The study of synoptic patterns that caused autumn and winter droughts in Khorasan Razavi Province

Drought is a weather condition which accompanies with lack of precipitation over a certain time periods that human activities in the area located under its impact will experience negative effects. In this research daily rainfall data of nine synoptic stations in Khorasan Razavi province in the static period of 1989 to 2008 were used. Standardized Precipitation Index for one-month and three-month scales was calculated and then drought and wet periods were determinated and counted. Monthly average of isobars, isotemps, iso- RH, and iso-PWC (6 months of autumn and winter) were considered for 20 years (1989-2008). Results showed that three cities, Sarakhs, Quchan and Gonabad in autumn and cities of Torbat heydariye, Sarakhs, Quchan, Kashmar and Gonabad in winter have experienced the most drought. Lowest dry autumn and winter periods, were related to the Kashmar and Nishabur stations respectively. Also consideration of every year drought situation of study period shows that in 2001, all stations have experienced different intensity (acute, severe and moderate) of this phenomenon, so this year was considered as the base year of droughts in the period under review and its synoptic maps were presented. The study of synoptic patterns in base year showed that despite of good conditions for precipitation (low temperature, high relative humidity and precipitable water content of cloud) in all months of autumn and winter, due to interaction of Siberian high pressure system with Azores high pressure system and strengthening of Siberian high pressure and its complete domination over the region, not only rainfall has not happened, but also local and orographic precipitation faced with constraints and therefore moderate, severe and acute droughts, occurred entire region of our study.