| چکیده انگلیسی مقاله |
 Recognizing the Synoptic Patterns of Wintertime Heavy Precipitation in the Southern Coast of the Caspian Sea    Gh. R. Janbaz Ghobadi. A. Mofidi. A. Zarrin  Received: 31 August 2009 / Accepted: 14 December 2010, 5-8 P    Gh. R. Janbaz Ghobadi,  Assistant Professor of Climatology, Islamic Azad University âNour Branch, Nour, Iran    A. Mofidi ( * ) , Assistant Professor of Climatology, Ferdowsi University of Mashhad, Mashhad, Iran.  e-mail: abbasmofidi@um.ac.ir   A. Zarrin , Center for Climatic Research, University of Wisconsin-Madison, WI 53706, USA.   Extended abstract  1- Intro duction  In southern coast of Caspian Sea the precipitation, as the most important climatic phenomenon, shows a complex temporal and spatial distribution. Such a complexity is due to the settlement of this area between the largest lake of the world in the north and a high mountain range in its south. The climate complexity is resulted when we add the successive eastward movement of westerly waves over the area throughout the year. Therefore, a unique climatic region is appeared in the southern coast of Caspian Sea due to Land-Sea-Atmosphere interactions in a variety of scales. In fact, this area is one of the unique locations in the world which migrating anticyclones can cause heavy precipitation with amount more than 200 -250 mm per day due to the shape, size and the location of the Caspian Sea (Mofidi et al. 2008).   In such an area it is not surprising to see a high frequency of heavy precipitation events during the cold period. In some meteorological stations the half of the annual precipitation amounts is related to the occurrence of heavy precipitation events. Although, many domestic researchers investigated the mechanism of heavy precipitation over the area, but the mechanism for occurrence of winter precipitation is less known and needs to be investigated in detail. Therefore, we plan to clarify how the heavy precipitation events occur during the winter season. In this way, we would like to answer to the following question: What are the most important synoptic patterns of winter heavy precipitation in southern coast of the Caspian Sea?   2- Methodology  In order to recognize the synoptic patterns of winter time heavy precipitation in southern coast of Caspian Sea, the precipitation data of eight synoptic stations in the southern coast of Caspian Sea is used for a 10-year (1994-2003) period. The 6 hourly NCEP/NCAR gridded reanalysis dataset of sea level pressure, geopotential height, vertical velocity, specific humidity, and zonal (U) and meridional (V) wind components at different levels were employed to determine the synoptic patterns of precipitation events. The data was used for 21 winter precipitation events and the maps were analyzed from 2 days before the occurrence of each precipitation event until the end of the precipitation period. Have been applied a synoptic approach called âEnvironment to circulationâ to investigate the events (Yarnal 1993). Also were used a percentage criterion to extract the heavy precipitation events through the abovementioned period.   3- Discussion and Conclusion  The results revealed that all the winter heavy precipitation of southern coasts of Caspian Sea can be classified into three main patterns including High pressure (HP), Low pressure (Low) and Coupling (CP) patterns. In all patterns, the anticyclonic circulation associated with the northerly winds in lower troposphere is dominated over the Caspian Sea. The findings showed that in High pressure pattern, there is a see-saw relation between the northern and southern Caspian Sea in the all precipitation days, due to the strong subsidence over northern Caspian Sea and a strong but shallow ascending air over the southern Caspian Sea. In High pressure pattern and in its peak time a continuous and extended north-north easterly flow stagnates over the Caspian Sea in lower troposphere.  The Coupling Pattern appears after passing a westerly trough over the area and in the beginning of penetration of a mid-troposphere ridge between the Caspian Sea and the Black Sea. In lower troposphere due to the settlement of a high pressure in the west and a low pressure in the east, the Caspian Sea area experiences a strong zonal pressure gradient over the sea. The results also indicate that unlike the High pressure pattern, in Coupling Pattern the anticyclonic circulation in southern Caspian Sea in lower troposphere reaches to its maximum value even though the negative vorticity just limited to lower troposphere at levels below 700 hPa. In contrast, the High pressure pattern only shows an increase in anticyclonic circulation in northern part of the Sea all the way from bottom to the top of the troposphere.  The strongest winter winds are occurred during the Coupling pattern and the Northerly winds of this pattern are stronger than the Northerly winds of High pressure pattern. Locating in the convergence region of the front of a cyclone, the Caspian Sea region experiences the maximum ascending of air in Low pressure pattern in an area from the southern coast of Caspian Sea to the Alborz mountain range in mid-troposphere. Also, in Low pressure pattern, unlike the High pressure and Coupling patterns, because of the convergence of the southern and northern moisture flows in the southern Caspian Sea coast, Caspian Sea is not the only source of precipitationâs moisture. The other far southern seas are also contributed in the moisture of heavy precipitation over the Caspian Sea coast.  The investigation of wind structure indicates that in all synoptic patterns a permanent northerly wind occurs in lower troposphere over the Caspian Sea region. Additionally, in all patterns the intensity of northerly wind has been increased to the eastward. However, the permanent wind direction in coupling mode has the most favorable conditions for transporting the moisture to the southern coast of Caspian Sea.  Key Words: Caspian Sea precipitation, Synoptic patterns, Extra-tropical anticyclone, Negative vorticity, Northerly wind.   References  Afshar-Moghaddam, Y. (1994). The Divergence and Vorticity Analysis over the Caspian Sea, MS. Thesis, Oceanography Department, Islamic Azad University-North Tehran Branch. [In Persian]  Alijani, B. (1993). 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