Investigation of synoptic aspects of severe floods in Aras river basin by using moisture flux convergence, fronting and divergence indices

Flooding is a phenomenon that most climatic regions experience. Floods often cause problems such as; Soil erosion, sediment accumulation and loss of life and property. Therefore, flood climatology plays an important role in reducing and preventing possible environmental damage. For study the synoptic climatology of floods in Aras river basin, daily data of 9 hydrometric stations and 6 synoptic stations during 1979-2014 have been used. Data on geopotential elevation, sea level pressure, specific humidity, omega index and Zonal (Meridional) wind components were also obtained from the US National Centers for Environmental Prediction/ Atmospheric Research. After reviewing the monthly and seasonal regime of the river and identifying the flood days, severe floods with discharge more than 700 m3/s in Khodaafarin hydrometer station were determined. In order to analyze the synoptic patterns of severe floods in Aras basin from the indicators; Moisture flux convergence function, fronting function and divergence function have been used. The results showed that during 4 severe floods in the region some atmospheric phenomena such as; a deep trough on the Black Sea, the Red Sea and the Persian Gulf and a cut off low on the west of the Caspian Sea at 500 hPa level in the region has been formed. At sea level, the formation of high pressure on the Caspian Sea and low pressure on Iraq and western Iran has caused the pattern of pressure changes in the region to increase and provide conditions for climbing. Applying the moisture flux convergence function showed that the source of moisture for these floods was mainly the Caspian Sea.