The study of synoptic-dynamic patterns and trends of snow cover changes in the northeast of Iran

This study investigates the most significant synoptic patterns associated with widespread snowfall in the eastern half of Iran.To achieve this, weather code data and snow depth records from synoptic stations in the eastern half of the country were obtained from the Iranian Meteorological Organization for the statistical period of 1371-1400 (1992-2021), focusing on the months of October to March.Days with simultaneous snowfall covering more than 70% of the study area were identified as widespread snowfall events.For the synopticdynamic analysis of these events, a classification method utilizing cluster analysis was employed.Maps of representative days were generated, including variables such as atmospheric temperature, moisture flux, geopotential height, vorticity, front formation, jet stream location, omega index, and meridional and zonal wind data.Additionally, trend analysis was conducted using the Mann-Kendall test.The results revealed that three primary synoptic patterns are responsible for widespread snowfall in the study area.These patterns include:(1) high-pressure systems over Siberia and central Europe coupled with lowpressure systems over eastern Iran; (2) high-pressure systems over western Iran paired with low-pressure systems over Sudan; and (3) high-pressure systems over central Europe combined with low-pressure systems over eastern Iran and Afghanistan.In all patterns, the intensification of meridional flows in the westerly winds, along with the formation of high-and low-pressure centers, creates blocking conditions that disrupt the westerly flow and promote upward air motion.The concentration of negative omega fields and positive relative vorticity advection, coupled with the positioning of northeastern Iran in the left exit region of the Subtropical Jet Stream, contributes to significant atmospheric instability and widespread snowfall in the region.Furthermore, the trend analysis indicated that, although there is no statistically significant trend in the number of snowfall days in northeastern Iran, the overall number of snowfall days has decreased over time.