مطالعه چرخندزايي در پشت به باد كوه هاي آلپ و اثر آن بر آب و هواي خاورميانه براي دوره يكساله

The formation and characteristics of lee cyclones over the Alps and their impact on the Middle East weather are investigated for a one year period (2002). The synoptic charts of 1. R. of Iran meteorological organization at different levels for this period as well as the ECMWF data have been used in this study. Relative vorticity at 500 hPa and thickness charts between 1000-500 hPa have been calculated in order to consider the effect of upper-level vorticity advectiori and low-level baroclinicity on the surface cyclogenesis. It is found that the lee cyclones have different seasonal characteristics, controlling cyclone formation and movement. The number of weak and intense cyclones in the Gulf of Genoa in the cold period of the year is significantly larger, especially as compared with the warm period. Conversely, the intensity of warm period cyclones is much higher than the cold period cyclones, as found in previous studies. The cold period cyclones can largely affect the Middle East and northwestern Iran due to their eastward movements, whereas the impact of warm period cyclones over Iran is very weak. The results suggest that the occurrences of intense snowfall in northwestern Iran in winter might be due to lee cyclones. During moderate periods of the year, the cyclones which are affected by the Black Sea might influence the weather of the Middle East and Iran.