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Atmospheric circulation patterns is the most important method of identification of environmental change that, it is useful For purposes such as weather forecasting, predict natural events (air pollution, floods, drought, etc). The aim of this study is to analyze the synoptic conditions of severe storms is Zab basin. Daily data of wind speed over the period 1364/01/01 to 1390/12/29 of three synoptic stations was taken from the department of meteorology. Given the scale of temporal and spatial distribution, wind speed and comprehensiveness of the three threshold size and calculate the 90th percentile wind speed, 40 days pervasive and severe storms were selected. Sea level pressure data on the network with a size 5.2°*5.2° that was located on Cornell 0-80 degrees east longitude and 0-80 degrees northern latitude from the database NCEP/NCAR were extracted. Matrix was formed in the dimensions of 40*864 with storm was on the rows and elevation data middle levels of the atmosphere, was on the columns. A principal component analysis was performed on data matrix elevation and six factors were identified that about 97.4% of the Pressure elevation changes of 500hp level was explained. To identify synoptic patterns, cluster analysis integration "ward's" was performed on these components. The results showed that, Cut off low synoptic pattern of the upper atmosphere with an abundance of 47.5% in May and December had the highest frequency. Irregularities in the movement and position of the polar vortex caused the jet stream and storm paths meridional winds greater control and troughs are driven towards the low latitudes deeper. As a result, the pressure gradient and energy exchange at its maximum reached compression Isobaric lines and Strong winds in the catchment area level has been created. All patterns are identified for the winds mainly from the west and southwest.

The Spatial Distribution of Atmospheric Conditions, Severe Storms Zab Basin in Iran (A Case Study of Cut off Low Synoptic Pattern)