The U.S.Global Change Research Program classifies the role of clouds and radiation as its highest scientific priority (CEES,1994). There are many excellent summaries of the scientific issues (IPCC, 1992; Hansen et al. 1993; Ramanathan et al. 1989; Randall et al. 1989; Wielicki et al. 1995) concerning the role of clouds and radiation in the climate system. These issues naturally lead to a requirement for improved global observations of both radiative fluxes and cloud physical properties. The CERES Science Team, in conjunction with the EOS Investigators Working Group representing a wide range of scientific disciplines from oceans, to land processes, to atmosphere, has examined these issues and proposed an observational system with the following objectives:
- For climate-change analysis, provide a continuation of the ERBE record of radiative fluxes at the top of the atmosphere (TOA), analyzed using the same algorithms that produced the existing ERBE data.
- Double the accuracy of estimates of radiative fluxes at TOA and the Earth's surface.
- Provide the first long-term global estimates of the radiative fluxes within the Earth ’s atmosphere.
- Provide cloud-property estimates which are consistent with the radiative fluxes from the surface to the top of the atmosphere.
To accomplish these goals, the CERES data products are divided into three major categories, as shown in the Standard Products Flow Chart: ERBE-like Products (top row), CERES Surface/TOA Products (middle row), and CERES Surface/TOA/Atmosphere (bottom row).
The ERBE-like products address the first objective of long-term continuity of the ERBE TOA fluxes. The TOA/Surface Products address the second objective and attempt to provide the most direct tie between surface radiative-flux estimates and TOA flux measurements. The TOA/ Surface/Atmosphere products focus on the last two objectives and attempt to derive an internally consistent set of atmosphere, cloud, and surface-to-TOA radiative fluxes, all within the context of a state-of-the-art radiative-transfer model. In this last case, the observed TOA fluxes are used as a direct constraint on the model calculations in order to implicitly account for non-plane-parallel and other poorly modeled atmospheric radiative effects. As in ERBE, CERES radiative fluxes will be separately determined for both clear-and cloudy-sky conditions.
One of the major advances of CERES over ERBE is the availability of high spatial and spectral resolution cloud imagers for cloud masking, cloud height, and cloud-optical-property determination (i.e., VIRS on TRMM, MODIS on Terra and Aqua). A second major advance is the use of one CERES scanner in a cross-track mode (global spatial coverage) and a second scanner in a rotating-azimuth-plane mode (complete angular sampling of viewing zenith and azimuth). The rotating-azimuth-plane scanner data will be combined with nearly simultaneous cloud-imager data to develop a new set of improved empirical models of the SW and LW anisotropy of radiation as a function of surface and cloud type. While it is estimated to take two years of these data to develop new angular models, they are expected to reduce instantaneous TOA flux errors by a factor of three from the ERBE levels.
A detailed listing of the CERES data products and their individual parameters can be found in the CERES Data Products Catalog (http://asd-www.larc.nasa.gov/DPC/DPC.html). Documentation of the Release 1 CERES analysis algorithms can be found in the CERES Algorithm Theoretical Basis Documents (ATBDs) Volumes 0 through 12, where each volume covers one of the CERES Data Products and its associated technical algorithms. A summary of the CERES experiment can be found in Wielicki et al. Bulletin of the American Meteorological Society, May1996.
SOURCE: EOS Data Products Handbook, Volume 2, 2000, p. 26. |
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