Abstract

Abstract Extreme precipitation in every given point is defined as rare and abnormal precipitations in which could happened far from the center of frequency distribution. The high extreme and upper tail of distribution of precipitation has been taken a lot of experts’ attentions. The thresholds values for extreme precipitations that have been chosen are varied by geographical locations. One of the most usage indices is based on deciles of any given distributions. The current paper has analyzed tempo-spatial characters of extreme precipitation of northwest of Iran based on daily observations database created from of 729 synoptic, climatology and rain gauges stations by using90th percentiles of precipitation distribution . The period under investigation is 41 years which started in 1967to 2007(14975 days). The 90th percentiles of precipitation probability distribution have been estimated for every single pixel as extreme threshold. The numbers of pixels have experienced the extreme threshold or more, had been calculated. Many kinds of coverage of this character have been recognized. Based on analyzing temporal and trends characters it has cleared that there is a significant decreasing trend in 20-30 percent coverage of extreme precipitation. Meanwhile there are stationary in other extreme coverage except for 60% -70% coverage’s. The coverage of 40%-5% and 50% - 60% in March is the second order in extreme precipitation events. Monthly analyses showed the months with most extreme (two times in every month) are April, May, March and October r. Keywords: Precipitation Threshold, Extreme Precipitation, Normal, Precipitation Center of Gravity. Northwest of Iran.