On the Determination and Characteristics of Effective Roughness Length for Heterogeneous Terrain

A new method is proposed for the effective roughness length (ERL) in heterogeneous terrain basedon the principle of equalization of momentum or heat fluxes calculated by the drag coefficient parameter-ization scheme used in the ECMWF numerical model. Compared with the area-weighted logarithmicallyaveraged ERL (drag coefficient), the newly calculated ERL (drag coefficient) is about 40% (16%) largerwith a roughness step of 2.3. These differences reach their maximum values when the ratio of smooth torough surface is 60% to 40%. Since the determination by this method is not sensitive to the atmosphericstratification, it is suitable for use in climate models.