عنوان مقاله [English]
Hails have an important role in creating sudden floods and makes great damages the agriculture art of Iran and Fars province. In order to mitigate the resulting damages, it is necessary to understand the synoptic conditions of such destructive phenomenon. Daily heavy hail data of 14 synoptic stations in this province were used during a four-year statistical period (2006-2009) to study dynamic and synoptic conditions of hail flow in Fars province. Then three samples of heavy hails in this region were identified and selected during the statistical period. Then using NCEP/NCAR centers data, we plotted sea-level synoptic maps, 500 and 850 hecto-pascal levels, moisture, omega and rotation in GrADS software at each selected period and also plotted hail distribution in a peak day during three periods using GIS software. The data of Shiraz upper atmospheric data were used for measuring the instability indices of Shulter (Si) and Viting (Ki,) and eventually, by using the said maps and data, the synoptic conditions and hail patterns at each selected period were determined..
The results show that during each three selected period, falling hail in this province in March is due to the formation and strength of cutoff low-pressure systems (cold pool) at 500 hecto pascal levels and creation of low-pressure and instable systems at sea level. The study of atmospheric patterns showed that this phenomenon is due to strengthening and expanding low-pressure systems located at eastern red sea and Saudi Arabia. These low-pressure systems obtain moisture through passing over southern waters of our country and lead to instability and hail in south west, central and the study areas. In addition, falling cold air through middle atmospheric layers by cold pit phenomenon, provides conditions to expand such southern low-pressure systems and formation of instable flows and its movement towards Iran and this region. Therefore, cold weather flow from middle levels along with great temperature variations in lower and middle atmospheric layers can cause instability and great ascent of air, formation of cumulonimbus and heavy hail in selected periods in Fars province