تحليل همديدي موقعيت استقرار پرفشار سيبري و مسيرهاي ورودي آن به كشور ايران در فصل سرد

Analysis of Synoptic Syberian High Pressure Position and Entry Paths into Iran in Cold Season

پرفشار سيبري از توده هاي هواي بزرگ‎مقياس جهان است كه روي پهنه وسيعي از سياره زمين اثر مي‎گذارد و به‎دليل نقش دوگانه آن در سواحل شمالي ايران و ساير قسمت‎هاي كشور اهميت زيادي دارد. در پژوهش حاضر مسير ورود پرفشار سيبري به ايران در فصل سرد با روش سينوپتيكي مطالعه شد. در اين مطالعه، نقشه‎هاي فشار سطح متوسط دريا (slp)، طي دوره آماري 2000 تا 2010 براي شش ماه سرد سال با قدرت تفكيك مكاني 5/2 درجه از پايگاه داده (NCEP/NCAR) دريافت شد. به‎منظور سنجش اعتبار داده‎ها، سال 2000 به‎منزله سال نمونه مورد ارزيابي قرار گرفت و تاييد شد. سپس داده‎ها وارد نرم‎افزار GIS شده و در سه بخش شناسايي هسته، محور گسترش و مسير ورود، تجزيه و تحليل شدند. نتايج پژوهش نشان داد، هسته مركزي سلول پرفشار سيبري در اوايل پاييز روي تبت شكل گرفته و با نزديك‎شدن به فصل زمستان، به محدوده بين درياچه بايكال و بالخاش منتقل مي‎شود. زبانه پرفشار سيبري در ابتداي پاييز از سمت شرق وارد ايران‎شده و تا دامنه‎هاي شرقي البرز گسترش مي‎يابد، ولي با شروع فصل زمستان و انتقال هسته‎هاي مركزي به عرض‎هاي بالاتر، پشته فشاري اين سامانه از شمال شرق وارد ايران شده و گاهي تا درياي عمان گسترش مي‎يابد؛ روند كلي گسترش پرفشار سيبري نيز شرقي ـ غربي بوده و در زمستان گسترش هسته‎ها به عرض‎هاي بالاتر بيشتر است؛ به‎گونه‎اي كه تا 40 درجه طول جغرافيايي را دربرمي‎گيرد؛ در حاليكه در فصل پاييز هسته‎ها روي فلات تبت قرار دارند و به‎دليل توپوگرافي خاص منطقه، توده پرفشار محدودشده و قلمرو عملكرد آن كاهش مي‎يابد.

Introduction Syberian high-pressure is one of the most important systems in climatology. Unique features of this system include expansion zone, the intensity of central pressure, and its temperature and moisture characteristics. Its entry into Iran causes a sharp drop in temperature with occasional frosts and sometimes some extensive damages. Thus, more detailed study and learning more about the activities, entry routes and influence area of this system is necessary for Iran. The first sign of forming this high-pressure mass is a closed curve around Lake Baikal. Due to its proximity to Iran, the system affects the climate of Iran during the year. Methodology Fluctuations and movements in a particular system have been surveyed in the present study in an 11-years period. To investigate the influence of the Syberian high-pressure, maps of 45-115 degrees eastern longitude and 20-70 degrees northern latitude are used. Besides, the data for the cold months have been used. To determine the exact position of the Syberian high-pressure, data were analyzed in ArcGI. In analyzing the maps, the average position of the central core, the expansion model and the main axis of the high-pressure were reviewed. The central nucleus of this high-pressure was identified by using the first closed curve in each of the cold months. The Pressure-driven expansion and entry route into Iran also were recorded. Results and Discussion The results indicate that the central core of the Syberian high pressure cell has been started in early autumn over Tibet and by approaching to winter it is gradually dissipated in an area between Lake Baikal and Balkhash. The high pressure mass arrives over Iran at early autumn and expands, then, towards eastern slopes of Alborz. But by approaching cold season and transfer of the central cores to higher latitudes, pressure of this mass arrives on Iran from the northeast and expands to Oman Sea. In autumn, nucleuses are created in Tibet Plateau and by decreasing in temperature they move to higher latitudes and are also more extended. By approaching cold season in northern hemisphere, the nucleuses move to higher latitudes and are gathered in Syberia land. In this season, it seems that temperature of Syberia land has not provided necessary conditions for creating this air mass, while position of the nucleuses in winter has moved to south of Syberian Sahara. In this season, negative energy on Syberia is near to maximum and temperature is at minimum. Penetration of cold arctic streams from higher latitudes has also increased this coldness and on the other hand, snow on this area has increased reflection and has decreased the temperature in underneath layers. Conclusion Main pattern of Syberian high pressure is east-west. This line is more extended when the nucleus is on Baikal and Balkhash lakes. But when it is on Tibet Plateau, its extension line isn’t expanded significantly in the east and west. It can be concluded that this high pressure mass directly influences entrance of cold air mass into Iran. In addition, it has direct influence on quality and vastness of rainfall in northern coasts of Iran. Present study indicates that in most of the cases, pressure of central nucleus is near to 1035 hp.