عنوان مقاله [English]
Extreme precipitation plays a fundamental role in the formation of some hazards that could have severe social-economic consequences. Studying and determining the factors that lead to the occurrence of such extreme events in the city of Tehran as the capital of the country is important. In this study, precipitation in twenty-four hours that were equal or higher than the 95th percentile of precipitation during 1976-2005, considered as an extreme event. The data required for the selected cases was taken from the NCEP/NCAR archive. Among the identified cases, two 8th of November 2005 with 15mm and 1st of February 1994 with 39mm of precipitation in 24 hours have been adapted for a detailed study. To better understand the atmospheric circulation leading to the events, synoptic conditions 48 hours before and 24 hours after the precipitation were examined by means of surface, 500-hPa and 250-hPa weather charts. Consequently, two different circulation patterns were recognized. In the first one, a center of high pressure over the Caspian Sea that extends toward the Black Sea and occasionally tongues toward central part of the country was observed. In the second pattern, there were two high pressures, one over the North West of the Mediterranean Sea and the other over the central Asia. At the same time, a trough from the Red Sea extended toward the east of the Mediterranean Sea and combined with a dynamic low-pressure system that accompanied with a trough in the middle of the troposphere. It has also been realized that the sub tropic jet stream axis passes over the Tehran and a meridional branch of polar jet stream forms over the Mediterranean Sea have a significant role in increasing precipitation. The study also revealed that if the Red Sea trough reaches higher latitudes, the precipitation over Tehran would be heavier.