International Workshop ADVANCES IN STATISTICAL HYDROLOGY May 23-25, 2010 Taormina, Italy EXTREME RAINFALL IN THE MEDITERRANEAN by Carmine De Luca (1) , Pierluigi Furcolo (1) , Fabio Rossi (1) , Paolo Villani (1) and Claudia Vitolo (1)(*) (1) Department of Civil Engineering, University of Salerno, Fisciano, Italy (cdeluca@unisa.it) (*) now at Halcrow Group Limited, Crawley,United Kingdom (vitoloc@halcrow.com) ABSTRACT 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. Keywords: extreme rainfall, Power Extreme Value, regionalization technique 1 INTRODUCTION The Mediterranean region is characterized by the occurrence of short and very intense storms. These phenomena often cause landslides and floods with considerable loss of human life and enormous economic damage. These events (known as extreme events) are very rare when referring to specific sites. However their frequency is not negligible in a regional context. This seems to be more evident when analysing the annual maxima for rainfall and flow data series. In fact, there are some values much higher than the others and called "outliers" which don't seem to come from the same sample of data. Figure 1 shows the data for some extreme events in Southern Italy occurring between 1949 and 1963. In the '90s the National Group for Defence from Hydrological disasters has developed a National Program for assessing floods in Italian watercourses, involving dozens of researchers in different operative units representing all Italian regions. A methodology was developed under this program, now known as the VAPI procedure (Rossi and Villani, 1994), used in different Italian Regions for the assessment of floods. This procedure is based on: - The TCEV probabilistic model (Rossi et al., 1984). This model is particularly suitable for describing the annual flood variability. It explains in a sensible way the presence of the extraordinary values that characterize the series; - Use all the available hydrological information including rainfall data sets. - The hierarchical regionalization technique, it is able to minimize the uncertainty in the estimation of regional parameters. This technique estimates the parameters with higher sample variance on increasingly large areas, and then by a higher number of data; De Luca et al., Extreme rainfall in Mediterranean 1