Effect of Swell on Wind Stress for Light to Moderate Winds

Abstract

AbstractBuoy observations from a 1999 Gulf of Mexico field program (GOM99) are used to investigate the relationships among friction velocityu*, wind speedU, and amount of swell present. AU–u*seaparameterization is developed for the case of pure wind sea (denoted byu*sea), which is linear inUover the range of available winds (2–16 m s−1). The curve shows no sign of an inflection point near 7–8 m s−1as suggested in a 2012 paper by Andreas et al. on the basis of a transition from smooth to rough flow. When observations containing more than minimal swell energy are included, a differentU–u*equation forU< 8 m s−1is found, which would intersect the pure wind-sea curve about 7–8 m s−1. These two relationships yield a bilinear curve similar to Andreas et al. with an apparent inflection near 7–8 m s−1. The absence of the inflection in the GOM99 experiment pure wind-sea curve and the similarity of the GOM99 swell-dominated low wind speed to Andreas et al.’s low wind speed relationship suggest that the inflection may be due to the effect of swell and not a flow transition. Swell heights in the range of only 25–50 cm may be sufficient to impact stress at low wind speeds.