Synoptic analysis of 500 hpa flow patterns in rainy Spring’s Arasbaran region

Arasbaran is one of most important regions in terms of agriculture, economics and tourism in northwest of Iran that it usually receives the most yearly spring precipitation. To recognize the synoptic patterns of the 500 hpa, geopotential  height data were driven for coordinates 00-70 ˚ E and 15- 65˚N in rainy springs (1972-1976-1979-1981-1986).To select the most important component using principal components  analysis, a matrix S mode with dimensions 386×610 was used. The results showed that more than 92% of the total variance can be explained by 13 components. After clustering procedure on the 13 components, six circulation patterns were obtained. In 5 cases of the extracted patterns, there's a really high center of east Mediterranean and south east Europe, the most important component was detected during the wet spring. Other major factors will be referred to the short-wave atmospheric middle level that at this time of year is usually active between the Caspian Sea and the Black Sea and cause wet periods in spring in this area.The systems accompanying low active centers of located in Central Asia and their short waves bring instability and precipitation in the spring to this area.