Extreme rainfall in the Mediterranean

This paper examines the recent advances in the analysis of extreme rainfall in the Mediterranean region. In the ‘90s the National Group for Defence from Hydrogeological Disasters for Italian rivers developed a regional methodology that later became the VAPI procedure (Rossi and Villani, 1994). The procedure is based on the TCEV probabilistic model (Rossi et al., 1984). The TCEV distribution assumes individual rainfall or flood peaks can be expressed as a mixture of two exponential components. A regionalized TCEV distribution was shown to accurately reproduce the observed distribution of skewness, a statistical test particularly useful for flood frequency analysis. Recently, advances in meteorology showed that the variability of extreme rainfall can depend on the presence of several meteorological processes with different characteristics. The baroclinic cyclogenesis is the dominant meteorological phenomenon in the Mediterranean. When a baroclinic wave impacts on an orographic obstacle, like the Apennines, it causes a baroclinic lee cyclone generated at the lee of the obstacle. The baroclinic structure generates ordinary extremes (more frequent and less severe on average). The other two mechanisms exhibit a greater variability and tend to generate more severe and less frequent extremes: the first mechanism consists of isolated convective cells of small spatial scale (tens of kilometers) mostly associated with warm season thunderstorms; the second mechanism consists of tropical cyclones, defined as hurricane-like cyclones in the Mediterranean environment, which have a spatial scale greater (hundreds of kilometers) than the isolated convective cells. This phenomenon strongly depends on the interaction between the atmosphere and the sea. In this study, a new methodology is proposed based on: i) procedure for “a priori” identification of meteorological structure of the events. In this procedure, three homogeneous time series can be identified in each site. Moreover, each of the series can be considered independent of the others and then studied separately; ii) the application of Power Extreme Value (PEV) distribution, a parametric family of power law transformations. The PEV distribution is applied to a single series of homogeneous data by considering a hierarchical approach to estimate the regional parameters. Finally, an application of the approach on the maximum annual daily rainfall time series in Campania is provided.