نوع مقاله : مقاله تحقیقی (پژوهشی)
1 دانشجوی دکتری، مؤسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
2 استاد، مؤسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
3 استادیار مؤسسه ژئوفیزیک دانشگاه تهران
عنوان مقاله [English]
Precipitation is a favorable phenomenon for human due to the water resources for agriculture, drinking water and health purposes. Because of the importance of precipitation, understanding the mechanism of this phenomenon in synoptic, dynamics and climatology has always been of interest for researchers. Although heavy rainfall is useful for water supply, it has destructive effects, such as floods. Heavy rains show the mechanism of precipitation more clearly because the pressure generating systems are more clearly detectable.
In this study, mean sea level pressure pattern of severe precipitation in southwestern Iran over a period of 30 years was identified and classified. Moreover, the area and time of research were defined, and using the Climate Prediction Center (CPC) precipitation data, the 30-years extreme precipitation with 99th percentiles was determined. There were 64 cases of extreme precipitation from 1989 to 2018 with 99th percentiles.
Classification of extreme precipitation systems based on mean sea level pressure field was performed with a spatial resolution of 2.5° data from the NCEP-NCAR center. Extreme precipitation in southwestern Iran at warm time of year was classified using factor analysis technique. In this study, it was shown that the pressure systems that cause extreme precipitation in southwestern Iran during the warm season are classified into five main patterns:
(A) The first dominant pattern is created by the Red Sea trough and the low-pressure center of Saudi Arabia. It should be noted that in this pattern, the role of the Red Sea trough is more prominent than the low-pressure Saudi Arabia. In some cases, Saudi Arabia has formed along the Red Sea trough, reflecting the influence of the Red Sea on creation and development of the Saudi Arabian low-pressure center.
(B) The second dominant pattern is formed by the-low pressure of Saudi Arabia and the low-pressure center in eastern Iran. The low-pressure center in the eastern part of Iran has an important role in the precipitation of Iran. It is present in all patterns, but first one. The presence of this low-pressure center has created atmospheric fronts in Iran. The rainfall region of this pattern shows that the cold front activity of this low-pressure center causes precipitation.
(C) The third pattern is similar to the second one, but a high-pressure center located in the north and northwest of Iran. The presence of this high-pressure center indicates the cold air advection behind the low-pressure cold front of eastern Iran.
(D) The low-pressure center of the eastern part of Iran and the high-pressure center in the north and northwest of Iran exist in the fourth pattern. The difference between this pattern and the third one is elimination of Saudi low-pressure center in this pattern. Therefore, the dynamic effects are similar to the third pattern and the extreme precipitation caused by this pattern is due to the cold front of the low-pressure system of the eastern part of Iran.
(E) The fifth pattern, like the third one, consists of low-pressure systems in eastern Iran and Saudi Arabia. The difference between these patterns is presence of low-pressure of Cyprus in the fifth pattern. The low-pressure of Cyprus is cause of classical precipitation pattern (formation of the occluded front) in this pattern. The occluded front in this pattern has expanded from low-pressure eastern of Iran to low-pressure in Cyprus and converted into a secondary cold front over several stages of the frontogenesis process.
In this research, the southwestern precipitation systems of Iran during the warm season are classified into five patterns. The most important factor of precipitation in the southwestern part of Iran is the Red Sea trough with 48% frequency of occurrence. Therefore, it can be concluded that almost half of the extreme precipitation of southwestern Iran during the warm season is related to this area that extends from the Red Sea trough to the west of Iran.