A model of occult deposition applicable to complex terrain

A model is presented of the turbulent deposition of cloud water and dissolved species to regions of complex topography where cloud forms in contact with the ground. A three‐dimensional airflow model (FLOWSTAR) which includes a treatment of atmospheric stability and boundary‐layer turbulence is used. Versions of the model are described which include either simple calculations of liquid‐water content or detailed calculations of the cloud microphysics. Both a simple equilibrium model of cloud‐water chemistry and a detailed model including the explicit calculation of the chemical processes in individual droplet size categories are used. The deposition velocities of the cloud droplets to the terrain are calculated from recent measurements of cloud‐water deposition to forest and moorland. It is found that good estimates of deposition may be obtained using the simplest cloud‐water and chemistry models with deposition velocities parametrized in terms of a mean droplet radius. The results indicate that the deposition is a very strong function of position within the terrain. The implications of this for the deposition of phytotoxic species to the flora and the deposition of radio‐nuclides following a nuclear accident are considered.