A data-based assessment of the dependence of short-duration precipitation on elevation

Variability of precipitation with elevation is often related to the presence of an orographic effect interacting with prevailing arrival directions of wet air masses. This effect is commonly recognized to be responsible for the increase with elevation of the annual precipitation amounts measured at the ground level. However, the variability with elevation of heavy rainfall of short duration is poorly investigated in hydrology, despite the importance of short-duration events in hydrological applications. Analyzing a database of 567 time series of annual maximum sub-daily rainfall in northern Italy, we find the relation of extreme precipitation with elevation to be a function of the event duration. In particular, it emerges that the intensity of rainfall decreases with elevation for very short durations (i.e., 1–3 h), while the negative slope of the intensity-elevation regression lines tends to decrease when considering events of longer duration (i.e., 12–24 h), at least in the western part of the alpine chain. A combined use of kriging and regression techniques is then proposed to account for the effect of elevation and longitude in the spatial interpolation of sub-daily rainfalls.