Heavy precipitation events in Italy: lessons from the HyMeX SOP1 campaign

The first Special Observation Period (SOP1) of HyMeX (Hydrological cycle in the Mediterranean eXperiment) was held in Fall 2012 and focused on the observation and real-time numerical simulation of heavy precipitation events (HPEs) and floods in the northwestern Mediterranean. Nine of the twenty intensive observation periods (IOPs) involved the three Italian target areas, enabling unprecedented monitoring and analysis of intense precipitation systems over the north-east (NEI target area), Liguria and Tuscany (LT), and central Italy (CI). In the present work, we aim to summarize the improved understanding of the mechanisms responsible for HPEs over Italy, highlighting the main findings emerging in the HyMeX campaign by means of conceptual models and through the identification of relevant mesoscale features. For NEI, two categories of events (Upstream and Alpine HPEs) have been identified, which differ mainly in the temporal evolution of the stability of the upstream environment and of the intensity of the impinging flow (i.e., the Froude number). Some mesoscale features (e.g., the presence of a shallow pressure minimum in the eastern Po Valley) were identified as fundamental to adequately simulate the detailed evolution of severe convection episodes. For LT, local-scale or large-scale convergence lines appear fundamental to trigger and sustain convection. Cold pools formed via evaporation of precipitation also played a major role in determining the position of the trigger. The accurate representation of the moisture structure below 2 km is a key to an accurate simulation of the timing and location of precipitation. For CI, a high low-level moisture content and marked low-level convergence over the Tyrrhenian Sea were critical to maintaining deep convection in some IOPs. Also, an elevated moisture plume from the Tropics has been noted to enrich the mid-level humidity, thus reinforcing the intensity of the updrafts.