The relation between circulation types and regional Alpine climate. Part I: synoptic climatology of Trentino

A comprehensive data set of surface temperature, rainfall, solar radiation and wind measurements was composed to investigate the relation between synoptic circulation types and the climate of Trentino, a mountainous region in the South-Eastern Alps. Synoptic patterns are classified over different levels by means of an existing classification method according to their degree of zonality, meridionality and vorticity. Distinct seasonal anomalies of mean daily temperature, total daily rainfall, daily solar irradiation and mean daily wind intensity are associated with most circulation types. Their magnitude varies not only among weather types, but also within the same type for different isobaric levels and seasons. The largest positive temperature anomalies are observed in correspondence with high-pressure and westerly circulations, whereas southerly circulation in the upper levels and low-pressure systems at lower altitudes produce the largest positive deviations of daily rainfall. Daily solar irradiation and mean daily wind intensity are larger than average with northerly circulations. Particular emphasis is given to extreme meteorological events, which occur with preferential circulation types. In fact, the frequency of occurrence of extremes of any variable for each synoptic pattern rarely coincides with the climatological value. Even though extremes in daily rainfall are observed with almost all synoptic circulations, most of them occur with the south-westerly circulation at 500 hPa, a frequent configuration contributing more than 50% to the total rainfall. Moreover, they occur with a larger variety of weather types at the lower levels and in spring and summer with respect to the upper levels and the colder seasons. The large number of statistical indices presented in this paper reveals that the climate of Trentino is strongly influenced not only by the synoptic circulation, but also by the mesoscale mechanisms resulting from the interaction of the large-scale flow with the local orography.