Comparison of the DRAIS and EURAD model simulations of air pollution in a mesoscale area

Summary

In this paper the results of simulations of air pollution carried out with the mesoscale model system KAMM/DRAIS are presented. They are compared with results of the European scale model EURAD which have been provided by the EURAD-Group, Cologne. With this comparison it is intended to analyse to what extent better resolution of topography and emission data used by the mesoscale model effects the model results. The simulations have been carried out for July 15, 1986, a typical summer day. The model domain contains south-west Germany and part of Alsace with a resolution of 5 km. The emissions for this resolution have been derived by a combination of the coarse EURAD emission data with the data of the TULLA experiment which are available on a much finer grid. The initial and boundary conditions for the species concentrations are determined from the results of the EURAD model. This coupling introduces the long range transport of pollutants into the mesoscale simulation.

The meteorological and concentration data of the EURAD model are compared with the corresponding DRAIS model results. The mesoscale flow field is characterized by the channeling along the Upper Rhine Valley, which is not resolved in the EURAD model. The concentration distributions of both models are similar during midday, because of the strong vertical mixing. In the night and especially, in the morning and evening hours the spatial distribution is much better represented by the DRAIS model results. The better resolution of the emissions and the topography in the DRAIS model compared with the EURAD model (80 km grid size) becomes really noticeable. The difference of the ozone concentrations between cities and the, surrounding areas and between the Rhine Valley and the limiting mountains are in the order of 30 ppb as compared to a few ppb in the EURAD simulation. In the morning NO concentrations of about 200 ppb are simulated in the area between Heilbronn and Stuttgart. The EURAD model provides only about 5 ppb. Comparisons with measurements show that the DRAIS simulations are more realistic than the EURAD model results. The features mentioned are also found in an evaluation of the concentration variations in areas corresponding to a grid cell of the EURAD model. Two completely different areas are selected to demonstrate the possible range of the concentration variation. In the area around the City of Stuttgart the ozone concentration in the morning and the evening varies between zero ppb and 50 ppb, approximately. The mean value is nearly the same in both simulations.