Studying and identifying of Dynamic synoptic patterns affecting droughts occurrence in Semnan province

Drought as an anomaly atmospheric is a natural phenomenon that rooted in the natural climate variability and most importantly, the general circulation of the atmosphere. It can be said with regard to the general circulation of the atmosphere control systems are cyclones, anti-cyclones and air fronts that control middle latitudes of the Earth’s climate. Therefore, it should be sought the causes of droughts and abnormal changes in weather patterns. One of these disorders is anti- cyclones. Review and identify the structural nature of the phenomenon as one of the atmospheric anomalies can be very effective in identifying dry periods. In order to explain the mechanism of anticyclones in the event of widespread drought in Semnan province, the regional structure of atmosphere was investigated. In this regard, rainfall statistics of 7 synoptic stations and 27 meteorological stations of country meteorological organization were used. To identify the dry periods, it was used the Standardized Precipitation Index (SPI). Then to explain the structure of anticyclones, it was used the data of geo-potential heights, sea level pressure, Omega, zonal and meridional wind components belonging to the National Center for Environmental Prediction / atmospheric sciences (NCEP / NCAR). The results showed that anticyclones in the form of four patterns affect area drought event. The establishment of a strong ridge over the area with negative vortices, weather extreme dynamic descent, along with a sharp drop in humidity and exposure the province under negative volubility are the main reasons for this phenomenon. Reducing the moisture needed and ascending flow values current and entering the tab of the Siberian high pressure to central of Iran has made the atmospheric turbulence to be minimize.