Statistical – Synoptic Analysis of the First Effective Precipitation in the Western and Northwestern of Iran

One of the most important issues related to rainfed wheat cultivation is the date of the First Effective Precipitation (FEP) which can provide soil moisture for cultivation. The current study aims to evaluate the variability and probable trend in the date of the first effective precipitation and the synoptic conditions that lead to its occurrence in northwestern and western Iran. Therefore, precipitation data were extracted from Iran’s Meteorological Organization. First precipitation with a value of at least 5 mm and duration of one to two consecutive days, associated with another precipitation occurring less than 10 days apart, was selected as the first effective precipitation. Then, the trend of first effective precipitation was investigated using Man-Kendall test and linear regression method. Finally, geopotential height, wind speed and humidity data (ERA-Interim) at 850 and 500 hPa levels were obtained from ECMWF website to identify synoptic patterns of the first effective precipitation by cluster analysis method. The findings reveal that more than 90% of the first effective precipitation has occurred during October. Accordingly, Ardebil, Sanandaj, Khorramabad, Kermanshah, Zanjan, Orumieh, Tabriz and Hamadan stations had earliest climatic average of the first effective precipitation date during the period of 1987 to 2016, respectively. Despite the high annual variations regarding the occurrence date of first effective precipitation, evaluating the trend of the first effective precipitation by Mann-Kendall test at the significant level of 5% show that there is no significant trend in all studied stations. Furthermore, synoptic patterns including the short trough, omega block, cutoff low and the Mediterranean long trough are identified as responsible patterns on the occurrence date of the first effective precipitation. Among the patterns, omega block has been associated with widespread rainfall at the studied stations due to greater access to moisture and unstable conditions caused by the cold weather.