تعیین الگوی همدیدی بارش‌های شدید و حدّی پاییزه در سواحل جنوبی دریای خزر

In order to determine the synoptic pattern of heavy and extreme precipitations on the southern coast of the Caspian Sea, precipitation characteristics of the Caspian region were investigated by the use of eight synoptic stations data. Considering the percentage thresholds, the days with heavy and extreme precipitations were determined for a 10 year period (1994-2003). Since more than 75% of days with heavy precipitation were concentrated in autumn, the threshold of 10% of autumn precipitation values (52.8mm) were considered and 28 days with heavy precipitation were determined in the 10 year period. The 6-hour and daily mean NCEP/NCAR 2.5×2.5 reanalysis data of pressure, vertical velocity, specific humidity and zonal and meridional wind components at different levels and outgoing longwave radiation(OLR) from two days before of precipitation days were employed.
The results revealed that all the heavy and extreme precipitations of the southern coast of the Caspian Sea can be classified into three main patterns including High pressure, Low pressure and Coupling. 16 days (57.1%) out of 28 days with heavy precipitation were the result of the existence of high pressure pattern and negative vorticity related to the influence of a high pressure tongue on the southern coast of the Caspian Sea. In contrast, there were only 6 days (21.4%) with low pressure pattern. 6 days with heavy (21.4%) precipitations belonged to the coupling pattern which included a high pressure center over the west-northwest of the Caspian Sea and a low pressure center over the east of it. The southern half of the Caspian Sea has mainly negative vorticity due to the influence of anticyclonic circulation in this pattern, However, the precipitation was the result of convergence of cyclonic and anticyclonic flows over the Caspian Sea.
The results indicated that the main mechanism of precipitations was the local convection occurrence at the southern extremity of the Caspian Sea in both high pressure and coupling patterns, whereas, the existence of a Cyclone was the main mechanism of precipitation in Low pressure pattern. However, the maximum ascend (highest negative omega) in both High Pressure and Coupling patterns occurs in the lower troposphere, while in Low Pressure pattern it occurs in a thick layer in mid-troposphere. The existence of a high pressure center over the west of the Caspian Sea and a negative vorticity over the sea at lower levels, associated with the strong and prevailing north-south flows are the main factors causing heavy and extreme autumn precipitation in all synoptic patterns.