This review considers theoretical and experimental knowledge of rough-wall turbulent boundary layers, drawing from both laboratory and atmospheric data. The former apply mainly to the region above the roughness sublayer (in which the roughness has a direct dynamical influence) whereas the latter resolve the structure of the roughness sublayer in some detail. Topics considered include the drag properties of rough surfaces as functions of the roughness geometry, the mean and turbulent velocity fields above the roughness sublayer, the properties of the flow close to and within the roughness canopy, and the nature of the organized motion in rough-wall boundary layers. Overall, there is strong support for the hypothesis of wall similarity: At sufficiently high Reynolds numbers, rough-wall and smooth-wall boundary layers have the same turbulence structure above the roughness (or viscous) sublayer, scaling with height, boundary-layer thickness, and friction velocity.

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