عنوان مقاله [English]
In order to study the Synoptic structure of the atmospheric circulation patterns of effects on the semi-easterly snowy days of Iran an environmental-cyclic approach from synoptic climatology was used. For this purpose, the first snow days based on current weather codes were obtained (WW) from the Meteorological Organization of Sistan and Baluchestan and South Khorasan province. Based on these days, variables such as sea level pressure, temperature, geopotential height, orientation and zonal components of the wind, thickness, streamlines in various levels were taken from the website of National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP / NCAR) with a resolution of 2.5 by 2.5 degree and the synoptic maps were prepared. The results of this study showed that snowfall in this region is a low-frequency phenomenon
because of the geographical location and the type of dry and dry climate and in the formation of rainfalls in the form of solid or liquid (snow or rain) elevation has a greater effect than latitude. Meanwhile, snowfall can be affected by the influence of synoptic systems such as Siberian high and its integration with the high pressure of Europe to advection the cold air and simultaneous transmission of wet and warm air will be obtained by the low-pressure systems on the Arabian Sea, the Gulf of Bengal and the Gulf. It is necessary to state that the formation of Siberian high pressure in the lower latitudes is much more effective for snowfall. In this situation, the central pressure of this system is about 1040 hPa and line 1030 hPa affects East of Iran. In addition, the study showed that when the major part of the country (especially on the Zagros Mountains) is under the influence of local high-pressure, with an anticyclonic circulation, it causes to transmission of a wet stream of Caspian Sea to the region. Other results this study are the role a strong jet stream with a speed of 65 m/s at level 300 hPa in the southeast and south Iran and the output axis of the jet stream is associated with a southwest-northeast axis. Snowfalls occur in thickness of 5300 to 5400 meter and temperature of about 3 degrees Celsius in the area.