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Edward C. Monahan

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Edward C. Monahan

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The variation of oceanic whitecap coverage with wind speed was determined from the analysis of group, of five or more photographs taken, along with measurements of wind speed and air and water temperatures, during each of 71 observation periods at locations on the Atlantic Ocean and adjacent salt water bodies. The fraction of the sea surface covered by whitecaps is always <0.1% for wind speeds V<4 m sec−1. For winds from 4 to 10 m sec−1 the maximum percentage of the sea surface covered by whitecaps is given by W=0.00135 V 3.4.

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Edward C. Monahan and IognáidÓ Muircheartaigh

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The optimal power-law expression for the dependence of oceanic whitecap coverage fraction W on 10 m elevation wind speed U as determined by ordinary least squares fitting applied to the combined whitecap data sets of Monahan (1971) and Toba and Chaen (1973), is W = 2.95 × 10−6 U 3.52. The equivalent expression, obtained by the application of the technique of robust biweight fitting, is W = 3.84 × 10−6 U 3.41. These expressions fit the combined data set better than any of the previously published equations.

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Edgar L. Andreas and Edward C. Monahan

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In high winds, the sea surface is no longer simply connected. Whitecap bubbles and sea spray provide additional surfaces that may enhance the transfer of any quantity normally exchanged at the air–sea interface. This paper investigates the role that the air bubbles in whitecaps play in the air–sea exchange of sensible and latent heat. Bubble spectra published in the literature suggest that an upper bound on the volume flux of bubbles per unit surface area in Stage A whitecaps is 3.8 × 10−2 m3 m−2 s−1. This estimate, a knowledge of whitecap coverage as a function of wind speed, and microphysical arguments lead to estimates of the sensible (Q bS) and latent (Q bL) heat fluxes carried across the sea surface by air cycled through whitecap bubbles. Because Q bS and Q bL scale as do the usual turbulent or interfacial fluxes of sensible and latent heat, these bubble fluxes can be represented simply as multiplicative factors f S and f L, respectively, that modulate the 10-m bulk transfer coefficients for sensible (C H10) and latent (C E10) heat. Computations show, however, that even the upper bounds on the bubble heat fluxes are too small to be measured. For 10-m wind speeds up to 20 m s−1, f S and f L are always between 1.00 and 1.01. For a 10-m wind speed of 40 m s−1, f S and f L are still less than 1.05. Consequently, for wind speeds up to 40 m s−1—a range over which it should be safe to extrapolate the models of sea surface physics used here—the near-surface air heated and moistened in whitecap bubbles seems incapable of contributing measurably to air–sea heat and moisture transfer.

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Edward C. Monahan and I. G. O'Muircheartaigh

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Edward C. Monahan and Iognaid G. O'Muircheartaigh

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Edward C. Monahan and David K. Woolf

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Kaylan Randolph, Heidi M. Dierssen, Alejandro Cifuentes-Lorenzen, William M. Balch, Edward C. Monahan, Christopher J. Zappa, Dave T. Drapeau, and Bruce Bowler

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Traditional methods for measuring whitecap coverage using digital video systems mounted to measure a large footprint can miss features that do not produce a high enough contrast to the background. Here, a method for accurately measuring the fractional coverage, intensity, and decay time of whitecaps using above-water radiometry is presented. The methodology was developed using data collected in the Southern Ocean under a wide range of wind and wave conditions. Whitecap quantities were obtained by employing a magnitude threshold based on the interquartile range of the radiance or reflectance signal from a single channel. Breaking intensity and decay time were produced from the integration of and the exponential fit to radiance or reflectance over the lifetime of the whitecap. When using the lowest magnitude threshold possible, radiometric fractional whitecap coverage retrievals were consistently higher than fractional coverage from high-resolution digital images, perhaps because the radiometer captures more of the decaying bubble plume area that is difficult to detect with photography. Radiometrically obtained whitecap measurements are presented in the context of concurrently measured meteorological (e.g., wind speed) and oceanographic (e.g., wave) data. The optimal fit of the radiometrically estimated whitecap coverage to the instantaneous wind speed, determined using robust linear least squares, showed a near-cubic dependence. Increasing the magnitude threshold for whitecap detection from 2 to 4 times the interquartile range produced a wind speed–whitecap relationship most comparable to the concurrently collected fractional coverage from digital imagery and previously published wind speed–whitecap parameterizations.

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