Numerical simulation of the impact of orography on active synoptic weather systems over Iran


Orography plays a substantial role in the formation and evolution of many atmospheric phenomena. Observations indicate that two mountain ranges to the southwest of the Red Sea and west of Iran (Zagross mountain range) play a crucial role in the formation, evolution and activities of synoptic weather systems over Iran. Observations also, indicate that there are considerable differences between the amount of precipitation over the west and east of the Zagross mountain range.
In this paper, the effects of those two mountain ranges on the formation, evolution and activities of a synoptic system over Iran between 23rd and 30th of December 2004 have been studied. To do this fifth-generation of PSU (Pennsylvania State University) /NCAR (National Center for Atmospheric Research) Meso- and Micro-scale Modeling system (MM5) was used. To provide a clear picture of the development and evolution of the system an area bounded to 10 to 50 degrees north and 20 to 70 degrees east was selected. A rectangle grid by 101 zonal grid points and 81 meridional grid points with a distance of 55.5km between the grid points was implemented for simulation and modeling purposes and the study of dynamical and thermodynamical processes involved in the development and evolution of the system. Betts-Miller, Grell, Blackadar, and MRF schemes were used to model different physical, thermodynamical and dynamical processes.
Model results show that the elimination of the mountain range to the southwest of the Red Sea leads to widen the existing low pressure over the southwest area of the Red Sea towards the east. It also leads to a decrease in the small high pressure cell that is believed to play a substantial role in the formation of the so-called Red Sea trough towards the north of the Red Sea and southeast of the Mediterranean Sea, where many of the weather systems that pass over Iran during the winter form. Therefore, it can be concluded that elimination of that mountain range will destroy part of the mechanism essential for the formation of some weather systems that are important for Iran.
Model results also state that the elimination of Zagross mountain range could lead to the enhancement of precipitation over the central part of the country, east of the range.