Dynamic and Synoptic Conditions of Heavy Snowfall Occurrence in Tehran Province, Case study: January 2018

Snowfall follows different patterns in various regions. Severity of precipitation in populated areas such as Tehran due to its effect on urban infrastructure and disruption of transportation is important in the urban management. In this study, heavy snowfall event in Tehran province investigated in January 2018. An important feature of this system was that the maximum snowfall occurred in the southern regions in comparison to the mountainous areas of the province. Synoptic-dynamic analysis and atmospheric simulation using the WRF model was done in this event. The establishment of a stationary front in the south of the Alborz Mountains for two days resulted in maximum system activity in the southern parts of the area. The formation of cold pressures over Russia and its expansion to the Iranian plateau in contrast to the low-pressure from the Mediterranean and Sudan to the central parts of Iran has caused a severe temperature gradient. Concurrent with warm front activity in the plains south of the province, maximum potential vorticity in the front of the trough, negative (positive) vorticity advection in the south of the Caspian Sea (south-west, some parts of the west and center of Iran) observed. The maximum tropopause folding (250 decameter) occurred in Tehran and the maximum moisture flux convergence observed in the southwest of the Alborz Mountains. The results of the 24-hour cumulative precipitation simulation showed that the model had underestimate for the rainfall in the study area and the position of the maximum of the precipitation forecasted incorrectly.