DRAG AND DRAG PARTITION ON ROUGH SURFACES

An analytic treatment of drag and drag partition on rough surfaces is given. The aims are to provide simple predictive expressions for practical applications, and to rationalize existing laboratory and atmospheric data into a single framework. Using dimensional analysis and two physical hypotheses, theoretical predictions are developed for total stress (described by the square root of the canopy drag coefficient), stress partition (described by the ratio tau(S)/tau of the streSS tau(S) on the underlying ground surface to total stress tau), zero-plane displacement and roughness length. The stress partition prediction is the simple equation tau(S)/tau = 1/(1 + betalambda), where beta = C(R)/C(S) is the ratio of element and surface drag coefficients. This prediction agrees very well with data and is free of adjustable constants. Other predictions also agree well with a range of laboratory and atmospheric data.