Journal of Geophysical Research 01/2011; 116(D19):D19209. · 3.02 Impact Factor
ABSTRACT: The Clouds and Earth's Radiant Energy System (CERES) provides coincident global cloud and aerosol properties together with reflected solar, emitted terrestrial longwave and infrared window radiative fluxes. These data are needed to improve our understanding and modeling of the interaction between clouds, aerosols and radiation at the top of the atmosphere, surface, and within the atmosphere. This paper describes the approach used to estimate top-of-atmosphere (TOA) radiative fluxes from instantaneous CERES radiance measurements on the Terra satellite. A key component involves the development of empirical angular distribution models (ADMs) that account for the angular dependence of Earth's radiation field at the TOA. The CERES Terra ADMs are developed using 24 months of CERES radiances, coincident cloud and aerosol retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS), and meteorological parameters from the Global Modeling and Assimilation Office (GMA0) s Goddard Earth Observing System DAS (GEOS-DAS V4.0.3) product. Scene information for the ADMs is from MODIS retrievals and GEOS-DAS V4.0.3 properties over ocean, land, desert and snow, for both clear and cloudy conditions. Because the CERES Terra ADMs are global, and far more CERES data is available on Terra than was available from CERES on the Tropical Rainfall Measuring Mission (TRMM), the methodology used to define CERES Terra ADMs is different in many respects from that used to develop CERES TRMM ADMs, particularly over snow/sea-ice, under cloudy conditions, and for clear scenes over land and desert.
ABSTRACT: Clouds and the Earth's Radiant Energy System (CERES) instruments on
Terra that is on a polar orbit are taking measurements of broadband
shortwave, longwave and window radiances since March 2000. Because the
CERES instruments can be operated under the rotating azimuth mode, they
can take radiance measurements over snow covered surface from a wide
range of viewing angles. Angular distribution models (ADM) for snow and
sea ice are developed in order to estimate top-of-atmosphere broadband
shortwave and longwave irradiance using measurements by CERES
instruments. Because of a large difference of angular dependent
radiances, ADM is divided in to three types based on surface type,
permanent snow, fresh snow, and sea ice. These ADM types are further
divided using MODIS-derived scene type; colocation of MODIS image with
CERES footprints provides scene identification of CERES footprints such
as cloud, snow, and sea ice fraction over a CERES footprint. The
shortwave permanent snow ADM depends on cloud fraction and shortwave
fresh snow and sea ice ADMs depend on both cloud fraction and snow and
ice fraction. The longwave ADMs depends on cloud fraction, surface
temperature, and temperature difference between the surface and cloud
top. ADM-derived albedo indicates that albedo over clear sky permanent
snow is approximately 0.65 and nearly independent of solar zenith angle.
Clouds increase the albedo over permanent snow; all-sky albedo over
permanent snow is approximately 0.7. As a part of an internal
consistency check of ADM-derived irradiance, ADM-derived irradiances and
irradiances computed by integrating measured radiances are compared.
AGU Fall Meeting Abstracts. 11/2003; -1:0560.
ABSTRACT: Clouds and the Earth s Radiant Energy System (CERES) investigates the critical role that clouds and aerosols play in modulating the radiative energy flow within the Earth-atmosphere system. CERES builds upon the foundation laid by previous missions, such as the Earth Radiation Budget Experiment, to provide highly accurate top-of-atmosphere (TOA) radiative fluxes together with coincident cloud and aerosol properties inferred from high-resolution imager measurements. This paper describes the method used to construct empirical angular distribution models (ADMs) for estimating shortwave, longwave, and window TOA radiative fluxes from CERES radiance measurements on board the Tropical Rainfall Measuring Mission satellite. To construct the ADMs, multiangle CERES measurements are combined with coincident high-resolution Visible Infrared Scanner measurements and meteorological parameters from the European Centre for Medium-Range Weather Forecasts data assimilation product. The ADMs are stratified by scene types defined by parameters that have a strong influence on the angular dependence of Earth's radiation field at the TOA. Examples of how the new CERES ADMs depend upon the imager-based parameters are provided together with comparisons with existing models.
ABSTRACT: This study introduces new CERES Angular Distribution Models (ADMs) for estimating shortwave, longwave and window top-of-atmosphere (TOA) radiative fluxes from broadband radiance measurements. By combining CERES measurements with narrowband, high-resolution imager measurements, up to a factor 4 improvement in TOA flux accuracy is achieved compared to TOA flux estimates from previous experiments, such as ERBE.
Geoscience and Remote Sensing Symposium, 2002. IGARSS '02. 2002 IEEE International; 07/2002
ABSTRACT: To estimate the earth's radiation budget at the top of the atmosphere (TOA) from satellite-measured radiances, it is necessary to account for the finite geometry of the earth and recognize that the earth is a solid body surrounded by a translucent atmosphere of finite thickness that attenuates solar radiation differently at different heights. As a result, in order to account for all of the reflected solar and emitted thermal radiation from the planet by direct integration of satellite-measured radiances, the measurement viewing geometry must be defined at a reference level well above the earth s surface (e.g., 100 km). This ensures that all radiation contributions, including radiation escaping the planet along slant paths above the earth s tangent point, are accounted for. By using a field-of- view (FOV) reference level that is too low (such as the surface reference level), TOA fluxes for most scene types are systematically underestimated by 1-2 W/sq m. In addition, since TOA flux represents a flow of radiant energy per unit area, and varies with distance from the earth according to the inverse-square law, a reference level is also needed to define satellite-based TOA fluxes. From theoretical radiative transfer calculations using a model that accounts for spherical geometry, the optimal reference level for defining TOA fluxes in radiation budget studies for the earth is estimated to be approximately 20 km. At this reference level, there is no need to explicitly account for horizontal transmission of solar radiation through the atmosphere in the earth radiation budget calculation. In this context, therefore, the 20-km reference level corresponds to the effective radiative top of atmosphere for the planet. Although the optimal flux reference level depends slightly on scene type due to differences in effective transmission of solar radiation with cloud height, the difference in flux caused by neglecting the scene-type dependence is less than 0.1%. If an inappropriate TOA flux reference level is used to define satellite TOA fluxes, and horizontal transmission of solar radiation through the planet is not accounted for in the radiation budget equation, systematic errors in net flux of up to 8 W/sq m can result. Since climate models generally use a plane-parallel model approximation to estimate TOA fluxes and the earth radiation budget, they implicitly assume zero horizontal transmission of solar radiation in the radiation budget equation, and do not need to specify a flux reference level. By defining satellite-based TOA flux estimates at a 20-km flux reference level, comparisons with plane-parallel climate model calculations are simplified since there is no need to explicitly correct plane-parallel climate model fluxes for horizontal transmission of solar radiation through a finite earth.
J. Climate. 22:748-766.