On modeling dry deposition of long-lived and chemically reactive species over heterogeneous terrain

An explicit multi-layer subgrid-scheme was developed for a meso-gamma/beta-scale model to consider subgrid-scale surface heterogeneity, dry deposition, biogenic and anthropogenic emission of trace gases. Since dry deposition measurements of highly reactive trace species are scarce we try to evaluate this scheme by heuristic principles. The results of simulations conducted for a 5x5 km(2) resolution with and without this scheme are evaluated by using results of a model run with 1x1 km(2) resolution, which is taken as a `grand thruth' and which has the same resolution as the subgrid. The explict multi-layer subgrid scheme provides a similar distribution of dry deposition fluxes as the much more computationally expensive simulation with the 1x1 km(2) resolution. Dry deposition fluxes determined from observations give evidence that the explicit multi-layer subgrid scheme which does not require a constant flux approximation for a layer of several decameters leads to an improvement in determining the exchange between the atmosphere and the ground. Results of simulations with a microscale model show that the inhomogeneity at forest edges leads to an increase of the turbulent transports of up to a factor 4 compared to horizontally homogeneous terrain, which is assumed to be the conditions of the subgrid cells (and which is usually the assumption for the entire grid cell in mesoscale models). Inhomogeneity inside an extended stand of trees causes an overall increase of 5-10% with high local extremes, i.e. such an inhomogeneity results to an underestimation of dry deposition in meso-gamma/beta-scale models. The effects are most pronounced for a wind direction perpendicular to the forest edge.