The ‘roughness’ of the sea surface in light winds

Abstract

If above some surface one observes under neutral conditions a logarithmic velocity distribution, it is possible to assign a ‘roughness length’ z0 to the surface using the wellhyphen;known ‘wall layer’ law (see, for example, Monin and Yaglom [1971]): where u* = 7lpar;τ0/ρ)1/2 is friction velocity, k is Karman's constant, and z0 is roughness length. It is in this manner that the roughness of the sea surface has been determined in many observations, notably those summarized by Roll [1965] and those published by Sheppard et al. [1972]. We see that z0 is a parameter describing the flow in the universal (logarithmic) portion of the wall layer, even if its value is determined by processes close to the wall, either in a viscous sublayer or in what one might call a ‘rough sublayer,’ within which the flow is irregular. These processes establish where the edge of the sublayer is and what the velocity is there, providing a lower boundary condition for (1).