Simulation of Pollutant Transport in Complex Terrain with a Numerical Weather Prediction–Particle Dispersion Model Combination

A new scaling approach, based on the convective velocity obtained from the sun-exposed eastern slopes and thus suited for steep and narrow Alpine valleys, is investigated with respect to pollutant dispersion. The capability of the new method is demonstrated with the operational emergency response system of MeteoSwiss, which consists of the COSMO (COnsortium for Small-scale MOdelling) numerical weather prediction model coupled with a Lagrangian particle dispersion model (LPDM). The new scaling approach is introduced to the interface between COSMO and LPDM, and is compared to results of a classical similarity theory approach and to the operational coupling type, which uses the turbulent kinetic energy (TKE) from the COSMO model directly. For the validation of the modelling system, the TRANSALP-89 tracer experiment is used, which was conducted in highly complex terrain in southern Switzerland. The ability of the COSMO model to simulate the valley wind system is assessed with several meteorological surface stations, and the dispersion simulation is evaluated with the measurements from 25 surface samplers. The sensitivity of the modelling system towards the soil moisture, horizontal grid resolution, and boundary-layer height determination is investigated, and it is shown that, if the flow field is correctly reproduced, the new scaling approach improves the tracer concentration simulation when compared to classical coupling methods.