Little evidence for super Clausius–Clapeyron scaling of intense rainstorm properties with air temperature

Extreme precipitation is thought to increase proportionally to the rise in the water va-por holding capacity of the air at roughly 7 %◦C−1, the so called Clausius–Clapeyron(CC) rate. We present an empirical study of the variability in the rates of increase inprecipitation intensity with air temperature using 30 yr of hourly data from 50 stations5in an Alpine environment. The analysis is conducted on storm events rather than fixedtime resolutions, and divided into event subsets based on concurrent lightning strikesindicating the presence of convection. The average rates of increase in mean event in-tensity (7.4%◦C−1) and peak hourly intensity (5.1 %◦C−1) for 90th percentiles are closeto the CC rate expected under fully saturated conditions. Super-CC rates reported by10other studies are an exception in our dataset. Events accompanied by lightning (con-vective events) exhibit significantly higher rates of increase than stratiform rain. Mixingof the two storm types exaggerates the relations to air temperature. The large spatialvariability in scaling rates across Switzerland suggests that both local (orographic) andregional effects limit moisture availability and supply in Alpine environments especially15in mountain valleys. A trend analysis shows that our estimate of the number of convec-tive events across Switzerland has steadily increased in the last 30yr. This significantshift towards more convective storms in a warming climate may as a consequence leadto stronger storm intensities and therefore higher risk connected with those events.ISSN:1812-2116ISSN:1812-210