A Climatic Investigation of the Relationship between Synoptic Factors and Hail Occurrence in Northern Greece during the Domination of 500 hPa Lows

Abstract

An objective detection and analysis of 500-hPa cyclones (lows) is performed during the warm period (15 April-15 October) of the year for the central and east Mediterranean region and especially for northern Greece. The NCEP/NCAR reanalysis gridded data of geopotential height and temperature are employed in the detection of lows and in the calculation of the various dimension, shape and instability parameters. The parame-ters are used in the identification of hail days during a period of 13 operational hail seasons in northern Greece where the National Hail Suppression Program is conducted. The estimated conditional probability (8%) for hail occurrence under low domination indicates that hailfalls are rather rare and lacking severity. When however, the PVA advection centers, which usually accompany the lows are considered, hailfalls are more frequent (20%) and severe. This is attributed to the increased low-level instability at the PVA centers in the absence of extended cloud covers. Hail-related lows are larger and bear a greater resemblance to circles than the ordinary lows. They are usually moderate or negligibly elongated in the northeast to southwest direction and originate at the east coast of Adriatic Sea.