بررسی الگوهای گردش جوی روزهای همراه با بارش برف سنگین در غرب ایران

Heavy snowfall in the cold period will cause a lot of damages and troubles for society such as collapse of building installations, and confusions in road network and fuel distributions.  Freezing event will also make worse the intensity of damages. Therefore, identifying the mechanisms of Atmospheric Circulation Pattern (ACP) which form the heavy snowfall and freezing events will certainly help to manage the risk and mitigate the impacts of the crisis.  The main objective of this research was identification of Circulations Patterns (CP) related to snowy days in the basins located in the west and southwest parts of Iran.  In this study, the geographical location of the study area is  West of Iran in 30- 37degree  North latitude and 45 57  - 51 40  East longitude. The region consists of provinces of Kurdistan, Kermanshah, Hamedan, Lorestan, Ilam, Zanjan, Markazi, Chahar Mahal Bakhtiari, Kohgiloye Boyer and parts of West Azerbaijan. In this study, the days which have at least 15 cm of snow in 24 hours, are known as days with heavy snowfall.To choose widespread snowy days, the days that were reported in three stations at least, 3 adjacent provinces and the amounts of the snow were more than 15 cm, are known as heavy snow days. In this research, to analyze the synoptic patterns of heavy snowfall days, daily data of 500 hp level and sea level pressure in 2/5 degree from reanalysis data of NCEP were extracted. All effective systems covered the studied region. The region consisted of 408 point of (20-60) degree in north and (20-80 ) degree  in east altitude .To classify and analysis the synoptic patterns of heavy snowfall days Cluster  Methods  Analysis and Principal Component Analysis were used. To draw average maps of air circulation patterns, pressure data and daily resulted geopotential height of factor analysis and cluster analysis were used. Then four circulation patterns were obtained. Frequency of days with heavy snow – in fourth patterns - showed patterns of CP1 (13 days), CP2 (17 days), CP3 (39 days), and CP4 (12 days). During the under study period, the most frequency belonged to CP3 pattern – 39 day- and the least is belonged to CP4 pattern -12 days. The results obtained of researching synoptic patterns of heavy snowfall in west and south-west of Iran: On days with heavy snowfall in the South West and West of Iran usually Siberian  high pressure system extended widely and one toungue towerd east of Aral lake and other toungue developed to south till Tibetan also some times its toungue extend on the Caspian sea and north part of Iran to Europa. Azores high pressure center is usually reinforced and its tongue extends to the Mediterranean Sea, central europa and africa and other tongue semi- west Iran and Mesopotamia. Extension of Siberian high pressure in north part of Iran with cold high pressure in east Europe created Azore high pressure belt.the system turning clockwise will cause falling the cold air. While it is snowing heaviling,the blocking system create  between  azore  - Siberian high pressure in the east europa and its tongue covers western north and west of Iran. Sudan tongue of low pressure is active.this low pressure with orient east northern or north extened to north of Iran and create a partly strong tongue. Low pressure and high pressure tongues, to other hand warm air and cold air, hit together in west of Iran. Most strong ridge in east Europe was located .this ridge mostly extened tiltly from east north of Moscow to south of Red sea. -subtropical High height center in west Mediterranean and  Tibetan Plateau reinforced then moved to upper latitudes and caused strong ridges in Asia and europe with axis northern-southern These ridges and low height centers in west of Iran and region of Mesopotamia reinforce ridge in east of Mediterranean and extend to lower latitudes. The ridge cause cold advection the region. In these situation,Polar wandering  with axis of northern- southern or eastern north- western south extended to lower  such as nourth of caspean sea and west of Iran and in the rigon  caused falling cold weather. Ridges of Mediterranean (western-east) in europe and Tibet (northen-southern) in Asia is leaded to tilt in axis of ridge towerd lower latitudes. In such days, studied stations have intense tempreture lapse rate.