Hail in Northeast Italy: Climatology and Bivariate Analysis with the Sounding-Derived Indices

In this work the hailpad data collected during the April September period of the years 1992-2009 by a network of approximately 360 stations on the plain of the Friuli Venezia Giulia region (northeast Italy) were analyzed. The frequency distribution of the hail cases is studied for the different years, for the yearday, and for the time of day, considering four 6-h-long periods of the day. Hail is found to be more frequent during the 1100-2300 UTC time interval and in the months of June and July even though during the 18 years analyzed there does not seem to be a defined trend in hail probability. The spatial distribution shows a maximum of probability along the foothills in particular in the northwest. In the middle of this area there is also the Udine-Campoformido radiosounding station, which performed four soundings per day. Each case (6-h-long period) is then associated with the 52 indices derived from the sounding launched at the beginning of that period. These sounding-derived indices are not all statistically independent, as found by looking at their correlation matrix and aggregating the most-correlated indices in three groups. A diagnostic bivariate analysis between each index and the hit hailpads in 6 h was done as a preliminary attempt to evaluate the utility of these indices for forecasting hail. It is found that some measures of instability (like updraft, hail diameter, and lifted index) seem to have more skill than the other indices when classifying the hail-event occurrence. When estimating the number of hit hailpads, the best correlation is obtained by the indices belonging to the "lifted index family."