Classifying severe rainfall events over Italy by hydrometeorological and dynamical criteria

Raingauge data over Italy for the period January 2006-February 2009 have been used to classify severe rainfall events into two types using a recently developed methodology. The types are defined as either long-lived and spatially distributed (Type I) if lasting more than 12 h and larger than 50 x 50 km(2) or brief and localized (Type II) if having shorter duration or smaller spatial extent. A total of 81 events were identified, with 51 classified as Type I and 30 as Type II. The work presented here examines the hypothesis that the two types of event are associated with different dynamical regimes distinguished by differing degrees of control of convective precipitation by the synoptic-scale flow. For each of the 81 events, a time-scale for convective adjustment is computed, based on gridded hourly precipitation rates derived from rain-gauge data and ECMWF analysis (ERA-Interim) of convective available potential energy (CAPE). Values of the convective adjustment time-scale, tau(c), shorter than 6 h indicate convection that is responding rapidly to to the synoptic environment (equilibrium), while slower time-scales indicate that other, presumably local, factors dominate. It was anticipated that tau(c) > 6 h would correspond to brief and localized Type II events, while tau(c) < 6 h would indicate Type I events. This hypothesis was largely confirmed, with 45 of the 51 Type I events having time-scales shorter than 6 h and 20 of the 30 Type II events having time-scales longer than 6 h. Copyright (C) 2011 Royal Meteorological Society