Identifying the Changes in Precipitation Regime over Iran during Recent Decades

One of the most important features in analyzing the climatology of any region is to study the precipitation as the most variable climatic element. The main objective of this study is spatial and temporal analysis of inter-annual variability of frequency of month-to-month precipitation over Iran using harmonic analysis. To do so, long monthly precipitation time series data from 33well-scattered synoptic stations are studied with basic harmonic analysis in two statistical periods (1980-1992, 1993- 2005) in order to identify the effects of climate change and climate variability on precipitation. First, relative frequency of precipitation for each month is calculated and finally characteristics of monthly precipitation over the whole country are analyzed in form of six harmonics. The harmonic analysis showed that the first three harmonics are the most important ones for explanation of inter-annualvariability of precipitation over the country. As the first harmonic represents a single annual cycle with the largest amplitude in comparison to the other harmonics, so that this harmonic revealed more than 70 percent of monthly precipitation variability in most of regions except Northwest and Southeast of the country in both studied statistical periods. Also, it is observed that the rate of spatial and temporal changes of first Harmonic in both statistical periods is less than second and third harmonic. This represents higher significant of first harmonic and it’s less variability in explanation of precipitation changes over the country. In contrast, comparing the results of two statistical periods indicate spatial and temporal displacement in second and third harmonics caused by the effects of climate change and climate variability which represents more variability in these two harmonics in comparison with first harmonic.