Apparent heavy tails of sub-daily precipitation explained by the coexistence of lighter-tailed processes

Extreme value theory is routinely applied to estimate rainfall frequency for several accumulation periods. Typically, it is found that sub-daily precipitation has power-type tails, meaning that the probability of observing increasingly large magnitudes decreases as a power law. Physical arguments, however, suggest it should have lighter, stretched exponential, tails. Here, we reconcile these perspectives showing that part of the contradiction is caused by precipitation process heterogeneity. We examine hundreds of sub-daily precipitation records in the Greater Alpine Area, for which a classification of storms into homogeneous types is available. We find that an apparent heavy-tail behavior is reported at scales of 1–6 hr, and is explained by the coexistence of stratiform and convective processes, both characterized by stretched exponential tails. Our results challenge the assumptions which justify the use of extreme value theory for sub-daily precipitation, with important implications for how design values are determined.