Dry Deposition in Complex Terrain under Special Consideration of the Heterogeneous Characteristics of the Surface
Within the framework of this project numerical modules were devolped that allow to determine the dry deposition and biogen emission of trace gases as well as the required fluxes of sensible and latent heat in complex terrain with heterogeneous surface characteristics. In doing so, the mosaic approach (Mölders and Raabe 1996) and the explicit subgrid scheme (Mölders et al. 1996 ) that describe the exchange of energy and water between the atmosphere and the system soil-vegetation-water were adapted for the treatment of trace gases (Hinneburg and Mölders 1999, 2000, Mölders et al. 2000, Hinneburg 2000). Moreover, these schemes were vertically extended within the lowest model layer. This finer horizontal and vertical subgrid allows to consider the antropogenic emissions in a more realistic location within the grid-cell of the coarse grid. Thus, the subgrid heterogeneity of emission and thus concentration distributions can be considered more realistically. This vertical subgrid also allows to overcome the constant flux assumption for a layer of several meter thickness. Note that in most mesoscale Chemical Transport Models the first layer above ground is about 30 m thick or thicker (e.g., Mölders et al. 1994). The constant flux assumption is not valid for such large a layer due to chemical reactions within this layer (see e.g., Spindler et al. 1996). Thus, using the vertical subgrid and calculating the chemical reactions on each layer of the vertical subgrid and in each grid cell of the horizontal subgrid allows to consider the strongly changing profiles of trace gases. In the lowest subgrid layer, which is only several centimeters thick, dry deposition is determined by the constant flux approach as suggested, for instance, by Kramm et al. (1994).
Another important advantage of the explicit subgrid is that the subgrid scale surface heterogeneity can be taken into account more realistically. Since dry deposition and biogenic emissions strongly depend on the undelaying surface and thus vegetation, it has to be assumed that these processes can be simulated more realistic by applying the explicit subgrid strategy than by assuming one homogeneous vegetation for the entire grid-cell only. Total dry deposition and biogenic emission within a grid-cell should be improved as compared to the formerly used assumption of determining these quantities by assuming that the dominant land-use type is representative for the entire grid-cell.
For the sake of simplicity, the triade NO-NO2-O3 was dealt with in the testing phase. The results obtained by applying the explicit subgrid scheme within the framework of a chemical model show that dry deposition may be strongly affected by subgrid-scale surface heterogeneity.
The deposition modules were developed within the framework of GESIMA. In close cooperation with the institute for tropospheric research, the modules were now also implemented into METRAS/MUSCAT. This implementation showed that it is easy to switch off modules which are not needed (e.g., because the implementor's model includes already a module with this function) or to use other chemical modules within the framework of the deposition module package developed within the framework of this project. The module is also available to other German modellers of the TFS-program. It can be downloaded from the TFS-data-bank. The report (Mölders et al. 2000) provides help for implementation and can be downloaded as a zipped postscript-file.
Furthermore, numerical experiments were carried out within the framework of model validation to systematically quantify the connections between the energy and water cycle in terrain with heterogeneous surface characteristics. Sensitivity studies were preformed on the general impact of different degree of heterogeneity on the water and energy fluxes (Friedrich and Mölders 1998, Friedrich et al. 2000).
Poster (pdf-file) presented at the AGU meeting in Washington 2002
Example of dry deposition of ozone south east of Berlin on July 27 1994
Acknowledgements:
This project was funded within the framework of the Tropospheric Research Program (TFS) by the BMBF under contract LT2.D.2. We wish to express our thanks to R. Wolke and E. Renner (IfT) for help in testing the dry depostion module package also within the framework of a model different from GESIMA.
Co-workers:
Dr. Detlef Hinneburg, Dr. H.-D. Haenel, Dipl. Met. K. Friedrich, stud. met. C. Stolle
Principal investigator:
Dr. habil. Nicole Mölders