ANGULAR-DEPENDENCE OF OBSERVED REFLECTANCES - A COMPARISON WITH PLANE-PARALLEL THEORY

In this study, a direct comparison between plane parallel model calcul ations and one year of Earth Radiation Budget Satellite (ERBS) shortwa ve scanner observations over ocean between 30 degrees S and 30 degrees N is performed. Considering only cloud-contaminated pixels, plane par allel model calculations are first normalized to observations at nadir at high Sun elevations on a pixel-by-pixel basis by adjusting cloud f raction and cloud optical depth. These are then used to generate plane parallel model reflectances as a function of view, solar zenith, and relative azimuth angle, which are directly compared with the observati ons on a statistical basis. At moderate to high solar elevations (thet a(0) less than or similar to 63 degrees), the relative view angle depe ndence of plane parallel reflectances remains, on a statistical basis, within approximate to 10% (relative difference) of that of the observ ations. For larger solar zenith angles, however, observed and plane pa rallel reflectances show systematic differences at all view angles tha t increase with increasing solar zenith angle. Provided atmospheric ef fects above the cloud are accounted for in the calculations, observed reflectances exceed plane parallel values in the backscattering direct ion by roughly a constant amount at all view angles for theta(0) great er than or similar to 63 degrees. In the forward scattering direction, observed reflectances generally fall within the range of plane parall el model values (for a range of model assumptions) but show a very dif ferent view angle dependence; observed reflectances level off at view angles between 60 degrees and 73 degrees, whereas the calculations inc rease steadily. When stratified by pixel brightness, the plane paralle l model generally provides a better approximation to the observed refl ectance dependence on view angle for darker (i.e., optically thinner) pixels than for brighter (optically thicker) pixels. For the brightest pixels, reflectance differences are largest at nadir in the backscatt ering direction and at oblique view angles in the forward direction. O verall, the relative dependence on azimuthal angle is similar for the observations and plane parallel model, irrespective of cloud thickness . Neglecting pixel area expansion with view angle in the calculations is shown to have only a minor influence. Finally, the marked differenc e in the reflectance dependence on solar zenith angle between observat ions and calculations is suggested as the likely reason why the princi ple of directional reciprocity applied to satellite measurements break s down at ERBS pixel scales.