Synoptic analysis of the onset of the earliest widespread winter precipitation in Iran(except the Caspian Sea coastal region)

Different local and temporal distribution of rainfall across the Earth has been amongst the determining parameters in many civilizations’ fate through history. Rainfall fluctuations and changes in climate phenomena are the monumental causes of these anomalies. Scientists have done a large number of researches to find the causes of the onset of widespread precipitation and its effects on agricultural crops, forecasting the onset of widespread rainfall, the teleconnection on widespread rain and the presentation of proper cultivation calendar for winter cultivation. Cold season brings about a great amount of widespread rainfall in many places in Iran. However, the basic consumption of water is during the warm season not only in civil but also in agricultural sectors. Therefore, being aware of the onset of widespread winter precipitation (OWWP) and its year to year fluctuations is a pressing issue. To examine and analyze the pattern of atmospheric circulation for the earliest OWWP,  a 33-year period of rainfall data from 50 synoptic stations and one rain gauge station within four areas of Iran were analyzed. To find the early onset of the widespread rainfall, the average daily precipitations of all stations through each area were calculated. Furthermore, OWWR has two particular features per year. Firstly, the day having the higher standard deviation than the average rainfall of all the areas’ stations would be the onset day. Secondly, there should be at least two consecutive days having precipitation in more than 50% of all stations in the given area. As a result, there was a specific onset of rainfall for each area during 33 years. Moreover, if the OWWR in one year had started before the rainfall onset of that year, it would have been the year with the early onset. Consequently, the geopotential and vorticity maps of 500 and 1000 hPa showed the presence of a significant mridional component of atmospheric circulation in the earliest years with abundance of blocking highs and cut-off lows especially in the East Mediterranean and Iran. In addition, the anomalous positive relative vorticity in the earliest years lasted until the end of winter but the negative anomaly of relative vorticity showed the lowest amount compared with the other years. Finally, the result of Tukey’s test (HSD) showed a meaningful correlation (at 5% confidence level) between the OWWP and the total amount of rainfall in those above mentioned earliest years. In conclusion, 1974 in the northwest part, 1976 in the west, 1991 in the south and southwest and 1982 in the center and east parts of Iran presented the earliest widespread winter rainfall during those 33 years. In those mentioned years, the polar vortex had the highest positive anomaly and the high pressure of Saudi Arabia had its most optimal conditionon the Arabian Sea which caused a wet year for Iran. Also in the mentioned years, heavy rain and flood have been seen in the semi arid areas of the East Mediterranean, Saudi Arabia and Sienna desert.