An inert tracer dispersion scheme for use in a mesoscale atmospheric model

An Eulerian inert tracer dispersion model was developed for use in association with a meteorological model. A truly horizontal diffusion scheme was used in a terrain-following co-ordinate system and a “one-side” diffusion scheme was used at terrain bounded grid points. An implicit scheme was used for vertical diffusion to ease the time step restriction imposed by high-resolution vertical levels. The model was incorporated into a mesoscale meteorological model and inert tracer distribution in and around a partially idealised urban source area was simulated. Comparison with observations showed that the simulations captured the diurnal variation of tracer concentration well. The magnitudes of the concentrations in the urban area varied inversely with the urban heat flux, largely due to the effect the latter had on the depth of the mixing layer. The horizontal distribution of the tracer was influenced by the orographically induced wind field at the mesoscale. The tracer model may be incorporated into other meteorological models with slight modification or run as an independent model as long as suitable wind field and vertical diffusivity are specified