Open AccessA threshold for wind‐wave growth
Author(s) -
Donelan Mark A.,
Plant William J.
Publication year - 2009
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2008jc005238
Subject(s) - wind speed , wind stress , fetch , wind wave , roughness length , gravity wave , surface wave , capillary wave , wind profile power law , mechanics , atmospheric sciences , wind gradient , infragravity wave , log wind profile , surface roughness , meteorology , geology , optics , physics , wave propagation , longitudinal wave , mechanical wave , oceanography , quantum mechanics
Measurements in a closed, recirculating wind‐wave tank using variable wind speeds showed that wind waves in the gravity‐capillary range exhibit a threshold in their growth. Surface wave height variance spectral densities in the wave number domain were measured for gravity‐capillary waves using both radar backscatter and a wavelet transform technique applied to a laser probe. The measurements showed that when the wind speed was slowly ramped up, a threshold wind speed or friction velocity was required to produce waves. Turning the wind on suddenly showed that the wind stress did not grow as rapidly as the wind since the surface waves supporting the stress grew relatively slowly. Changing water temperature or current in the water caused a pronounced change in the wind speed threshold but not in the friction velocity threshold. Changes in fetch of as much as a factor of 2 had no discernible effect on the thresholds. The results imply that wind speed, being a condition imposed on the air‐water interface, causes wave growth, while friction velocity, being a result of air‐water interactions, is closely related to surface roughness, hence radar cross section, and changes during wave growth.