Synoptic Analysis of Chill Winds in the West and Northwest of Iran during 1980-2005

The chill wind index is represented to demonstrate relationship between human discomfort due to the joint effect of wind and temperature. Parts of Iran, such as Northwestern sometimes experience very extreme cold weather and frost associated with violent winds which can be detrimental to human health and other flora and fauna. Therefore, the main objective of this study is evaluating of the chill wind index as one of the significant indices of body comfort and finding the synoptic patterns which are related to this index in the Northwestern of Iran. To do so, the climatic data for the January, February and December months of a 26 years statistical period (1980-2006) were extracted from the synoptic stations of Northwestern of Iran and data include pressure, temperature, wind speed components (U&V) in different levels and in hourly scale which were extracted for sample days with the highest chill wind index from NCEP/NCAR. Composite maps of temperature advection, pressure, wind vector and speed in middle levels and lower levels (850Hpa) and sea level were generated by GRADS software. Also, for calculating the chill wind index the Sipel-Passele’s equation was used. The results illustrate that chill winds mostly happened under the two patterns: Deep trough and blocking. Cold advection in both patterns is provided from three sources: High pressure in Northern Africa and Southern Europe, Deep trough over the Siberia (north of the Black Sea) and weak high pressure over the southern part of the Black Sea.