Document Type : Full length article
Associate Prof., Geography and Climatology, Shahid Beheshti University
MA Student, Climatology, Shahid Beheshti University
Siberian high-pressure is one of the most important systems in climatology, which has been focused by all climatologists because of its influence area and expansion zone.Unique features of this system including the expansion zone,the intensity of central pressure,which sometimes amounts to more than1075hp pressure at its core,its temperature and moisture characteristics,the influence area of its tabs that sometimes covers Central Europe,Pacific Ocean in the East and from the Arctic areas to southern Iran even the Arabian Peninsula,have attracted the attention of many researchers.The proximity of Iran with this powerful system in which more than half the year,either directly or indirectly be affected by this system,and its entry into the country,causes a sharp drop in temperature,with occasional frosts and sometimes the damage is extensive,so Iran requires more detailed study and learn more about the practices, entry routes and influence area. Despite extensive studies indirectly addressed to this system, but it has not been studied as a professional .Siberian high-pressure is one of the large air masses that affects a wide area of the planet.During the cold period in the vast territories of Eastern and Central Asia,Siberia,and especially because of the large distance of water resources,the lack of moisture and the clear sky due to long wave radiation, tremendous energy lost as a result of extremely cold air near the ground and causes a high-pressure centers.The first sign of forming this high-pressure mass is a closed curve around Lake Baikal in September that gradually increasing its intensity during the cold season so that in December the central pressure is 1035hp.At this time,it occupies most of Iran's regions that Orbit at 30 degrees.
This high-pressure begins through the North East and gradually will occupy the whole area of the Central Plateau.Due to its proximity to Iran,the system will affect the climate of Iran during the year.During the expansion of the high- pressure mass, the air temperature will drop,so dry and cold weather will be dominant in Iran .
Since only fluctuations and movements in a particular system have been focused in this study, therefore, for 11 years, our expectations were met. To investigate the role of the Siberian high-pressure limited maps 45 to 115 degrees east longitude and 20 to 70 degrees north latitude by a period of eleven years ( between 2000 to 2010) is used. Besides, the data for the cold months ( October, November, December, January, February and March ) have been used.Converted to mean sea level pressure(slp) data used from the database of the United States National Center for Atmospheric Research and National Center for Environmental Prediction (NCAR / NCEP) with a 2/5 × 2/5 spatial resolution level . To determine the exact position of the Siberian high-pressure, the data were analyzed in ARC GIS. The nucleus ,the route and the expansion axes formed layers of digital and converted to interpretative maps in climatology. In analyzing these maps,the average position of the central core, the expansion model and the main axis of Siberian high-pressure in an eleven-year period were reviewed.At this stage, the central nucleus of the Siberian high-pressure were identified by using the first closed curve in each of the cold months. The Pressure-driven expansion and entry route into Iran (the closed end of the last curve around the high-pressure center ) also were determined. To ensure the accuracy and the pattern of spread,the high pressure central core in 2000,was mapped daily in each of the cold months.
Results and Discussion
The results shows that the central cores of the Siberian high pressure cell has started in early autumn over Tibet and by approaching the winter,It is gradually dissipated to an area between Lake Baikal and Balkhash.Siberian High pressure mass arrives at Iran at early autumn and expands to eastern slopes of Alborz.But by approaching cold season and transfer of central cores to higher widths,pressure stack of this mass arrives at Iran from the northeast and expands to Oman Sea.In autumn,firstly nucleuses are created in Tibet plateau and by decreasing of temperature,move to higher widths and are more extended.By approaching cold season in northern hemisphere,nucleuses move to higher widths and are gathered in Siberia land.In this season, it seems that temperature of Siberia land has not provided necessary conditions for creating this air mass,while situation of nucleuses in winter has moved to south of Siberian Sahara.
In this season,negative energy on Siberia is near to maximum and temperature is at minimum.Also penetration of cold arctic streams from higher widths has increased this coldness and on the other hand, snow on this area has increased reflection and has decreased the temperature in underneath layers.
Main pattern of high pressure Siberian is west-east or in better words,long high pressure line is generally west-east. This line is more extended when the nucleus is on Baikal and Balkhash lakes,so sometimes it includes more than 40 grades.But when it is on Tibet Plateau,its extension line,due to topographic reasons, isn’t extended significantly in the east and west.
The pattern of the Siberian High– pressure mass is east-west and in the winter when the cores are in higher widths,it is much broader,so that it takes up to 40degrees longitude.While in the autumn that cores are on the Tibetan Plateau, due to the topographical factors, it hasn’t expanded considerably in east and west. High pressure Siberian is one of influential and important air masses on climate of a vast area of Iran, especially in cold season or raining season. It can be concluded that this high pressure mass directly has important role in entrance of cold air mass to Iran,Especially premature and post mature cold air mass and winter cold air.In addition,It has direct influence on quality and vastness of raining in northern coasts of Iran.This phenomenon can be seen in pattern which is found through periodical as well as daily analysis of high pressure central nucleus.During these studies this has been found that In most of the cases,pressure of central nucleus is near to 1035 hp.