Spatial and temporal distribution of Relative Vorticity in Iran

In order to investigate the spatial and temporal variations of relative vorticity in Iran, we have extracted the mean monthly relative vorticity data from the NCEP Reanalysis data site for the window of 45 to 62.5 degrees east and 25 to 40 degrees north for the 1971-2005 period with 2.5 degrees resolution. The mean monthly maps of the extracted data were drawn and described. The results showed that in winter season most of the country is under the positive relative vorticity indicating the existence of instability over the country and invasion of westerlies resulting in rain. The spring season was the same as winter, but toward the end of the season the negative vorticity was invading the country from the south. So that, in summer most of the country was occupied by the negative voriticity and resulted stability. During this season only the southern coasts of the Caspian was out of the negative vorticity occupation. During fall season the winter conditions were dominating the country from the north.