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- Author or Editor: Philip D. Nightingale x
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Abstract
The emission and reflection properties of a rough sea surface are investigated, with particular emphasis on the wavelengths and viewing geometry relevant to the Along Track Scanning Radiometer. The authors start from Fresnel's equations for a flat water surface and calculate the effect of changing sea state on direct emissivity and reflectivity. The authors then investigate the role of surface-emitted surface-reflected (SESR) radiation, which enhances emissivity at high wind speeds. The effect of foam and whitecaps at high wind speeds is referred to briefly in the appendix but essentially remains an unknown quantity.
Radiative transfer calculations that employ emissivity models also have to consider the reflection of downwelling radiance from the atmosphere. Although energy conservation requires that reflectivity is 1 minus emissivity, the variability of the sky brightness with zenith demands that one consider also the angular distribution of the reflected radiance. Additionally, the extended statistical model is used to investigate what one may call surface-reflected surface-reflected (SRSR) radiance.
It was found that the SESR and SRSR effects, and the reflection of the anisotropic sky radiance, together act to cancel the first-order effect of reduced emissivity with increasing wind speed, such that the approximation of constant emissivity and specular reflection is essentially valid for the Along Track Scanning Radiometer viewing geometry. Finally, parameterizations are derived for the variable emission and reflection of a rough sea surface that are suitable for fast radiative transfer models.
Abstract
The emission and reflection properties of a rough sea surface are investigated, with particular emphasis on the wavelengths and viewing geometry relevant to the Along Track Scanning Radiometer. The authors start from Fresnel's equations for a flat water surface and calculate the effect of changing sea state on direct emissivity and reflectivity. The authors then investigate the role of surface-emitted surface-reflected (SESR) radiation, which enhances emissivity at high wind speeds. The effect of foam and whitecaps at high wind speeds is referred to briefly in the appendix but essentially remains an unknown quantity.
Radiative transfer calculations that employ emissivity models also have to consider the reflection of downwelling radiance from the atmosphere. Although energy conservation requires that reflectivity is 1 minus emissivity, the variability of the sky brightness with zenith demands that one consider also the angular distribution of the reflected radiance. Additionally, the extended statistical model is used to investigate what one may call surface-reflected surface-reflected (SRSR) radiance.
It was found that the SESR and SRSR effects, and the reflection of the anisotropic sky radiance, together act to cancel the first-order effect of reduced emissivity with increasing wind speed, such that the approximation of constant emissivity and specular reflection is essentially valid for the Along Track Scanning Radiometer viewing geometry. Finally, parameterizations are derived for the variable emission and reflection of a rough sea surface that are suitable for fast radiative transfer models.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
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