The role of topography on the simulation of Sistan wind structure in the east of Iranian Plateau

In this study, the interaction between atmosphere and earth surface and its effect on the simulation of Sistan wind structure in the East of Iranian plateau is investigated. For this purpose, four experiments have been carried out with RegCM4, with horizontal resolution of 20 km. In non-topography experiments, the model was implemented in three different conditions. The results indicated that the Sistan wind is a multi-scale climate phenomenon which will be affected by topography both in wind intensity and in wind direction in the lower troposphere. In the synoptic scale, the pressure gradient which dominates between Pakistan heat low and the Turkmenistan anticyclone (Caspian Sea high pressure) can create a large scale background northerly flow in the lower troposphere which will be passing through the whole area in the east of Iranian Plateau. Furthermore, in meso to regional scales, the topography of the area would be responsible for creation and maintenance of a Low Level Jet (LLJ) through a mechanical and thermal forcing. the mechanical forcing of mountains are responsible for appearance of two LLJ cores across eastern borders of Iran which their preference locations would be around Atishan Desert in the north and upstream of Hamon Lake in the south. As a general result, by eliminating the topography in all non-topography experiments, the LLJ core will disappear on upstream of Hamon Lake as the most important mechanical forcing of the mountains. However, eliminating only the southern Khorasan Mountains will accelerate the north LLJ core in the Atishan Desert, while the LLJ core on the upstream of Hamon Lake will disappear over the Iran Borders at the same time. To evaluate the influence of thermal forcing of the mountains on Sistan wind structure, the total heating, as a residual term of thermodynamic equation, is calculated. The results indicate that mountains have a significant role to building a local low level circulation in the east of Iranian Plateau.