Atmospheric patterns associated with hailstorm days in the Ebro Valley, Spain

Hail is the most important high-impact weather phenomenon in the Middle Ebro Valley (MEV). Hailstorms lead to major economic losses and cause a great social impact. The local character of hailstorms is a serious obstacle to their forecasting. The aim of this study is to establish an objective classification of atmospheric patterns related to hail events in the MEV in order to improve the forecasts. A multivariate statistical scheme comprising Principal Component Analysis in T-mode and a Cluster Analysis is used. The database comprises 260 hailstorm days from 2001 to 2008. The reanalysis data from the NCEP has been used, and the initial ensemble of atmospheric fields is formed by the geopotential height and temperature at 500 hPa and 850 hPa, and the relative humidity at 850 hPa. After the CA, the hailstorms days are classified into five clusters. The different atmospheric circulation patterns corresponding to each cluster are analyzed, revealing well defined individual characteristics. The atmospheric patterns corresponding to the main hailstorm days of each cluster have been studied, and their relationship with the spatial distribution of hailstorms in the study area.