The Case Study of Synoptic Systems that Caused Heavy Snow in Tehran City in February,2003

Heavy snowfall and its aftermath is an example of atmospheric phenomenon which can cause serious harm to many parts of the natural and human environment. In order to identify the effective synoptic characteristics and patterns at the events of heavy snow in Tehran in the present study, the snowfall data are collected from the Mehrabad airport weather station archives over the years 2000-2009. From the total number of 150 snowy days that had been recorded, synoptic maps have been created for 19 February, 2003, using data from the GFS, with an accuracy level of 1.0o×1.0o, by GrADS software. Temperature advection maps have also been created using CFSR data. Studying the synoptic maps at different atmospheric levels revealed that during the period of heavy snowfall in February, 2003, the formation of low-pressure over the Mediterranean Sea and on the other hand the formation of high-pressure and its ridge over Caspian Sea are visible and the transition of the two systems causes intensive pressure gradient over the area under the study. Simultaneously at the level above, a low and trough forms that the transition of this trough to the aforementioned area leads to the creation of snowfall. Analyzing the amounts of absolute vorticity and the vertical motion reveals advection of absolute vorticity and the negativity of the vertical motion () which imply the instability and ascending air mass that will eventually result in the occurrence of heavy snowfall. Studying the amounts of temperature advection at 200 hPa also represents warm advection.