摘要：The aim of this research was to create an appropriate classification of atmospheric patterns related to hail fall events and their climatology features in Zab catchment. In this research, in order to achieve the above mentioned aims, station hail fall data of three synoptic stations of Zab catchment (codes related to hail fall events which was 90-99 out of 100 codes related to the current weather) in a 22 year statistical period (1993-2014) were used. Also, data of NCEP/NCAR station were used to analyze and identify synoptic circulation patterns coincided with these events. By cluster analysis on the data of sea-level pressure and geo-potential height (500 hPa), three circulation patterns including weak, moderate and intense cases were identified related to the hail fall. Finally, the day mostly correlated to the other days was chosen as the representative of the circulation pattern of that cluster and became the basis of the research analysis by performing intragroup correlation between each cluster. Results showed that hail fall in west of Iran occurs under three main conditions, including: 1. Strengthening and expanding low pressure system in Sudan, 2. Strengthening and expanding the European and Siberian high pressures, 3. Region location in east Mediterranean trough. So that, on hail fall events, moisture advection through low pressure tongue of Sudan along with cold air loss through the European and Siberian anticyclone high pressure tongue led to the formation of instable conditions on the west front of Iran and the location of west Iran in front of Mediterranean trough provided the condition for the formation of strong upward flows and exacerbated instability and hail fall
