Some features of ground rain patterns measured by radar in north Italy

Abstract

The radar records of rain observed during 1980 by the weather radar facility of Spino d'Adda in north Italy were used to generate a large data base composed of about 16,000 ground rain cells characterized by a reflectivity factor greater than 34 dBZ. This large sample of rain elements, collected at the low altitude of 1.5 km from the ground, was analyzed in order to supply the statistical information required for rain attenuation modeling. The point rainfall rate distribution calculated against this data base, as well as the cumulative distributions of maximum, mean, and root‐mean‐square of rain rate referred to an individual rain area (cell), were found to be power law functions of the rain rate. The study of the horizontal extent of cells, based on the entire cell collection, leads to the conclusion that the probability distribution of the linear extent D of a rain cell with a given rate intensity R exceeded over the cell is an exponential function of D, while a power law relates the probability distribution of cells with a given size D to the rain rate exceeded in the cell. The empirically derived formula for calculating the absolute probability of a rain cell with a given intensity and size is proposed and found adequate at least for the observed region and period.