On the relationships between precipitation changes and circulation types variations in north west of Iran

An attempt is made to examine rainfall variability over the some mountainous area of Iran in realation to 1000 hPa atmospheric circulation in winter. This area is in the intersection of the two main mountain range of Iran. Daily precipitation series from 7 stations have been used for the period 1977-2006, along with the classification scheme of daily circulation types at 1000 hPa for the same period. A daily catalogue of 15 circulation types was constructed for the study perid. Five anticyclonic types, three cyclonic and seven types which almost are combination of them are defined. The seasonal frequency and the trends of circulation types have been calculated. It was found that there is a general weak positive trend of cyclonic and anticyclonic circulation types but a negative one for the combinations. The trends of wet days and precipitation total amounts have also been calculated and analysed. A decreasing tendency of winter precipitation is observed for all stations. Concerning the frequency and intensity of rainfall per circulation type, a decreasing tendency is evident for the stations (except 4 circulations which have increased).