Spectral characterization of fine‐scale wind waves using shipboard optical polarimetry

Abstract

Fine-scale sea surface waves are of profound importance to a number of air-sea interaction processes. Due to a number of reasons, there exists a great degree of difficulty in obtaining quality in situ observations of these waves. This paper presents the application of a shipboard wave-sensing method toward the following quantifications: regime-specific contribution to sea surface slope and sensitivity to wind speed increases. Measurements were made via polarimetric camera, resolving waves with wavelengths ranging from 0.21 to 0.003 m (30 rad/m < k < 1750 rad/m). The gravity-capillary regime was found to contribute the bulk of mean square slope during stationary wind conditions and supply the majority of mean square slope growth during periods of increasing wind speed. Capillary waves were found to contribute approximately 5% of the overall surface roughness. Furthermore, capillary waves were found to be the least sensitive to increases in wind speed. This implies that such waves saturate at low wind speeds (% 3 m/ s) and slow wind speed increases (% 0.02 m=s 2 ). The slight roughness contribution from capillary waves and significant contribution from gravity-capillary waves offers insight for scientists in the remote sensing field and important information for the formation of new wave models.

Water surface slope spectra in nearshore and river mouth environments

“…The re-processed open-ocean results of [9] (figure 4) are also given for consideration; however, the inside-inlet values appear to be closest to those results. For higher wind speed values, a gravity-capillary regime peak (≈  rad/m) appears for both outside and insideinlet cases.…”

“…The spectral shape of the near-capillary regime (the high-wavenumber portion of the gravity-capillary regime) is shown in figure 5. The parameter n (i.e., B(k) ∼ k n ) is plotted against wind speed U  rather than wind stress magnitude τ so that the results from past observations [6,9] can be shown in their native form in order to avoid invoking a wind stress parameterization. Whereas the spectra from [9] show good agreement with those of [6], those observed for this work are generally steeper, falling off like k − .…”

“…The parameter n (i.e., B(k) ∼ k n ) is plotted against wind speed U  rather than wind stress magnitude τ so that the results from past observations [6,9] can be shown in their native form in order to avoid invoking a wind stress parameterization. Whereas the spectra from [9] show good agreement with those of [6], those observed for this work are generally steeper, falling off like k − . None of the spectra shown have a discernable steepening or shallowing trend with wind speed, though there is a regime between U  = 7 m/s and U  = 9 m/s for which the scatter in outside-inlet spectral fall-off is greater, with some cases exhibiting k −. behavior.…”

“…The U  reported in this work is the 10-m, neutral wind speed calculated using the eddy covariance techniques, while the wind stress, τ, comes directly from the along-and across-wind covariance components of the Reynolds stress (equation 1). transformed onto the ocean surface in order to maintain a uniform scale across the image under a timevarying camera look angle [9].…”

“…For this reason, techniques that are able to retrieve synoptic, spatial measurements of wave structure are preferred for the study of short ocean waves [6]. The polarimetric slope sensing (PSS) technique is an optical method designed to accomplish such a task [7]; it has been used successfully to acquire short-scale wave structure from aboard a moving vessel [8,9]. This has the advantage of adapting this relatively new field-ready technology to heterogeneous ocean surfaces where both spatial and temporal variability are first order concerns.…”