A CLIMATOLOGICAL STUDY OF THUNDERSTORM ACTIVITY IN THE PO VALLEY

After a description of the monthly distribution of thunderstorm occurrence in the Po Valley and in the Northern Adriatic region and an identification of the homogeneous areas in terms of the contemporary occurrence of the phenomenon, this study examines the dynamic and thermodynamic characteristics of the atmosphere that are conducive to the development of thunderstorms. The study was carried out using ground-level and radiosonde observed data, as well as objective analyses of mean sea level pressure, geopotential height and vertical velocity at different upper-air levels. The period considered, not always homogeneous for the different types of data, refers to the years 1985-1988. The main result that emerges from this study is that thunderstorms are, in the majority of cases, associated with synoptic-scale dynamical forcing, as for example the passage of fronts (generally cold) in the Po Valley, which are also almost invariably connected with depressions affecting the entire Northern Italy. Only a small number of thunderstorms can be attributed to pure local thermodynamic causes, such as for example moist-static instability due intense heating. An analysis of the thermodynamic indices indicates that a suitable vertical distribution of humidity, temperature and static stability can certainly favour thunderstorm activity, but only when the larger scale dynamical forcing is favourable (from the subsynoptic scale to the mesoscale) will the organized convection develop in a sustained manner.