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  • Author or Editor: Hans C. Graber x
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Charles L. Vincent
,
Hans C. Graber
, and
Clarence O. Collins III

Abstract

Buoy observations from a 1999 Gulf of Mexico field program (GOM99) are used to investigate the relationships among friction velocity u *, wind speed U, and amount of swell present. A Uu *sea parameterization is developed for the case of pure wind sea (denoted by u *sea), which is linear in U over the range of available winds (2–16 m s−1). The curve shows no sign of an inflection point near 7–8 m s−1 as 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 different Uu * equation for U < 8 m s−1 is 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.

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Henry Potter
,
Hans C. Graber
,
Neil J. Williams
,
Clarence O. Collins III
,
Rafael J. Ramos
, and
William M. Drennan

Abstract

One of the scientific objectives of the U.S. Office of Naval Research–sponsored Impact of Typhoons on the Ocean in the Pacific (ITOP) campaign was improved understanding of air–sea fluxes at high wind speeds. Here the authors present the first-ever direct measurements of momentum fluxes recorded in typhoons near the surface. Data were collected from a moored buoy over 3 months during the 2010 Pacific typhoon season. During this period, three typhoons and a tropical storm were encountered. Maximum 30-min sustained wind speeds above 26 m s−1 were recorded. Data are presented for 1245 h of direct flux measurements. The drag coefficient shows evidence of a rolloff at wind speeds greater than 22 m s−1, which occurred during the passage of a single typhoon. This result is in agreement with other studies but occurs at a lower wind speed than previously measured. The authors conclude that this rolloff was caused by a reduction in the turbulent momentum flux at the frequency of the peak waves during strongly forced conditions.

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