Synoptical Survey of Extreme precipitation in North West of Iran A case study of Extreme precipitation in Uromieh Station

IntroductionPrecipitation systems have a main role in water supply, so identifying their characteristics such as understanding their formation, water resources and locating their activities are really important. The interaction of Sea Surface Temperature and precipitation in different regions of the Earth has been emphasized by many researchers. Studies show that high-latitude jet stream is an important factor in developing mid-latitude systems. The purpose of this research is to understand the synoptically structure of the extreme precipitation pattern in the North West of Iran and can be used as a diagnostic factor in the weather forecasting.Materials and MethodsPrecipitation data source are obtained from I.R of Iran Meteorological Organization. For investigating synoptically pattern and anomaly for extreme precipitation in the North-West of Iran, mean sea level pressure, 500-hpa geopotential height, 300-hpa jet stream from data analysis of National Centers for Environmental Prediction / National Center for Atmospheric Research (NCEP / NCAR) are considered by using advanced very high resolution radiometer (AVHRR) data for analyzing sea surface temperature. Some parameters such as Mean sea level pressure and 500-hpa geo-potential height with their anomaly, 300-hpa level of jet stream and water temperature were investigated and analyzed.the day with the highest records of rainfall during the study period. The 6-hourly upper level atmospheric variables for the days 4 to 6 of January, 2008 were extracted from the NCEP reanalysis data web site. These variables include: sea level pressure maps, geo-potential height of upper level pressure levels, the U and V components of upper level winds, vertical motion (Omega), Jet stream wind speed, and air temperature. These upper level data were obtained for a greater window of 30° W to 80° E and 0° to 70°N.Results and conclusionThe daily rainfall of January 6, 2008, was the heaviest daily rainfall ever recorded in the southwestern part of the country. The amount of rainfall ranged from 3 millimeters in the most eastern part to 50+ millimeters in the most southern part of the region. The analysis of the weather maps of the surface, 850, 500, and 300 hPa. levels revealed the following results. During the pre- rain days the region was under the stable upper level conditions; the Siberian High had been spread over the region on the surface and the upper westerly ridge and was established at the 500 hPa level. On the rainy day of January 6, 2008, the surface high pressure center has been retreated. On the upper level a deep westerly trough has moved over the area and had created very unstable conditions. On 850 hPa level the study region no humid winds recorded. But during the rainy day the wind from west and over the Persian Gulf added the required moisture. The instability indices of the atmosphere indicated strong instability on January 6, 2008. The wind maps showed the intrusion of the Persian Gulf moisture to the region from west and south. Another important factor was the presence of the westerly jet core over the region which accelerated the uplift of moist air. In brief, the southward penetration of a deep westerly trough at the upper atmosphere, intensive instability of the atmosphere, and the intrusion of moisture from the nearby water bodies caused the heavy rain of January 6, 2008. The results confirmed the previous findings that when a deep trough penetrates to the south of Iran lifts the moist air of the water bodies of the region and causes very intensive torrential rains.