The 13th Symposium on Boundary Layers and Turbulence

### P2A.8ESTIMATION OF FORM DRAG USING SATELLITE DATA

Job W. Verkaik, Wageningen Agricultural Univ, Wageningen, The Netherlands

Form drag on isolated obstacles usually accounts for a large portion of the total surface stress, even if the area-fraction of the obstacles is very small. Simple form drag models can deal with height and length scales, associated with only one type of obstacle. More sophisticated models need detailed information on the distribution of the obstacles, their height, width, and porosity. These are essential to compute the roughness density, the main parameter describing the form drag. The roughness density (Ai) is the sum of the silhouette areas (width x height) of all obstacles on the surface, divided by the total area under consideration. From land-use information, derived from Landsat images, roughness densities have been estimated using a very simple algorithm. To all land-use types with significant vertical extent, a height and porosity is assigned. The width of the obstacles is prescribed by the resolution of the satellite images (30 m). The roughness density is computed for 30 degrees wide wind direction sectors for the Cabauw site and the meteorological site of the Wageningen Agricultural University (WAU). This was done by summing the product of the area-fractions, height and porosity for the different land-use types within a 10 km radius of the measuring site. The resulting roughness densities are compared with roughness lengths derived from the standard deviation of horizontal wind speed, which yields an effective roughness length, representative for an area of about 5 km. A strong correlation is found between this roughness density and the effective roughness length (z0eff), suggesting a very simple relationship between z0eff and Ai: ln (z0eff / z0s) = a * Ai. Here z0s is the roughness length of the surface without obstacles. This method is successful for open terrain where the roughness elements can be considered as isolated obstacles. At the WAU-site, where in some sectors large patches of obstacles, densely packed together, are present, this relation does not hold. The mutual sheltering effect of the obstacles becomes important