Airborne spectral measurements of surface anisotropy during SCAR-B

During the Smoke, Clouds, and Radiation-Brazil (SCAR-B) deployment, angular distributions of spectral reflectance for vegetated surfaces and smoke lay ers were measured using the scanning cloud absorption radiometer (CAR) moun ted on the University of Washington C-131A research aircraft. The CAR conta ins 13 narrowband spectral channels between 0.3 and 2.3 mu m with a 190 deg rees scan aperture (5 degrees before zenith to 5 degrees past nadir) and 1 degrees instantaneous field of view. The bidirectional reflectance is obtai ned by flying a clockwise circular orbit above the surface, resulting in a ground track similar to 3 km in diameter within about 2 min. Although the C AR measurements are contaminated by minor atmospheric effects, results show distinct spectral characteristics for various types of surfaces. Spectral bidirectional reflectances of three simple and well-defined surfaces are pr esented: cerrado (August 18, 1995) and dense forest (August 25, 1995), both measured in Brazil under nearly clear-sky conditions, and thick smoke laye rs over dense forest (September 6 and 11, 1995). The bidirectional reflecta nces of cerrado and dense forest revealed fairly symmetric patterns along t he principal plane, with varying maximal strengths and widths spectrally in the backscattering direction. In the shortwave-infrared region the aerosol effect is very small due to low spectral optical depth. Also, these backsc attering maxima can be seen on the bidirectional reflectance of smoke layer over dense forest. These detailed measurements of the angular distribution of spectral reflectance can be parameterized by a few independent variable s and utilized to retrieve either surface characteristics or aerosol microp hysical and optical properties (e.g., size distribution and single-scatteri ng parameters), if proper physical and radiation models are used. The spect ral-hemispherical albedo of these surfaces is obtained directly by integrat ing all angular measurements and is compared with the measured nadir reflec tance. Using CAR nadir reflectance as a surrogate for spectral-hemispherica l albedo can cause albedos to be underestimated by 10-60%, depending on sol ar zenith angle.