Non super-Clausius-Clapeyron scaling of convective precipitation extremes

Short duration precipitation extremes pose a risk to human lives and infrastructure and may be strongly affected by climate change. In the last two decades, several studies reported extreme rainfall intensity to increase with temperature at rates exceeding the thermodynamic Clausius-Clapeyron (CC) rate. Two explanations were given for this so-called “super-CC rate”: (i) convective precipitation — arising from thunderstorms — might be strongly invigorated with temperature; (ii) a statistical shift from low-intensity stratiform to higher-intensity convective-type rainfall might accentuate the super-CC scaling rate with temperature. Here we use high spatio-temporal resolution lightning records in Europe to revisit the super-CC hypothesis 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 super-CC rate and both stratiform and convective precipitation extremes increase merely at the CC rate — thus refuting hypothesis (i). Harbouring a statistical superposition of rain types, mesoscale convective systems (MCSs), which play a dominant role in generating precipitation extremes, do feature a super-CC rate as a consequence of a dramatic increase in their convective fraction with dew point temperature above 14 degrees Celsius.