On the effective roughness length for use in numerical three-dimensional models

Abstract

We present analytical and numerical calculations of the effective roughness length (ERL) over a flat surface with varying roughness elements, for use in large-scale models. It is shown that ERL is mostly determined by the roughest elements present inside the averaging domain and that, more surprisingly, the ERL increases as the first level of the numerical model gets closer to the surface and its altitude approaches the value of the largest local roughness length. This effect further increases the drag coefficient, in addition to the well-known increase due to the lowering of the first model level.

Comments and further analysis on effective roughness lengths for use in numerical three-dimensional models

“…These results could be used to specify effective roughness lengths for use in large-scale models. Some of our conclusions differ significantly from those reached recently by Andre and Blondin (1986).…”

Average fluxes from heterogeneous vegetated regions

“…Figures 8, 10, 12 and 14 show that regionally averaged momentum flux is increased by local advection, which is in qualitative agreement with studies by Andre and Blondin (1986), Taylor (1987), Mason (1988) and Claussen (1991). The physical explanation of the momentum flux increase by local advection is that in the case of a variable surface roughness, the rougher zone is exposed to higher wind speeds and the smoother zone to lower wind speeds relative to the adjusted flow.…”

Surface drag and turbulence over an inhomogeneous land surface

“…In this case, some method of averaging the roughness lengths of different types of surface is required. This problem of calculating an effective roughness length for an area consisting of a patchwork of surfaces with different roughness lengths has recently been discussed by Andre and Blondin (1986), Taylor (1987) and Mason (1988). Taylor (1987) and Mason (1988) have suggested somewhat different ways of forming the average of surface roughness lengths to give an estimate of the effective roughness length of the inhomogeneous surface.…”