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Richard E. Payne

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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Robert M. Thompson Jr., Steven W. Payne, Ernest E. Recker, and Richard J. Reed

Abstract

Data from a dense network of ship observations are used to study the structure and properties of westward-moving wave disturbances observed in the eastern Atlantic Intertropical Convergence Zone (ITCZ) during Phase III of the GAPP Atlantic Tropical Experiment (GATE). Comparisons are made with similar disturbances found in the ITCZ of the western Pacific. Wave fields are determined by fitting low-order polynomials to the ship data with use of the method of least squares.

The wave structures in the two regions are found to be similar in many respects, the principal difference being in the divergence field and associated vertical motion. Unlike in the Pacific a multi-layer divergence pattern exists in the eastern Atlantic, leading us to hypothesize the existence of three main cloud populations with outflow levels near 800, 500 and 250 mb. The soundings for the Atlantic exhibit lesser parcel instability then the Pacific soundings in agreement with the reduced vigor of the convective cells and the greater tendency for multiple cloud layers. The strongest upward motion (∼150 mb day−1) occurs in and somewhat ahead of the wave trough, as in the Pacific, but at a much lower level (800–700 mb). A secondary maximum appears near 350 mb, where the primary maximum appears in the Pacific. The maximum precipitation rate of 22 mm day−1 is observed in the region of strongest upward motion. The rate decreases to 4 mm day−1 in the region of suppressed convection near the wave ridge. Vertical eddy flux of total heat is largest at the 800 mb level in the wave trough (225 W m−2) and produces cumulus heating and cooling of about 5°C day−1 above and below the maximum, respectively.

A nearly balanced moisture budget for the inner ship array or B-scale area was obtained from the fitted fields when data from both outer and inner ships were employed in the fitting. In particular, two individual waves and the composite or average wave yielded sufficiently accurate budgets to encourage their use in quantitative studies of interactions between synoptic-scale and convective-scale systems. The residual in the heat budget suggests a radiational cooling rate of 0.9°C day−1. The surface energy budget indicates a net radiative flux at the surface of 129 W m−2 of which 106 W m−2 was used for evaporation and 12 W m−2 for sensible heat flux to the atmosphere, leaving 11 W m−2 for heating of the ocean mixed layer. The heat exchange between ocean and atmosphere underwent a pronounced variation with passage of the synoptic disturbances, causing sea surface temperatures to be 0.3°C warmer ahead of the wave troughs than behind. Precipitation rates employed in the budgets were based on radar measurements; surface sensible and latent heat fluxes were computed by the bulk aerodynamic method with use of temperatures, humidities and winds from the booms of four B-scale ships; and net radiation at the surface was obtained from measurements made aboard the same four ships.

The kinetic energy of the waves was provided by the barotropic conversion process (conversion from zonal kinetic energy), the baroclinic conversion being negative and thus a sink for the eddy kinetic energy. Likewise, the generation of eddy available potential energy was negative, implying that latent heat release opposed, rather than contributed, to the wave growth. The described conditions are quite unlike those in the western Pacific ITCZ where condensation heating provides the source for the wave energy and the barotropic conversion constitutes a weak sink.

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