Numerical experiments using Version 3 of the Regional Climate Model (RegCM3), coupled with the Biosphere-Atmosphere Transfer Scheme (BATS), are performed to study the role of topography and land cover of the Zagros ranges in the simulated mesoscale meteorological features over Iran. The simulation domain covers the area between 34 and 63 degrees East and 24 and 44 degrees North. The simulation period is the winter 1999: The horizontal resolution is set at 45 km × 45 km. The boundary conditions are determined from the (United States) National Center for Environmental Prediction (NCEP) - National Center for Atmospheric Research (NCAR) reanalysis.
Three sets of numerical simulations are performed in different conditions, with changing the Zagros topography and land-use. For the first (control) run, the present topography and land-cover, as derived from the GTOPO30_3MIN and GLCC3MIN_BATS data, are used. For the second run, the Zagros ranges are flattened and the surface-level elevation of the grid points in the area is set using a linear interpolation between the grid points on the west and on the east of the ranges. The simulations show that excluding the Zagros does not change the average regional precipitation considerably, but changes the precipitation distribution by decreasing it in the west and increasing it in the central and eastern parts of the country. In line with theoretical expectations, the analysis also demonstrates the important role the Zagros topography plays in the formation of vorticity and divergence/convergence fields in the lower levels of the atmosphere. For the third run, the vegetation cover of the Zagros area was replaced with that of the desert. The results show that such a change results in only a minor decrease in the precipitation amount and the temperature of the eastern areas of the domain.