Wind wave spectral observations in Currituck Sound, North Carolina

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Abstract

[1] We examine a set of 1626 high-resolution frequency-direction wind wave spectra and collocated winds collected during a 7-month period at a site in Currituck Sound, North Carolina, in terms of one-dimensional spectral structure and directional distribution functions. The data set includes cases of shore-normal winds in broad-fetch conditions as well as winds oblique to the basin geometry, with all fetches of order 10 km or less. Using equilibrium-range scaling, all one-dimensional spectra have a spectral peak region, an equilibrium range of finite bandwidth following an f−4 slope at slightly higher frequencies, and a high-frequency tail that falls off more rapidly than f−4. For shore-normal winds, spectral peakedness appears to be high and approximately constant for young waves, low and approximately constant for old waves, and steeply graded for intermediate inverse wave ages in the range 1.0 < u10/cp < 1.7. Equilibrium-range bandwidth seems to be narrow for young waves and increases with increasing wave age. Directional distribution functions in shore-normal winds are symmetric about the wind direction, narrow at spectral peaks, and broad at high frequencies with distinct directionally bimodal peaks, consistent with other observations. In oblique-wind cases, directional distribution functions are asymmetric and directionally sheared in spectral peak regions, with peak directions aligned with longer fetch directions. At high frequencies, directional distributions are more nearly symmetric about the wind direction. One-dimensional spectra tend to have reduced spectral peakedness and highly variable equilibrium-range bandwidths in oblique-wind conditions, clearly indicating a more complex balance of source terms in these cases than in the more elementary situation of shore-normal winds. These complications are not without consequence in wave modeling, as any bounded or semibounded lake or estuary will be subject to oblique winds, and current operational models do not deal well with conditions like those we find here.

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