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

An Eulerian inert tracer dispersion model was developed for use in associat ion with a meteorological model. A truly horizontal diffusion scheme was us ed in a terrain-following co-ordinate system and a "one-side" diffusion sch eme was used at terrain bounded grid points. An implicit scheme was used fo r vertical diffusion to ease the time step restriction imposed by high-reso lution vertical levels. The model was incorporated into a mesoscale meteoro logical model and inert tracer distribution in and around a partially ideal ised urban source area was simulated. Comparison with observations showed t hat the simulations captured the diurnal variation of tracer concentration well. The magnitudes of the concentrations in the urban area varied inverse ly with the urban heat Bur, largely due to the effect the latter had on the depth of the mixing layer. The horizontal distribution of the tracer was i nfluenced by the orographically induced wind field at the mesoscale. The tr acer 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. (C) 2000 Elsevier Science Ltd. All rights reserved.