Synoptic-dynamic analysis of extreme precipitation in Karoun River Basin, Iran

In the present study, synoptic-dynamic aspects of extreme precipitation in Karoun River Basin were analyzed using two types of data, namely (1) grid views of Iran’s daily precipitation as registered in 1434 stations and (2) atmospheric data including sea-level pressure (SLP), geopotential height (HGT) for 1000, 850, and 500 hPa, temperature, and U&V wind components for a 54-year statistical period (1960–2013). In order to identify extreme precipitation, three criteria were used: the precipitation events should exceed 95th percentile threshold, have a minimum of 50% coverage with spatial continuity, and last for at least two consecutive days. The results showed that extreme precipitation of study area are affected by atmospheric patterns of the Caspian Sea low pressure-European migratory high pressure, Eastern Mediterranean low pressure-Central Iran low pressure, the Eastern Mediterranean low pressure-Siberian-Tibetan high pressure, and Sudanic low pressure-gigantic European high pressure. In all these patterns, the cyclonic motion is observed at all of the atmosphere levels, which indicates the effect of the atmosphere dynamic mechanisms at the time of occurrence of extreme precipitation. At 300 hPa level, the left side of the jet stream, the left exit of the subtropical jet stream, and the right entrance of the polar front jet stream were located over the study area.