Momentum, heat and water vapour transfer to and from natural and artificial surfaces

Recent atmospheric and wind tunnel data on the heat, mass and momentum transfer above natural and artificial surfaces indicate a general increase in the quantity kB −1 (= In(z o /z p ), where z p is the roughness length appropriate to the quantity p ) between aerodynamically smooth and fully rough flow. Below a roughness Reynolds number, u*z o /v , of about 100, kB −1 appears to be a single valued function of u*z o /v , closely approximated by the assumption of a molecular sub‐layer for heat and water vapour fluxes. Beyond u*z o /v = 100, there is no unique value of kB −1 , but rather the observations suggest that the behaviour falls into either of two categories. For surfaces comprised of arrays of widely spaced solid roughness elements, kB −1 continues to increase (with some evidence for a plateau being reached by about u*z o /v = 1,000, when kB −1 ≃ 10), whilst for surfaces comprised of randomly distributed fibrous roughness elements, kB −1 gradually decreases with increasing roughness Reynolds number beyond about 100.