Convective precipitation extremes may not increase beyond the Clausius-Clapeyron expectation

Flash floods arising from short-duration precipitation extremes are costly for the population, and their frequency and intensity could increase with global warming (Fowler et al., 2021). Understanding the mechanisms leading to extreme precipitation is thus essential. A common hypothesis for precipitation extremes is that they scale with temperature according to the thermodynamic Clausius-Clapeyron (CC) law. However, increases in short-duration precipitation extremes beyond the CC expectation (or super-CC) were reported in multiple regions. The super-CC scaling is currently understood as the combination of two effects: (1) an invigoration of convective precipitation through convective cloud feedbacks; (2) a statistical effect resulting from a shift in rain type, from light stratiform to heavier convective-type precipitation, with increasing temperatures.

This work revisits these hypotheses by identifying convective precipitation at an unprecedented high resolution (5 km spatially and 10 min temporally). For this, we employ the EUropean Cooperation for LIghtning Detection (EUCLID) lightning dataset to define convective precipitation and combine it with weather station data from the German weather service (Deutscher Wetterdienst, DWD). We show that while (total) extreme precipitation increases with a super-CC rate, the scaling of both convective and stratiform-type precipitation extremes is in accordance with the CC law. We thus conclude that the super-CC rate is explained by the statistical shift in rain type alone and refute any mechanistic origin. 

Mesoscale Convective Systems (MCSs), which dominate extreme precipitation events in Europe (Da Silva & Haerter, 2023), are known to contain both a convective and stratiform region (Houze, 1997). By tracking MCSs over Germany, we show that MCS extreme precipitation also features a super-CC rate, which we relate to a dramatic increase in their convective fraction for dew point temperatures exceeding 14 degrees Celsius. 

References:

Da Silva, N. A., & Haerter, J. O. (2023). The precipitation characteristics of mesoscale convective systems over Europe. Journal of Geophysical Research: Atmospheres, 128, e2023JD039045. https://doi.org/10.1029/2023JD039045

Fowler, H.J., Lenderink, G., Prein, A.F. et al. Anthropogenic intensification of short-duration rainfall extremes. Nat Rev Earth Environ 2, 107–122 (2021). https://doi.org/10.1038/s43017-020-00128-6

Houze, R. A. Stratiform precipitation in regions of convection: A meteorological paradox? Bulletin of the American Meteorological Society 78, 2179 – 2196 (1997). https://doi.org/10.1175/1520-0477(1997)078<2179:SPIROC>2.0.CO;2