Albedo of the Sea Surface

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  • 1 Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543
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Abstract

An experimental study of the albedo of the sea surface for shortwave solar radiation has been carried out on a fixed platform. Fifteen-minute totals of upward and downward irradiances were recorded continuously for four months over a wide range of atmospheric and sea conditions. The resulting albedo values, the ratio of upward to downward irradiance, are expressed in terms of a particularly convenient pair of parameters, sun altitude and atmospheric transmittance (T). The latter is defined as the ratio of observed downward irradiance to the irradiance at the top of the atmosphere and has not been used before in describing albedo. Examples of albedo values are 0.061±0.005 for heavily overcast skies (0.0<T≤0.1), indicating isotropic radiance distribution, and a range for clear skies (T>0.65) of 0.03 for high sun to as large as 0.45 at sun altitudes <10°. The uncertainty in the values is less than 7% for sun altitudes >25° and increases to 25% for very low sun attitudes. The effect of wind, through surface roughness, is shown to be small but predictable. Effects of whitecaps are not noticeable at wind speeds up to 30 kt, the highest observed in the study.

Application of the results is made to climatological studies of the absorption of solar energy by the surface waters of the ocean. Monthly average albedos, are calculated for the Atlantic Ocean to compare with Budyko’s latitudinally dependent values, and it is shown that although the sets of results agree within 10% at latitudes up to 40°, there are discrepancies at higher latitudes as high as 100%. Finally it is shown with climatological albedo values calculated from the results of this study, that the accuracy of climatological estimates of solar energy absorbed in the ocean are now limited by the accuracy of climatological estimates of downward irradiance.

Abstract

An experimental study of the albedo of the sea surface for shortwave solar radiation has been carried out on a fixed platform. Fifteen-minute totals of upward and downward irradiances were recorded continuously for four months over a wide range of atmospheric and sea conditions. The resulting albedo values, the ratio of upward to downward irradiance, are expressed in terms of a particularly convenient pair of parameters, sun altitude and atmospheric transmittance (T). The latter is defined as the ratio of observed downward irradiance to the irradiance at the top of the atmosphere and has not been used before in describing albedo. Examples of albedo values are 0.061±0.005 for heavily overcast skies (0.0<T≤0.1), indicating isotropic radiance distribution, and a range for clear skies (T>0.65) of 0.03 for high sun to as large as 0.45 at sun altitudes <10°. The uncertainty in the values is less than 7% for sun altitudes >25° and increases to 25% for very low sun attitudes. The effect of wind, through surface roughness, is shown to be small but predictable. Effects of whitecaps are not noticeable at wind speeds up to 30 kt, the highest observed in the study.

Application of the results is made to climatological studies of the absorption of solar energy by the surface waters of the ocean. Monthly average albedos, are calculated for the Atlantic Ocean to compare with Budyko’s latitudinally dependent values, and it is shown that although the sets of results agree within 10% at latitudes up to 40°, there are discrepancies at higher latitudes as high as 100%. Finally it is shown with climatological albedo values calculated from the results of this study, that the accuracy of climatological estimates of solar energy absorbed in the ocean are now limited by the accuracy of climatological estimates of downward irradiance.

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