The purpose of this paper is to present the flow field with diffusion processes of air pollutant in a three dimensional numerical model. The model calculates a three dimensional wind field by using the mass consistent model that computes a total wind field from observed wind data on the ground several observatory sites. Therefore, this model doesn't calculate the three dimensional momentum equations, and it approximates the wind velocities in each mesh point using the variational principle. Then, it causes the reduction of total computational CPU time in wind field computations. In this model, the staggered mesh system of the u and v field variables location is employed in order to cancel the truncation errors and to increase the resolution of the topography. Once the wind velocities in each computing mesh are obtained, next atmospheric duffusion and dispersion processes are computed by the convection diffusion equation. In this paper, the results of the testing for diffusion processes of Yokkaichi area associated to the covergency of the wind velocity solution and the effect of the boundary conditions of the ground height are shown together with the description of the model.
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