Circulation type classifications for temperature and precipitation stratification in Italy

During the last few decades, weather circulation type classi?cations (CTCs) have been widely used to gaininsight into processes at the synoptic scale, but also for studying the relationship between atmospheric circulation and surfaceclimate variability. This study aims to evaluate the best performing CTCs based on COST733 software for the strati?cation ofdaily ground-level precipitation and surface air temperature across Italy by means of four statistical metrics. Six classi?cationmethods belonging to the four COST733 groups (threshold-based, PCA-based, leader algorithms and optimization algorithms)were investigated on 32 and 26 data time series derived from Italian weather stations for daily mean temperature and daily meanprecipitation, respectively. CTCs were computed using gridded mean sea level pressure and geopotential height at 500 hPaderived from the NCEP Reanalysis 2 dataset between 1979 and 2015 and tested on three different numbers of classes (8/9, 11/12and 18 circulation types). Evaluation metrics showed an evident seasonal variability and high-spatial heterogeneity re?ectingthe geographical complexity of the Italian territory. The study points out that the best classi?cation, both for temperature andprecipitation, is strongly dependent on the classi?cation variable (mean sea level pressure and geopotential height at 500 hPa)showing relevant differences between surface temperature and precipitation. A low number of circulation types (8/9) resultedas the most appropriate grouping for the Italian domain and the Principal Component Transversal and Simulated Annealingwere the best performing classi?cation procedures for ground-level precipitation and temperature strati?cation, respectively.