The Precipitation Field in Southern Italy: Climatology, Physical Factors and Forecasting Techniques

This chapter revisits some aspects of the precipitation field, including its prediction, over Calabria peninsula, which is the southwest tip of the Italian boot. Calabria offers an interesting scenario for mesoclimatic studies in spatial and temporal rainfall variability. The region is located in the Central Mediterranean Basin, which has a strong seasonal variability that mirrors in large precipitation differences among seasons. Moreover, the complex orography of the country leads to different exposures of the territory to the rain-bearing air masses and large differences in rainfall can occur in few tens of kilometres. After presenting the climatology of the rain field over the country and, specifically of heavy rainfall events (precipitation greater than 64 mm/day), the numerical analysis of an intense and destructive storm that occurred on 10-12 December 2003 was used to study the role of the physical factors involved. This case study was chosen because it is a prototype of heavy rainfall events occurring over the peninsula and more generally in the Mediterranean area. The factor separation technique was applied to this storm and the case study clearly shows the effect of the three main physical factors involved in intense and destructive rainstorm occurring over Calabria: (1) the orography, (2) the sea, and (3) the synoptic scale forcing. Results show that the Calabrian orography and the surface latent heat flux were important elements for the precipitation enhancement at the mesoscale. Humid marine air masses were advected by the synoptic flow toward the Calabrian steep coastal mountain ranges determining intense and abundant rainfall. The analysis of the storm also reveals the crucial role of the synopticscale forcing, which can be accounted by considering a single key variable: the potential vorticity (PV). In the extra-tropical atmosphere, intense cyclones often develop under upper-tropospheric jet streams that are characterized by anomalously high values of the PV, and accurate specification of the PV distribution is necessary for the successful prediction of hazardous weather events. The sensitivity of intense rainstorms to the PV streamer paves the way to a more advanced, yet simple, forecasting technique, based on a Limited Area Model Ensemble Prediction System (LEPS). The technique is particularly useful in data void regions as Calabria, where the presence of the sea makes ground based observations problematic. A LEPS forecast, whose resolution is able to represent structures of the precipitation field not resolved by global models, generates different scenarios compatible with model error and initial/boundary conditions error. For each scenario, a pseudo water-vapour image is computed by the temperature of the isosteric surface of 75 mg/kg. Pseudo water-vapour images are then compared with the METEOSAT water vapour image, before the time of the expected rainfall in the target area, to classify the scenarios' reliability. The precipitation forecast by these scenarios can be used to refine the deterministic forecast. The methodology was applied to the 10-12 December 2003 rainstorm. For this case it was able to discard the only scenario predicting comparatively lower rainfall, and to show the good performance of the unperturbed forecast.