Comparative analysis of the mechanism of autumn and winter rainfalls in the southwest of Iran

Autumn and winter rains play an important role in Iran's agricultural and livestock economy, as well as feeding underground water tables, especially in the western and southwestern regions of Iran. The continuous reduction of underground water reserves in Iran increases the importance of atmospheric precipitation especially during autumn and winter seasons. This importance is affected by the centrality of agriculture in Iran, as well as the supply of drinking water in small and large cities.
This sensitivity is more in the southern half of Iran and especially in the southwest, which has rivers full of water and it, is a region full of large dams.

In this research, the mechanism of rainfall in the two seasons of autumn and winter in southwestern Iran was investigated and compared. Then, the synoptic conditions of the longest and most intense rainfalls that occurred during the selected data period in the research area were identified. In this research, a rainy day was defined as a day when the weather station received at least half a millimeter of rain. Also, the duration of rainfall at each station was defined as the number of consecutive days of rainfall at that station. In first step, daily rainfall data were collected from 8 weather stations in the 30-year period (1987-2016) in southwest Iran. During the second step, separation of rainfalls according their duration using programming in MATLAB environment; it made it possible to select 40 precipitation samples. During the last step, synoptic patterns were designed from daily weather maps, including moisture flux, atmospheric height, and the position of jet-stream cores.

In this research, the analysis of patterns showed the following results 1) Sources of continuous and widespread rains in the autumn season, warm waters around Iran include the Red Sea, Gulf of Aden, Arabian Sea, Persian Gulf and Oman Sea. 2) The origin of the wet currents of these rains is from the central regions of tropical Africa. 3) During the winter season, these resources are generally limited to the Red Sea and the Gulf of Aden. 4)  Also, the analysis of the position patterns of negative omega cores at the level of 500 hPa indicates the conditions of inflammation and severe thermal gradient in the atmospheric column over the southwestern lands of Iran during the autumn season compared to the winter.

Based on the designed patterns, the western troughs in the eastern Mediterranean deepen during the winter season compared to other seasons and they provide the following conditions for rainfall in southwest Iran: 1) The path of precipitation systems also reaches the southwest of Iran. 2) Stronger cores are formed from jet-stream in the southwest of Iran. 3) More water areas such as the Mediterranean Sea, the Red Sea, the Gulf of Aden and the Arabian Sea contribute to the provision of rainfall moisture.