The Transition from Downward to Upward Air–Sea Momentum Flux in Swell-Dominated Light Wind Conditions

Fifteen hours of consecutive swell data from the experiment Flux, État de la Mer, et Télédétection en Condition de Fetch Variable (FETCH) in the Mediterranean show a distinct upward momentum flux. The characteristics are shown to vary systematically with wind speed. A hysteresis effect is found for wave energy of the wind-sea waves when represented as a function of wind speed, displaying higher energy during decaying winds compared to increasing winds. For the FETCH measurements, the upward momentum transfer regime is found to begin for wind speeds lower than about U 5 4ms21 . For the lowest observed wind speeds U , 2.4 m s21 , the water surface appears to be close to dynamically smooth. In this range almost all the upward momentum flux is accomplished by the peak in the cospectrum between the vertical and horizontal components of the wind velocity. It is demonstrated that this contribution in turn is linearly related to the swell significant wave height Hsd in the range 0.6 , Hsd , 1.4 m. For Hsd , 0.6 m, the contribution is zero in the present dataset but may depend on the swell magnitude in other situations. It is speculated that the observed upward momentum flux in the smooth regime, which is so strongly related to the cospectral peak at the dominant swell frequency, might be caused by the recirculation mechanism found by Wen and Mobbs in their numerical simulation of laminar flow of a nonlinear progressive wave at low wind speed

Sahlée, ErikUppsala universitet,Luft-, vatten- och landskapslära(Swepub:uu)ersah499 (author)
Smedman, Ann-Sofi,1942-Uppsala universitet,Luft-, vatten- och landskapslära(Swepub:uu)annssmed (author)
Rutgersson, Anna,1971-Uppsala universitet,Luft-, vatten- och landskapslära(Swepub:uu)aru27058 (author)
Nilsson, Erik O.,1983-Uppsala universitet,Luft-, vatten- och landskapslära(Swepub:uu)erini203 (author)
Kahma, KimmoFMI (author)
Drennan, WilliamUniversity of Miami, RSMAS (author)
Uppsala universitetLuft-, vatten- och landskapslära (creator_code:org_t)

By the author/editor: Högström, Ulf; Sahlée, Erik; Smedman, Ann-Sof ...; Rutgersson, Anna ...; Nilsson, Erik O. ...; Kahma, Kimmo; show more...; Drennan, William; show less...

About the subject

NATURAL SCIENCES: NATURAL SCIENCES; and Earth and Relate ...; and Meteorology and ...

Kungliga biblioteket hanterar dina personuppgifter i enlighet med EU:s dataskyddsförordning (2018), GDPR. Läs mer om hur det funkar här.
Så här hanterar KB dina uppgifter vid användning av denna tjänst.