Determination of extreme precipitation threshold and analysis of its effective patterns (case study: west of Iran)

Flash floods caused by extreme rainfalls are one of the most significant natural hazards. In the present study, the precipitation data of 69 meteorological and climatological stations with temporal intervals (1961-2010) were obtained to determine the threshold of extreme precipitation as well as analyzing its significant patterns in the western regions of Iran. To determine the threshold of extreme precipitation, the theory of extreme value method was applied. In this method, precipitation of 22 mm and more than that covers 30% of the area had been identified and extracted as extreme precipitation. Therefore, 119 extreme precipitation events during the study period had been identified. Then, four patterns were analyzed using cluster analysis. After that, network data of geopotential height levels of 200, 300, 400 and 500 hPa for these days, from re-analyzed data series of NCEP/NCAR in the range of 10 degrees-80 degrees E and 0 degrees-70 degrees N and in 13,460 cells 2.5 degrees x 2.5 degrees were extracted by GrADS software. The results of the study showed that the most important humidity source for precipitation was the Mediterranean Sea, the Black Sea and the Red Sea, respectively. The upward vertical speed at different levels, located on the east and southeast cyclones of upper levels, which matches low pressure of the Earth's surface, indicating unstable conditions in the region. Also, placing cutoff lows due to westerlies activities with warm and humid air advection at the surface and upper-level cold air were the main causes of severe atmospheric instability in the west of Iran.