Large-scale atmospheric circulation patterns associated with the occurrence of severe storms Zab basin

The aim of this study is to analyze the synoptic situation of severe storms of Zab basin. Daily data of wind speed, over the period 1364/01/01 to 1390/12/29 (9861 days) from three synoptic stations, was taken from the department of meteorology. Given the scale of temporal and spatial distribution, through threshold size of wind speed and comprehensiveness as well as calculation of the 90th percentile of the wind speed, 40 days of pervasive and severe storms were selected. Sea level pressure data on the network with a size 2.5° × 2.5° that was located on 00-80 degrees east longitude and 00-80 degrees northern latitude from the database NCEP/NCAR were extracted. A Matrix was formed in the dimensions of 864 columns in 40 rows on rows of days with stormy days on the rows and elevation data middle levels of the atmosphere, 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 "wards" was performed on these components. The results showed that the storms happen frequently during spring and autumn. April was the most prevalent (33%) and July and August had the lowest number of storms. Five synoptic patterns of atmospheric middle level, in the form of three major causes of severe storms in the Zab River Basin: Cut off low pattern, Shallow trough pattern of long wave and deep trough pattern of short wave. In these between, cut off low synoptic pattern of the upper atmosphere with an abundance of 47.5 percent 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 to have greater control and troughs to be driven deeper towards the low latitudes. Because of the special arrangement of relatively strong and contrasting surface synoptic, the pressure gradient and energy exchange at its maximum reached compression Isobaric lines and as a result, strong winds in the catchment area level have been created. The winds blow, almost in all identified patterns, from the west and southwest.