The transition from downward to upward air-sea momentum flux in swell-dominated light wind conditions

Ulf Högström, Erik Sahlée, Ann Sofi Smedman, Anna Rutgersson, Erik Nilsson, Kimmo K. Kahma, William M Drennan

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

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 = 4 m s-1. For the lowest observed wind speeds U < 2.4 m s-1, 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.

Original languageEnglish (US)
Pages (from-to)2579-2588
Number of pages10
JournalJournal of the Atmospheric Sciences
Volume75
Issue number8
DOIs
StatePublished - Aug 1 2018

Fingerprint

swell
momentum
wind velocity
air
momentum transfer
fetch
significant wave height
nonlinear wave
laminar flow
wave energy
hysteresis
sea
surface water
simulation
energy
experiment

Keywords

  • Air-sea interaction
  • Atmosphere-ocean interaction
  • Momentum

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

The transition from downward to upward air-sea momentum flux in swell-dominated light wind conditions. / Högström, Ulf; Sahlée, Erik; Smedman, Ann Sofi; Rutgersson, Anna; Nilsson, Erik; Kahma, Kimmo K.; Drennan, William M.

In: Journal of the Atmospheric Sciences, Vol. 75, No. 8, 01.08.2018, p. 2579-2588.

Research output: Contribution to journalArticle

Högström, Ulf ; Sahlée, Erik ; Smedman, Ann Sofi ; Rutgersson, Anna ; Nilsson, Erik ; Kahma, Kimmo K. ; Drennan, William M. / The transition from downward to upward air-sea momentum flux in swell-dominated light wind conditions. In: Journal of the Atmospheric Sciences. 2018 ; Vol. 75, No. 8. pp. 2579-2588.
@article{11d1a1ef37d74ce4b1e9cf9f1054106e,
title = "The transition from downward to upward air-sea momentum flux in swell-dominated light wind conditions",
abstract = "Fifteen hours of consecutive swell data from the experiment Flux, {\'e}tat de la Mer, et T{\'e}l{\'e}d{\'e}tection 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 = 4 m s-1. For the lowest observed wind speeds U < 2.4 m s-1, 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.",
keywords = "Air-sea interaction, Atmosphere-ocean interaction, Momentum",
author = "Ulf H{\"o}gstr{\"o}m and Erik Sahl{\'e}e and Smedman, {Ann Sofi} and Anna Rutgersson and Erik Nilsson and Kahma, {Kimmo K.} and Drennan, {William M}",
year = "2018",
month = "8",
day = "1",
doi = "10.1175/JAS-D-17-0334.1",
language = "English (US)",
volume = "75",
pages = "2579--2588",
journal = "Journals of the Atmospheric Sciences",
issn = "0022-4928",
publisher = "American Meteorological Society",
number = "8",

}

TY - JOUR

T1 - The transition from downward to upward air-sea momentum flux in swell-dominated light wind conditions

AU - Högström, Ulf

AU - Sahlée, Erik

AU - Smedman, Ann Sofi

AU - Rutgersson, Anna

AU - Nilsson, Erik

AU - Kahma, Kimmo K.

AU - Drennan, William M

PY - 2018/8/1

Y1 - 2018/8/1

N2 - 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 = 4 m s-1. For the lowest observed wind speeds U < 2.4 m s-1, 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.

AB - 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 = 4 m s-1. For the lowest observed wind speeds U < 2.4 m s-1, 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.

KW - Air-sea interaction

KW - Atmosphere-ocean interaction

KW - Momentum

UR - http://www.scopus.com/inward/record.url?scp=85050984115&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85050984115&partnerID=8YFLogxK

U2 - 10.1175/JAS-D-17-0334.1

DO - 10.1175/JAS-D-17-0334.1

M3 - Article

VL - 75

SP - 2579

EP - 2588

JO - Journals of the Atmospheric Sciences

JF - Journals of the Atmospheric Sciences

SN - 0022-4928

IS - 8

ER -