A Synoptic analysis for the occurrence of springtime heavy rainfall in the Northwest of Iran

The main purpose of this research is synoptic analysis for the occurrence of springtime heavy rainfall in the Northwest of Iran. Therefore, daily precipitation data were obtained from 15 synoptic stations for a period of 34 years (1981-2014). Then by using percentile index of above 95th, it was identified 98 days with heavy precipitation. So, to determine the synoptic patterns by applied the NCEP/NCAR reanalysis dataset, the maps of geopotential height, sea level pressure, streamlines, vorticity, and specific humidity at different levels were prepared and the regional pattern flow and atmospheric conditions two days before the occurrence of precipitation analyzed. The results indicate that heavy precipitations in North West of Iran are categorized into four synoptic patterns. At the first pattern, the main cause of heavy precipitation in North West is passing cyclone or low-pressure migratory systems and the regional dynamic convergence and its association with cyclonic circulation. The main factor for the transferring of moisture to the inner side of these cyclones is the anti-cyclone stationed in Saudi Arabia. At the second pattern, the couple blocking system in the middle level of troposphere system is caused heavy precipitation with stop-motion circulation systems in the middle and lower levels of the atmosphere. At the third pattern, a deep cyclonic circulation center has not been formed on their area and seems deepening a trough of extratropical and stimulates the local convergence condition dynamically, and is regarded as the main mechanism of precipitations occurrence.