Super-Clausius-Clapeyron scaling of extreme precipitation explained by shift from stratiform to convective rain type

Short-duration precipitation extremes pose a risk to human lives and infrastructure and may be strongly affected by climate change. In the past two decades, several studies reported that extreme rainfall intensity can increase with temperature at rates exceeding the thermodynamic Clausius-Clapeyron rate. Two explanations have been proposed for this: (1) convective precipitation-arising from thunderstorms-might be strongly invigorated with temperature; (2) a statistical shift from low-intensity stratiform rainfall to higher-intensity convective rainfall might amplify the scaling rate with temperature. Here we use high spatio-temporal-resolution lightning records in Europe to test these two hypotheses at the storm scale, that is, within 5 km spatially and 10 min temporally. We show that the statistical shift in rain type alone accounts for the observed super-Clausius-Clapeyron scaling rate, and when considered in isolation, both stratiform and convective precipitation extremes increase at the Clausius-Clapeyron rate-thus refuting hypothesis (1). Mesoscale convective systems, which play a dominant role in generating precipitation extremes, do feature a super-Clausius-Clapeyron scaling rate because of a substantial increase in their convective fraction with dew point temperature above 14 °C. Analyses of intensity-duration-frequency curves show that extreme sub-hourly storms are the most strongly intensified with higher dew point temperatures.