On the wave age dependence of wind stress over pure wind seas

[1] Data from five recent field campaigns are selected for pure wind sea, deep water, and fully rough flow conditions. The combined data set includes a wide range of wave ages, with high variability in both friction velocity and wave phase speed. These data, which are expected to follow Monin-Obukhov similarity scaling, are used to investigate the influence of wave age on wind stress. The relationship between the dimensionless roughness and inverse wave age is found to be z(o)/sigma = 13.4 (u(*)/c(p))(3.4), where z(o) is the surface roughness length, sigma is the standard deviation of the surface elevation, u(*) is the friction velocity, and c(p) is the wave phase speed at the peak of the spectrum. This relationship, which represents a significant dependence of roughness on wave age, was obtained using a procedure that minimizes the effects of spurious correlation in u(*). It is also shown to be consistent with the wave age relationship derived using an alternate form of the dimensionless roughness, namely, the Charnock parameter z(o)g/u(*)(2), where g is the gravitational constant.