Identifying the moisture source of precipitation in the southern coasts of the Caspian Sea

Abstract

This research was carried out to identify the moisture source of precipitation in the southern coasts of the Caspian Sea. To achieve this purpose, two categories of data were used. The first category was the environmental data comprising of daily precipitation of 10 meteorological stations of the southern coasts of the Caspian Sea during the period 1995–2017, equal to 8401 days. The second group was atmospheric data which included the variables of zonal (u) and meridional (v) components of wind, geopotential height (z), mean sea-level pressure (msl), and specific humidity (q) extracted from the ECMWF ERA-Interim database. The Z-score was based on and used to categorize different intensities of precipitation; days with precipitation were divided into five intensity groups: light, moderate, heavy, very heavy, and extreme precipitation. To recognize the atmospheric conditions at sea level, the combined SLP and HGT maps were plotted at 500 hPa. In addition, for identifying the moisture source, Vertically Integrated Moisture Flux Convergence (VIMFC) was used. In order to recognize the moisture source of the analysis region, the troposphere was divided into three layers of low (1000–850 hPa), middle (775–700 hPa), and upper layers (600–500 hPa) based on specific humidity. The results of the cluster analysis on the Euclidean distances of sea-level pressure in days with precipitation for each of the precipitation classes showed that the greater the high-pressure intensity on the Black Sea, the greater the flux density of the convergence of moisture due to the anticyclone movement of the system toward the south coasts of the Caspian Sea. This resulted in higher precipitation rates. The results of the analysis also illustrated that the Black Sea high pressure in the first layer played a key role in transferring moisture to the southern coasts of the Caspian Sea. The Siberian high pressure in one pattern (the first pattern) and also in combination with the Black Sea high pressure played a role in transferring moisture to the region analyzed for light precipitation phenomena. At high levels (the third layer), the Saudi Arabian high pressure played a key role in transferring moisture to the study area. The findings demonstrated that the most important source of moisture was the Caspian Sea at low levels, and at upper levels (third layer), the Red Sea, Black Sea, and Mediterranean Sea were the sources of moisture.