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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C05022, doi:10.1029/2007JC004453, 2008

Observed physical and environmental causes of scatter in whitecap coverage values in a fetch-limited coastal zone

Adrian H. Callaghan

Department of Earth and Ocean Sciences, National University of Ireland, Galway, Ireland


Grant B. Deane

Scripps Institution of Oceanography, University of California, San Diego, California, USA


M. Dale Stokes

Scripps Institution of Oceanography, University of California, San Diego, California, USA


Abstract

Meteorological and oceanographic data along with sea surface images were recorded at a fetch-limited coastal site to investigate the effect of physical and environmental conditions on whitecap coverage W. An automated image-processing technique allowed over 100,000 images to be analyzed for W. Data analysis showed that many processes influenced W. The presence of tidal currents appeared to have augmented values of W under certain specific conditions. Analysis of wave spectra indicated the ubiquitous presence of swell propagating northward. Scatter in W was markedly absent in mixed seas when the spectral intensity of the wind waves was of the same order of magnitude as the spectral intensity of the swell waves. Swell-dominated seas introduced much more scatter in W. W was approximately one third lower in swell-dominated seas than in mixed seas. Specifically, steep swell waves (steepness values greater than 0.01) that propagated opposite to wind wave direction appeared to have reduced W at wind speeds below approximately 7.5 m s−1, but this effect needs more investigation. The coastal site enabled the possibility of investigating physical and environmental effects on W that would otherwise have been more difficult to observe in the open ocean.

Received 17 July 2007; accepted 28 February 2008; published 17 May 2008.

Keywords: whitecaps; fetch limited; waves.

Index Terms: 4504 Oceanography: Physical: Air/sea interactions (0312, 3339); 4217 Oceanography: General: Coastal processes; 4560 Oceanography: Physical: Surface waves and tides (1222); 3360 Atmospheric Processes: Remote sensing.

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Citation: Callaghan, A. H., G. B. Deane, and M. D. Stokes (2008), Observed physical and environmental causes of scatter in whitecap coverage values in a fetch-limited coastal zone, J. Geophys. Res., 113, C05022, doi:10.1029/2007JC004453.