Beräkning av moln- och dimdepositionen i Sverigemodellen – Resultat för 1997 och 1998. Estimation of cloud and fog deposition by use of ”Sverigemodellen” – Results for 1997 and 1998

Typ: Rapport
Serie: Meteorologi 98
Författare: Valentin L. Foltescu / Christer Persson


The capture of cloud and fog droplets by vegetation as a possibly important deposition mechanism has been recognised lately after reports of increased forest decline with altitude. We are concerned with interception, impaction and stickiness of polluted cloud water and fog droplets to terrestrial surfaces, including forest canopies. This deposition is mediated by low-level clouds, in which parts of the terrain can be immersed, or by the presence of orographic and radiation fog. In this study the transport of cloud and fog droplets towards terrestrial surfaces is parameterised and the “occult” deposition (mediated by both clouds and fog) is calculated by a new approach, implemented in “Sverigemodellen” (MATCH-Sverige), a 3D Eulerian transport and dispersion model employed for making environmental assessments in Sweden. The model scheme is taking into account the mesoscale analysis (MESAN) of low-cloud coverage and cloud-base height, the visibility analysis, detailed topography information, land use, modelled atmospheric concentrations and the instantaneous deposition parameters for the investigated species (acidifying air pollutants: sulphate and oxidised/reduced nitrogen). The model scheme is validated by comparing with throughfall monitoring of sulphate deposition to coniferous forests at elevated sites in the Swedish mountains. It shows a rather good fit to the measurements, generally within 25%, which is deemed as encouraging. Discrepancies exist though and are subject of discussion. The percentage of occult deposition can be up to 115% of the sum of dry and wet deposition. Therefore we cannot ignore this contribution in the assessments of deposition of acidifying pollutants. The highest fluxes of cloud deposition are assigned to mountainous regions in Northern Sweden, while the highest fog deposition is calculated at a location in Jönköpings County, on the Sydsvenska höglandet.