In this paper, the results of the development of a three-dimensional model for simulating mesoscale phenomena in the mountainous coastal areas of Iran are presented briefly. The model is suitable for studying atmospheric disturbances, which are generated by land-sea surface temperature contrasts as well as by mechanical lifting and thermal forcing due to mountains and their interactions. The model is hydrostatic with a sigma coordinate along the vertical and based on non-linear time-dependent primitive equations. Initialization in the model creates a uniform prevailing wind. Parameterizations of the vertical and horizontal transport of eddy fluxes are incorporated. It also incorporates a unique formulation associated with the horizontal gradients of geopotential and surface pressure in the momentum equations. With this modification, the fatal computational error due to the spurious imbalance between the two terms, especially in the case of high irregular mountains and/or steep slopes, may be avoided or minimized. In the field of microphysics, explicit formula for evaporation from cloud droplets when they are advected to the unsaturated adjacent area, are added. This addition may give more reliable treatment of evaporation through preventing spurious local sudden cooling of evaporation.
First, the model was examined in a region with a bell-shaped mountain. Then, it was used to simulate wind field, cloud and precipitation for a smoothed real region, including part of the Caspian Sea, the complex Alborz Range and part of the central plateau. Since sufficient data for a realistic initialization were not obtained through the meteorological stations in the study area, the initial conditions were therefore assumed and defined as much in accordance with the available data in a specified day. As a consequence, output was inevitably based tightly on this initial condition.
)