Cloud characterization and clear-sky correction from Landsat-7

Landsat, with its wide swath and high resolution, fills an important mesosc ale gap between atmospheric variations seen on a few kilometer scale by loc al surface instrumentation and the global view of coarser resolution satell ites such as MODIS. In this important scale range, Landsat reveals radiativ e effects on the few hundred-meter scale of common photon mean-free-paths, typical of scattering in clouds at conservative (visible) wavelengths, and even shorter mean-free-paths of absorptive (near-infrared) wavelengths. Lan dsat also reveals shadowing effects caused by both cloud and vegetation tha t impact both cloudy and clear-sky radiances. As a result, Landsat has been useful in development of new cloud retrieval methods and new aerosol and s urface retrievals that account for photon diffusion and shadowing effects. This paper discusses two new cloud retrieval methods: the nonlocal independ ent pixel approximation (NIPA) and the normalized difference nadir radiance method (NDNR). We illustrate the improvements in cloud property retrieval enabled by the new low gain settings of Landsat-7 and difficulties found at high gains. Then, we review the recently developed "path radiance" method of aerosol retrieval and clear-sky correction using data from the Departmen t of Energy Atmospheric Radiation Measurement (ARM) site in Oklahoma. Nearb y clouds change the solar radiation incident on the surface and atmosphere due to indirect illumination from cloud sides. As a result, if clouds are n earby, this extra side-illumination causes clear pixels to appear brighter, which can be mistaken for extra aerosol or higher surface albedo. Thus, cl oud properties must be known in order to derive accurate aerosol and surfac e properties. A three-dimensional (3D) Monte Carlo (MC) radiative transfer simulation illustrates this point and suggests a method to subtract the clo ud effect from aerosol and surface retrievals. The main conclusion is that cloud, aerosol, and surface retrievals are linked and must be treated as a combined system. Landsat provides the range of scales necessary to observe the 3D cloud radiative effects that influence joint surface-atmospheric ret rievals. (C) 2001 Elsevier Science Inc. All rights reserved.