Orographic effect on extreme precipitation statistics peaks at hourly times scales

Preparedness to natural hazards in mountainous areas strongly relies on the knowledge of extreme rainfall probability. The presence of mountains influences the motion of air masses, thereby modifying the storms characteristics. Here, we use a novel statistical approach to quantify the orographic impact on the probability of occurrence of extreme rainfall of short duration (10-min to 6-hour). We find that mountains tend to decrease the mean annual maximum intensities at sub-hourly scales, thereby confirming the previously reported “reversed orographic effect”, and tend to decrease the tail heaviness, thereby decreasing the extremely high intensities such as the events occurring on average once in 100 years. The second effect is however non-monotonic, in that it increases between 10 minute and 1 hour and diminishes between 1 and 6 hours. Sub-hourly extremes could thus be higher than what can be estimated from hourly data alone, implying that the scaling assumptions typically adopted for risk assessment may systematically underestimate the risk of short-duration extremes