A scheme for drag partition over rough surfaces

We present a drag partition scheme for surfaces of various roughness densities. The total drag is partitioned into three components: a pressure drag, a skin drag due to momentum transfer to the surfaces of roughness elements and a surface drag due to momentum transfer to the ground surface. We introduce an effective frontal area index to characterize the mutual sheltering effect of roughness elements on drag partition and propose a method for the calculation of the drag components. The drag partition scheme is then applied to the calculation of bulk quantities such as roughness length and zero-plane displacement height. The results are compared with wind-tunnel measurements. Roughness length is found to increase with frontal area index λλ to a maximum at about λ=0.2λ=0.2, and then decreases with λλ. This finding is in agreement with observations. The drag partition scheme has applications to studying scalar exchange in boundary layers over rough surfaces, such as urban boundary layers.