climatology of iran precipitation by using harmonic analysis method

abstract one of the most variability of atmospheric parameter is precipitation, which changes seasonally. it is important to understand exact characteristic of rainfall seasonal variable. therefore, it is avoidable to study rainfall season in hamedan province, because hameden is one of the important centers of agriculture in iran. in this research, the seasonal variability of rainfall is considered in hamedan province. for this object, precipitation data spanning 30 years (1979-2009) were analysed to determine the onset and cessation, trend and seasonal variability of rainfall, based on the daily rainfall for there separated decades. the result of this study show the duration of rainfall is from early falls until end of spring in the first decade. this result indicated the duration of rainfall is long. in the second decade, there is no significant change in rainfall season, but only in some part of the province, the rainfall duration is shortfall and in the other part is long falls. nevertheless, in the third decade there is considerable movement, i.e. the onset of rainfall duration has been in winter and cessation of rainfall duration has been in summer. it express that rainfall season has moved to summer slowly. the trend of rainfall duration has increasing trend. on the other hand, the duration length of rainfall is longer than before. in order to classify, rainfall seasonal variability is interpolated and the cluster analysis is done base on the “oghlidos distance” and “ward” integrated method. according to this method, hamedan province is classified to 3 groups. the first group indicates the shortfall in the most parts, the second group indicates rainfall season onset is moved to winter, and in the third part the rainfall season hasn’t had change.