Classification of daily abundant rainfall patterns and associated large-scale atmospheric circulation types in Southern Portugal

The aim of this study is to carry Out two Objective classifications concerning the occurrence of episodes of heavy rainfall in Southern Portugal. The first one seeks to define the main spatial distribution patterns associated with abundant rainfall days in this region. Using a 15-year (from October 1983 to April 1998) precipitation data base corresponding to 98 sites in Southern Portugal, a sample is established by extracting the precipitation days in which the threshold of 40 min/24 h was exceeded in three or more stations, yielding a total of 199 days. It is found that the Algarve region is the one where episodes of heavy rainfall are most frequent and which exhibits the strongest torrential character. The highest frequency of days with heavy rain occurs in the fall and in the early part of the winter. Through the combined use of principal component analysis (PCA) and k-means clustering, four different rainfall patterns are identified, and each one exhibits specific characteristics. The second classification is obtained using the same Computational method (PCA coupled with k-means) and seeks to define the fundamental large-scale atmospheric circulation types associated with abundant rainfall in the region, based on sea level pressure (SLP) data. The data was collected from the NCEP/NCAR Reanalysis dataset, taking SLP as the most discriminating variable. Five Circulation types are thus obtained, indicating considerable diversity among the large-scale atmospheric circulation conditions that are conducive to the occurrence of heavy rainstorms in Southern Portugal. The article concludes by discussing the main links between the two classification schemes, in order to ascertain the influence of the large-scale atmospheric Circulation upon the spatial distribution patterns of heavy precipitation episodes. Copyright (C) 2007 Royal Meteorological Society.