Numerisk modell för global transport av spårämnen i atmosfären

Sammanfattning: The aim of this project was to develop a numerical model simulating the global transport of air pollution. The model currently used by SMHI, the Swedish Meteorological and Hydrological Institute, was intended to perform such simulations at regional scales. However, the usage of a grid based on longitudes and latitudes makes this model work poorly in global simulations. The problem with this grid is that the nodes are concentrated near the North and South Poles. Due to this characteristic, the Courant, Friedrich and Lewy-condition for numerical stability puts a sharp bound on the timestep length. In order to keep the transport algorithms stable, the timestep length has to be very small. To avoid too short timesteps, we adopted a new kind of grid. In principle, we defined the global grid on the surface of a cube. By replacing straight lines on the cube with great circles we took the curvature of the Earth into account. The result was almost equal distances between the nodes, and thus we were able to use an acceptable timestep length.