ARPA-FVG OSMER (Regional Meteorological Observatory) - Italy
Friuli Venezia Giulia (hereafter FVG) is a small region in the Northeast of Italy, bordered to the North by the Carnic and Julian Alps and to the South by the Adriatic Sea, with the lagoons of Grado and Marano. Due to its orography and geographical position, this region hosts several kinds of severe weather phenomena (heavy rain, tornadoes, severe thunderstorms, hail, etc.) (Morgan 1990; Bechini et al. 2001; Giaiotti and Stel 2001) not dissimilar from those of the Po Valley (Morgan 1973; Prodi 1976).
From 1974 to 1981 a first attempt was made to monitor the occurrence of hail in the plain of FVG (below 500 m above m.s.l over an area of nearly 4500 km2), using a network of volunteers who sent mail reports with the date and position of hailfalls. This network did not start and finish every year at the same month, for this reason the data collected can supply climatological information just for the summer months (i. e. June, July, August). In 1988 a second attempt, that still endures, started with the aid of polystyrene hailpads. This network has an almost regular grid with a mesh size of 4×4 km, it is managed by volunteers and it is active from April to September.
In this work these data are used to give a description of the average behavior of hail in the plain of FVG and to look for possible quantitative explanations. In particular, the monthly distribution of hail days and the 3-hourly distribution of the pads hit by hail are here computed. The spatial frequency of hail days (Stel and Cicogna, 1998) is here shown as well and it is interesting to notice that, even in such a relatively small area, it is not homogeneous but at least three maxima clearly stand out. The possible causes of these maxima, mainly related to the interplay between fluxes and orography, are here studied and presented. Moreover, in the areas of the three maxima, the hailstone size distributions are computed and compared, looking for possible differences.
The yearly frequency of hail days in FVG is then compared with those of other Italian Regions (Trentino Alto Adige, Emilia-Romagna) and European Countries (Greece, Austria, Bulgaria) and it is studied in relation with the main features of the general circulation of the Northern Hemisphere.
Bechini R., Giaiotti D., Manzato A., Stel F. and Micheletti S., 2001:
The June 4th 1999 Severe Weather Episode in San Quirino, Italy: a Tornado Event?. Atmos.
Res. Vol. 56/1-4 pp. 213-232.
Giaiotti D. and Stel F., 2001:
A comparison between subjective and objective thunderstorm forecasts. Atmos. Res.
Vol. 56/1-4 pp. 111-126.
Morgan G. M. Jr., 1973:
A General Description of the Hail Problem in the Po Valley of Northern Italy. J. of App.
Met. Vol. 12 pp. 338-353.
Morgan G. M. Jr., 1990:
La Grandine. ERSA pub. Gorizia -Italy- 145 pp.
Prodi F., 1976:
Hailstorms in the Po Valley of Northern Italy (1972-1973). Rivista Italiana di
Geofisica
Vol. III 1/2 pp. 21-31.
Stel F., Cicogna A., 1998:
The frequency of hail days in the plain of Friuli-Venezia Giulia: random or
organized phenomenon?. Proceedings of ECAC1998
The data concerning yearly frequency of hail days in areas different from FVG are kindly furnished by E. Eccel (Trentino Alto Adige, Italy), S. Nanni (Emilia-Romagna, Italy), O. Svabik (Austria), P. Simeonov (Bulgaria), M. Sioutas (Greece).