A Multiple-Cell Flat-Level Model for Atmospheric Tracer Dispersion over Complex Terrain

Abstract

A multiple-cell flat-level tracer dispersion model is developed for atmospheric pollution study. The horizontal domain may be constructed with multiple-sized cells for varied resolution. The sequence of cells is arbitrary, as in unstructured grids, as long as no holes are left in the horizontal domain, which may be tailored in shape according to local orography. The vertical levels are truly flat and the level spacing may vary from level to level. The surface orography is included by removing cells from the bottom of the three-dimensional cell block. The arrangement of wind velocity and tracer concentration is similar to the Arakawa C grid. Advection and horizontal diffusion are formulated on each cell face, using tracer concentrations in the two cells that share the cell face. Pointer-orientated numerical loops are used to facilitate the arbitrary horizontal cell arrangement and orographic variation of vertical levels. A second-order upstream upper limiter advection scheme is developed for this model and numerically tested to be positive-definite and mass conserving. Vertical diffusion is solved with an implicit scheme and simplified vertical diffusivity, which is parameterised as a function of the mixing layer depth. The model is fast, compact, easy to implement and highly portable. It is suitable for studies ofmesoscale and small-scale atmospheric tracer dispersion over complex terrain, including steep slopes. The model is used to simulate traffic pollution in London, UK, and is compared with available observations.