Surface drag coefficient dependence on the aerodynamic roughness of the sSea

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Analysis of logarithmic wind profiles measured to a height of 10 m on the south shore of Long Island indicates three distinct classes of aerodynamic roughness of the sea surface. On the basis of the roughness Reynolds number, defined as u/sub */z/sub 0//v, where u/sub */ is the surface friction velocity, z/sub 0/ the roughness length, and v the kinematic viscosity, the sea surface is aerodynamically smooth for u/sub */z/sub 0//v less than or equal to 0.15. There is a moderately rough transition regime for 0.15 < u/sub */z/sub 0//v < 4, the surface becoming fully rough for the roughness Reynolds numbers exceeding 4. The surface drag coefficient C/sub D/sub 6// obtained from the mean wind speed at a height of 6 m is found to vary from 0.75 x 10/sup -3/ for an aerodynamically smooth sea surface to 1.9 x 10/sup -3/ for a fully rough surface. Moderately rough flow conditions yielded a C/sub D/sub 6// of 1.15 x 10/sup -3/. Wind profile measurements from two other offshore or coastal sites showed good agreement. Mean wind speeds ranged from 3.5 to 10 m s/sup -1/. The drag coefficient did not vary with mean wind speed but was weakly dependentmore » on the atmospheric stability. Charnock's constant ..cap alpha.. in z/sub 0/ = au/sub *//sup 2//g, where g is the gravitational acceleration, was found to have a mean value of 0.016 for moderately rough conditions and 0.072 for fully rough conditions.« less