Synoptic Analysis of Hail Hazard in Bojnoord City, Iran

Hail is one of the most important atmospheric phenomenon, which occurred during Spring and Fall seasons due to atmospheric instability caused by cumulonimbus clouds and severe thunder storms. The most damages in agricultural sector is due to hail, which caused lost in horticultural and agronomy productions each year. In order to investigate synoptic mechanism on hail occurrence conditions, a cluster analysis was performed according to Bojnoord weather station data. To perform classification and subsequent calculations, the sea level pressure (SLP) and the geopotential height of 500 hPa for 26 days were extracted using programming in the Grad's software. Regarding the mechanism and performance of atmospheric systems in creating hail phenomena in the area, four patterns were identified in the formation of the hail, including: 1) low pressure in the Caspian Sea; 2) low pressure in Pakistan and high pressure in Azores; 3) high pressure Caspian Sea and low pressure in Saudi Arabia; and 4) high pressure in north of the Caspian Sea and Low Pressure of Pakistan. The results showed that the months of April and may had the most hail cases during the studied period. Overall, formation of low and high pressure systems in north and south of the region and enforcement of meridional flows cause a moisture advection from the Gulf of Oman and Caspian Sea in geopotential heights of 850 and 700 hPa. In addition, falling cold air from upper latitudes and lifting of warm air cause a gradient temperature on the area and hence occurrence of hail phenomenon. The region experience more hail phenomenon when low pressure in Pakistan and high pressure in Azores were dominant on the area.