SOME CONSIDERATION ABOUT THE EXTREME RAINFALL IN A MEDITERRANEAN AREA

In the time series of annual maximum daily rainfall there are outliers that require great attention, their study is often carried out with a regional analysis. About twenty years ago the National Group for Defence from Hydrogeological Disasters for the Italian rivers developed a regional methodology known as VAPI procedure (Rossi, F. and P. Villani, 1994). This procedure is based on: a) the use of TCEV distribution (Rossi et al., 1984); b) a three levels hierarchical regionalization procedure. The extreme rainfall can't be considered independent and identically distributed variables. The VAPI procedure solves this problem by using the TCEV distribution. It assumes that the individual rainfall can be expressed as a mixture of two exponential components: a) the first describes the ordinary events (more frequent and less severe on average); b) the second describes the extraordinary extremes (more severe and less frequent). In recent years, a new procedure for the regionalization of extreme rainfall has been implementing taking into account their different meteorological structures (De Luca et al., 2010). The baroclinic cyclogenesis process is the dominant meteorological phenomenon in the Mediterranean and generally produces ordinary extremes. The extraordinary events are due to two different components: isolated convective cells and tropical like cyclones, defined as hurricane-like cyclones in the Mediterranean environment. The first component affects areas of small extension then it is relevant only for urban basins. The second component consists of hurricane-like cyclones that have a greater extension and are strongly influenced by the interaction between the atmosphere and the sea. The new approach is based on: a) a procedure for “a priori” identification of meteorological structure of the events. With this procedure, it is possible to identify three homogeneous time series in each site. Each series can be studied separately because it can be considered independent of the others; b) the application of Power Extreme Value (PEV) distribution, a probabilistic model of the annual maxima of a marked Poisson process with marks distributed according to a stretched exponential distribution. With reference to the maximum annual daily rainfall time series of Campania we identified the series of events with baroclinic cyclogenesis by removing isolated cells and hurricane-like cyclones events. The results obtained are only preliminary results because the study is in progress for checking it and testing the application to other areas of the Mediterranean regions.