Incorporation of a high-roughness lower boundary into a mesoscale model for studies of dry deposition over complex terrain

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For flow over natural surfaces, there exists a roughness sublayer within the atmospheric surface layer near the boundary. In this sublayer (typically 50 z0 deep in unstable conditions), the Monin-Obukhov (M-O) flux profile relations for homogeneous surfaces cannot be applied. We have incorporated a modified form of the M-O stability functions (Garratt, 1978, 1980, 1983) in a mesoscale model to take account of this roughness sublayer and examined the diurnal variation of the boundary-layer wind and temperature profiles with and without these modifications. We have also investigated the effect of the modified M-O functions on the aerodynamic and laminar-sublayer resistances associated with the transfer of trace gases to vegetation. Our results show that when an observation height or the lowest level in a model is within the roughness sublayer, neglect of the flux-profile modifications leads to an underestimate of resistances by 7% at the most. Central to the United States and European dry deposition monitoring programs is an inferential technique for calculating the deposition velocity 14t of various trace gases and small particles. Essentially, the method estimates "v~ by applying a 'resistance' model which makes use of measurements of selected meteorological variables (Hicks et aI., 1987). The model needs values of wind speed (u), friction velocity (~), Obukhov length scale (L), air temperature and global radiation at a level just above the zero-plane displacement. One approach to obtaining these values in complex terrain where observations are sparse, and where they are likely to show significant horizontal variation, is to predict them with a prognostic mesoscale numerical model of the atmosphere. However, the presence of a high-roughness lower boundary such as a forest must be taken into account in the mesoscale model by following one of two approaches. The first involves a detailed parameterisation of the mean flow and turbulent fluxes within the forest (e.g., Deardorff, 1978; Garrett, 1983). The second approach is simpler and basically treats the forest as a very rough surface. The first model level is above the forest top, but the presence of the forest is felt by incorporating a zero plane displacement, a relatively large value of the aerodynamic roughness length z0, and modified Monin-Obukhov (M- O) functions to take account of increased mixing arising from flow around the roughness elements (trees). In this study, we have adopted the second and simpler approach.