An alternative approach to sea surface aerodynamic roughness

Abstract

The dependence of sea surface roughness on the wave parameters was investigated using measurements from the Humidity Exchange over the Sea Main Experiment (HEXMAX). This site was 18 m of water depth and was 9 km offshore in the North Sea. We examined the relationships among wave height (Hs), friction velocity (u*) and wave age (cp/u* or cp/U10N) using regression analysis and explored the relationship between dimensionless variables u*(gTp)−1 and U10N2(gHs)−1 by dimensional analyses, where g is the acceleration due to gravity (for which we use 9.81 m s−2); Tp is period of the waves at the peak of the wave spectrum, defined as Tp = 2πcp/g where cp is the phase velocity of the peak wave spectrum; and U10N is horizontal wind speed at 10‐m height under the neutral condition. When the derived results are combined with the neutral wind profile equation, we find that the relationship among friction velocity (u*), neutral 10‐m wind speed (U10N) and phase velocity (cp) is u* = a2/3U10N4/3/cp1/3 where a is an empirical constant, and a = 6.7 × 10−3 for this site. The relationship between aerodynamic roughness length (z0) and wave age and the relationship between the neutral drag coefficient (CDN) and wave age are derived accordingly. The present parameterization schemes were tested using an independent data set, and they work in moderate wind speed.