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Michel Legrand, Guy Cautenet, and Jean Claude Buriez

Abstract

The use of the mesoscale model described and qualified in Part I is arranged with radiative transfer codes for the simulation of the thermal infrared response of Meteosat from a Sahelian target. The sensitivity of the satellite response to various atmosphere and surface parameters, either relevant or extraneous to dustiness, is analyzed and physically interpreted throughout the daily cycle, considering especially the thermal impact of the dust at the ground surface. The most significant parameters, according to this criterion of sensitivity, are the amount of dust in the atmosphere and its radiative characteristics, and the ground surface emissivity in the satellite channel. If neglected, the atmospheric water vapor content may be a large source of error for the retrieval of dustiness from the satellite data. The theoretical results are discussed and compared with earlier published experimental work.

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Norman G. Loeb, Frédéric Parol, Jean-Claude Buriez, and Claudine Vanbauce

Abstract

The next generation of earth radiation budget satellite instruments will routinely merge estimates of global top-of-atmosphere radiative fluxes with cloud properties. This information will offer many new opportunities for validating radiative transfer models and cloud parameterizations in climate models. In this study, five months of Polarization and Directionality of the Earth’s Reflectances 670-nm radiance measurements are considered in order to examine how satellite cloud property retrievals can be used to define empirical angular distribution models (ADMs) for estimating top-of-atmosphere albedo. ADMs are defined for 19 scene types defined by satellite retrievals of cloud fraction and cloud optical depth. Two approaches are used to define the ADM scene types. The first assumes there are no biases in the retrieved cloud properties and defines ADMs for fixed discrete intervals of cloud fraction and cloud optical depth (fixed-τ approach). The second approach involves the same cloud fraction intervals, but uses percentile intervals of cloud optical depth instead (percentile-τ approach). Albedos generated using these methods are compared with albedos inferred directly from the mean observed reflectance field.

Albedos based on ADMs that assume cloud properties are unbiased (fixed-τ approach) show a strong systematic dependence on viewing geometry. This dependence becomes more pronounced with increasing solar zenith angle, reaching ≈12% (relative) between near-nadir and oblique viewing zenith angles for solar zenith angles between 60° and 70°. The cause for this bias is shown to be due to biases in the cloud optical depth retrievals. In contrast, albedos based on ADMs built using percentile intervals of cloud optical depth (percentile-τ approach) show very little viewing zenith angle dependence and are in good agreement with albedos obtained by direct integration of the mean observed reflectance field (<1% relative error). When the ADMs are applied separately to populations consisting of only liquid water and ice clouds, significant biases in albedo with viewing geometry are observed (particularly at low sun elevations), highlighting the need to account for cloud phase both in cloud optical depth retrievals and in defining ADM scene types. ADM-derived monthly mean albedos determined for all 5° × 5° lat–long regions over ocean are in good agreement (regional rms relative errors <2%) with those obtained by direct integration when ADM albedos inferred from specific angular bins are averaged together. Albedos inferred from near-nadir and oblique viewing zenith angles are the least accurate, with regional rms errors reaching ∼5%–10% (relative). Compared to an earlier study involving Earth Radiation Budget Experiment ADMs, regional mean albedos based on the 19 scene types considered here show a factor-of-4 reduction in bias error and a factor-of-3 reduction in rms error.

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Jean-Claude Buriez, Marie Doutriaux-Boucher, Frédéric Parol, and Norman G. Loeb

Abstract

The usual procedure for retrieving the optical thickness of liquid water clouds from satellite-measured radiances is based on the assumption of plane-parallel layers composed of liquid water droplets. This study investigates the validity of this assumption from Advanced Earth Orbiting Satellite–Polarization and Directionality of the Earth's Reflectances (ADEOS–POLDER) observations. To do that, the authors take advantage of the multidirectional viewing capability of the POLDER instrument, which functioned nominally aboard ADEOS from November 1996 to June 1997.

The usual plane-parallel cloud model composed of water droplets with an effective radius of 10 μm provides a reasonable approximation of the angular dependence in scattering at visible wavelengths from overcast liquid water clouds for moderate solar zenith angles. However, significant differences between model and observations appear in the rainbow direction and for the smallest observable values of scattering angle (Θ < 90°). A better overall agreement would be obtained for droplets with an effective radius of about 7–8 μm for continental liquid water clouds. On the other hand, changing the water droplet size distribution would not lead to a significant improvement for maritime situations. When horizontal variations in cloud optical thickness are considered by using the independent pixel approximation (IPA), a small improvement is obtained over the whole range of scattering angles but significant discrepancies remain for Θ < 80°, that is for large solar zenith angles in the forward-scattering direction. The remaining differences between various models based on the plane-parallel radiative transfer and POLDER observations are thought to be due to variations in cloud shape.

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