A Numerical Model of the Mesoscale Atmospheric Dynamics - Some Comparisons With Experimental Data
by
Kostadin Ganev
Institute of Geophysics, Bulgarian Academy of Sciences
Coauthors: Reneta Dimitrova (Institute of Geophysics, Bulgarian Academy of Sciences)

The present work demonstrates the performance of a numerical model of the mesoscale atmospheric dynamics by comparing the model results with data from some classic wind-tunnel experiments, as well as with some popular climatic schemes of local air flows in chosen regions in Bulgaria.

A three dimensional quasi-hydrostatic, non-divergent model of the mesoscale dynamics, based on the Businesque approximation, formulation of Guthman is developed in the Institute of Geophysics, Bulgarian Academy of Sciences for the purposes of studying local atmospheric phenomena (sea breazes, slope winds, local air flow disturbances by topography obstacles), local climate and it's changes due to anthropogenic impacts, air pollution in local scales, etc. The numerical treatment of the model equations is based on the splitting-up method, developed by Marchuk, according to which at each step the following simpler problems are successively solved:

- the first phase accounts for the transport along the trajectories and the turbulent exchange. The corresponding system of differential equations is solved by splitting-up along the spatial coordinates;

- the second phase accounts for the coordination of meteorological fields, which leads to Poison equations for the vertical velocity (three-dimensional) and the pressure disturbances at the upper boundary (two-dimensional).

A large number of numerical simulations of the turbulent flow, over two-dimensional hills and valleys, with different slope, were performed, varying the model parameters. The numerical results were compared with measurements from the EPA wind tunnel experiments RUSHIL and RUSVAL. The numerical experiments demonstrate the model sensitivity to the input parameters. An optimal set of parameters was chosen, for which the simulated fields agree very well with the experimental one in all the cases excluding the hills and valleys with the most steep slopes. The last cases are out of the domain of model's applicability outlined by the restrictions due to some physical assumptions in the model formulation, but the agreement is still reasonable. Numerical simulations of the local flow systems in the field of Sofia under stable stratification were also carried out. The general pattern of the air flows in the region in such cases is known from climate studies, air pollution measurements and some laboratory modeling. The numerical model performed well enough, being able to simulate the characteristic mesoscale wind field features for different directions of the background (geostrophic) wind.

Date received: February 1, 2000