High-Resolution Image

The Cloud Absorption Radiometer (CAR) flying aboard the University of Washington aircraft has provided a convenient and efficient means of obtaining complete bidirectional reflectance distribution function (BRDF) for several surface types as shown in this figure. The BRDF, from selected CAR channels, was obtained over marine stratocumulus clouds off the Skeleton coastline of Namibia (20.5¡S, 13.1¡E) in late winter (September 13, 2000), over Skukuza, South Africa (25.03¡ S, 31.51¡ E) on August 29, 2000, Etosha Pan in Namibia (19.0¡S, 16.0¡E) on September 16, 2000, and western Atlantic Ocean in the vicinity of Chesapeake Light (36.91¡N, 75.71¡W) on July 10, 2001 (more details can be found in Gatebe et al. 2003; 2005). In all polar plots, the viewing zenith angle is represented as the radial distance from the center, and azimuth as the length of arc on the respective zenith circle. The principal plane resides in the 0¡-180¡ azimuthal plane with the sun located in the 180¡ azimuthal direction. With this definition, the upper half circle represents forward scattering and the lower half circle back scattering. It is interesting to see the radiance field above various surfaces in unprecedented detail because of the high resolution of the CAR measurements both angular (1¡) and spatial (better than 4-10 m at nadir) coupled with high signal-to-noise ratio, and small quantization intervals (16 bits). For more details visit the CAR Web site.

BRDF is needed in remote sensing for the correction of view and illumination angle effects (for example in image standardization and mosaicing), for deriving albedo, for land cover classification, for cloud detection, for atmospheric correction, and other applications. It gives the lower radiometric boundary condition for any radiative transfer problem in the atmosphere and is hence of relevance for climate modeling and energy budget investigations.

The CAR will continue to acquire such measurements on new platforms such as South African Weather Service AeroCommander, where it was recently integrated successfully.

(submitted by Charles Gatebe, Michael King and Steve Platnick)