Analysis of the mesoscale structure of moist potential vorticity during an extreme event of snowstorm in Tehran

One of the most severe snowstorms in recent years occurred in the Tehran region on 27-28 January 2018 and led to roadblocks and closure of airports. In this study, the development of the snowstorm is investigated using a combination of synoptic and mesoscale analysis based on diagnostics of frontogenesis and various forms of moist potential vorticity. A surface cyclone, a midtropospheric trough, and an occluded front were the main ingredients of the synoptic environment of the snowstorm in the Tehran region. The high-resolution simulation is performed using the WRF model configured for two nested domains with horizontal grid spacings of 9 and 3 km using the ERA5 data for initial and boundary conditions. The simulation with grid spacing of 3 km makes it possible to investigate the effect of meso-gamma environmental conditions on the formation of heavy precipitation and snowbands. Results point to the presence of strong frontogenesis and intense negative saturation equivalent potential vorticity (SEPV) in the lower and middle troposphere during the development and mature stages of the snowstorm. As the snowstorm passed the region, the amounts of negative SEPV and frontogenesis became much weaker through most of the troposphere, meanwhile relative humidity and vertical motions reduced. Detailed analysis shows that conditional, inertial, and conditional symmetric instability all contributed to the formation of heavy precipitation in this snowstorm. Moreover, computation of the areaaveraged values of the terms contributing to negative SEPV indicates a considerable nonhydrostatic effect mainly by the term involving the meridional gradient of vertical velocity.