Analysis of a localized flash-flood event over the central Mediterranean

On 3 July 2006, an exceptionally heavy convective rainfall affected a small area in Calabria, Italy. A rainfall amount of 202 mm was recorded in 2.5 h, producing considerable damage and causing a localized flash flood. The Weather Research and Forecasting (WRF) model was used to analyze the instability present in the event and the related triggering mechanisms. The high-resolution simulation is able to correctly identify the position of the precipitation peak and to clarify the mesoscale processes involved, although it significantly underestimates the total amount of precipitation. Some sensitivity experiments confirm the importance of the choice of planetary boundary layer and microphysics parameterization schemes for a correct simulation of the event, showing a strong sensitivity to these numerical tests. Also, the need for high horizontal resolution emerges clearly: an accurate representation of the orography at small scales, is required to simulate the event in its correct location. Instability indices identified an extremely favorable environment for convection development, with very high values of CAPE and high moisture content at low levels. The low mountains near the rainfall peak play an important role in triggering the release of instability and controlling the location of rainfall; in particular, the peculiar morphology of the orography creates low-level wind convergence and provides the uplift necessary for the air parcels to reach the level of free convection. In this framework, nondimensional parameters, such as the Froude number, have been calculated to better understand the interaction of the flow with the orography.