausius-Clapeyron’ scaling relations of up to 14% per degree.

We analyse records from over 300 pluviograph stations in Australia with more than 10 years of record, to determine whether this hypothesis has empirical support, and whether regional trends are observed.

We analyse the scaling of historical rainfall intensity with temperature for a range of storm durations from 6 minutes to 24 hours. The influence of observed relative humidity and seasonal variation are also examined. We find that there are significant regional trends in scaling of extreme rainfall intensities with temperature, with most of Australia’s east coast showing a scaling of approximately 7% per degree for one-hour rainfall extremes, while parts of Australia show a negative scaling relationship, seemingly in contrast to the Clausius-Clapeyron relation.

We conclude that observed scaling of extreme rainfall intensities with temperature is not governed solely by the Clausius-Clapeyron relation. The results may have implications for projections of changes to extreme rainfall in a greenhouse-warmed atmosphere. Further investigations are required to determine whether available climate models and downscaling techniques reproduce the observed scaling behavior, and the extent to which historical temperature scaling relationships can be used as the basis for inferring how rainfall will change under a future climate.