On the effective roughness length for heterogeneous terrain

Several theories and a mesoscale model were applied in the calculation of the effective roughness length Z   eff oover heterogeneous flat terrain having varying proportions of land and water. Rules based on terrain classification tended to give large values of Z   eff o , while the simple logarithmic areal average of local roughness lengths gave small values compared with the model output. The most recent theories were quite close to the model results. These theories were then used to produce a map of Z   eff ofor 150 × 150 km 2 grid squares over Finland, based on a detailed knowledge of land use and water coverage.