Detection of continental-scale intensification of hourly rainfall extremes and implications for future changes

Temperature scaling studies suggest that hourly rainfall magnitudes might increase beyond thermodynamic expectations with global warming; that is, above the Clausius-Clapeyron (CC) rate of 6.5% °C^-1. However, there is limited evidence of such increases in long-term observations. Here, we calculate continental-average changes in the magnitude and frequency of extreme hourly and daily rainfall observations from Australia over 1990-2013 and 1966-1989. Observed changes are compared to the uncertainty from natural variability and to expected changes from CC-scaling as a result of global mean surface temperature change. We show that increases in daily rainfall extremes are consistent with CC scaling, but are within the range of natural variability. By contrast, changes in the magnitude of hourly rainfall extremes are close to or exceed double the expected CC-scaling, and are above the range of natural variability, exceeding 3xCC in the tropical region (north of 23°S). These continental-scale changes in extreme rainfall are not explained by changes in the El Niño-Southern Oscillation or changes in the seasonality of extremes. Our results indicate that CC-scaling on temperature provides a severe underestimate of observed changes in hourly rainfall extremes in Australia, with implications for assessing the impacts of extreme rainfall.

We extend this study to examine some commonly used metrics of change in extreme rainfall for the same dataset: (i) trends in high percentiles, (ii) change in high percentiles between two time periods; and their relation to changes in dew point and near-surface air temperature for both individual gauges and extreme regions. We use this framework to estimate changes in extreme rainfall for future time periods.