The effects of the Alborz mountain range on the development of synoptic weather systems


In this paper by using MM5 modeling system, the effects of the Alborz mountain range on the development of synoptic weather systems on the leeward side of the mountains have been studied. The primary aim was to see if the cyclones are produced and intensified (cyclogenesis), however the cyclogenesis was found not to be marked and hence the focus of the study was on the effects of the mountain on weather systems reaching this area.
The extent of the simulation domain is from 24 E to 74 E and 14.6 N to 53.6 N with horizontal grid resolution of 25 kilometer, and 23 half-sigma levels in the vertical. The effect of terrain elevation was considered by smoothing the elevation typically by 50 percent (near the peaks by 70 percent and about 10 percent near the topographic borders). We found that the north-westerly weather system(24th to 27th Dec.2005) crossing the Alborz mountain range showed a sharp difference between the case with topography and that without it. These changes were particularly observed in potential temperature field, Froude number, vertical speeds and precipitation. The vertical speeds as well as potential temperatures were found to be reduced markedly. Also the 48-hours accumulative precipitation was found to be reduced too. By removing the topography, the Froude number of the flow was substantially increased; indicating that wave activity over the area was reduced. Typical Froude numbers for the case with topography is about 0.9, which is a favorable condition for formation of Lee waves, but without topography it appears to be much larger than 1, which is not a favorable condition for wave formation.
We also found that the westerly and south-westerly weather systems moving to this area were hardly influenced by the Alborz mountain range. This may be due to the fact that these systems are greatly weakened by the smaller topographic features along their path as well as losing most of their moisture over the Zagros mountain range, although this needs further study.