Synoptic analysis of extreme rainfall in North Khorasan Province

Due to the short period of statistical data for other stations only the rainfall level of Bojnurd station was used to determine the coherent pattern of extreme precipitation in North Khorasan. In order to determine the patterns of rainfall, data related to sea level pressure, specificity, vertical velocity (omega), wind component (U), wind velocity component (V), geopotential height and air temperature at different levels, collected four times a day, were used. Also, Outgoing Longwave Radiation (OLR) prepared daily by the National Center for Environmental Forecast from two days before the rainfall was entered into GrADS software along with the previous parameters and analyzed using the software’s capabilities. The findings show that the prevailing atmospheric patterns during the occurrence of rainfall is the existence of a nave with a north-southwest axis in the southern part of the Caspian Sea, which causes instability, raising and transferring moist air to the studied area, causing heavy rainfall in the area. According to the vertical maps of atmospheric moisture, it was determined that the highest amount of moisture in the atmosphere was at lower levels of the troposphere. By studying the maps of the convergence nuclei of moisture at different levels, the role of the Caspian Sea in providing the moisture of such heavy rainfall is visible. The status of the atmospheric river at upper levels of the atmosphere also contributes to more severe ascent of air, increased instability, and elevated atmospheric turbulence in the region. On the other hand, falling cold air has higher latitude, which creates a frontier, increases its intensity, and stretches the frontier to the upper levels of the atmosphere.