Statistical - Synoptic analysis on the climatic variability of the rainfall regime in the mid-west of Iran with emphasis on the occurrence of severe droughts

Weather and climate hazards such as drought can seriously threaten natural ecosystems and human societies. The phenomenon of drought, due to its creep character and its non-sensible nature, affects the human environment more than any other hazards. In this study, daily rainfall data from a 26-year period was obtained for 16 stations in the mid-west of Iran from the website the Meteorological Organization of Iran. Then the drought periodic occurrences were calculated using the Standard Precipitation Index (SPI) in the periods of 1 month (monthly), 3 months (seasonal), 12 months (annual) and 24 months (long periods). To identify the daily and periodic atmospheric patterns of droughts, the dates that were as dry days were selected first. Using these dates, an array with 547 × 693 dimensions was prepared using geopotential heights data of 500 Hpa in MATLAB software. Maximum occurrence of drought occurred in the 12-month time scale and minimum frequency was in 1-month time scale of SPI. The reason for this is the most sensitivity of short-term SPI to humidity changes and the high contribution of monthly rainfall in changes of SPI. Most occurred droughts in the study area have mild to moderate nature. By increasing the time scale of the SPI, the number of dry spells and periods decreases, but their duration increases. Surveying the synoptic patterns showed that the patterns of ridge, zonal (in the daily patterns of the east-west stretch and the semi-longitudinal extension) and trough are the main patterns in the occurrence of the dry spells and periods in mid-west of Iran respectively.