Abstract

One of the most important natural disasters are storms that each year, causing financial losses and Johnny are Frequency. Sometimes damage to the extent that the economic system, social disrupts a country. The aim of this study is to analyze the synoptic conditions of severe storms is Zab basin. In this study, given the scale of temporal and spatial distribution, wind speed and comprehensiveness of the three threshold size and calculate the 90th percentile wind speed, Forty days pervasive and severe storms were selected. Sea level pressure data and the elevation data of 500hp level from the database NCEP/NCAR were extracted. Matrix was formed that storm was on the rows and elevation data middle levels of the atmosphere, was on the columns. Then, principal components that explain the variation in height level pressure hp500 were identified. To identify synoptic patterns, cluster analysis integration "ward's " was performed on these components. The results showed that five synoptic pattern 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. The most frequent traffic synoptic pattern of a synoptic pattern of middle levels and 60 % of the patterns assigned to itself, causing severe storms in the Zab River Basin. Irregularities in the movement and position of the polar vortex caused the jet stream and storm paths meridian winds greater control and troughs are driven towards the low latitudes deeper. 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 has been created. 1.