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Estimation of SW Flux Absorbed at the Surface from TOA Reflected Flux

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  • 1 Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
  • | 2 Meteorological Research Institute, Tsukuba, Ibaraki, Japan
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Abstract

Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known.

Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends strongly on SZA and moderately on precipitable water and cloud type. The linear relationship provides a simple parameterization to estimate surface-absorbed flux from satellite-measured reflected flux at the TOA. Unlike other models, the present model makes explicit use of the SZA. Precipitable water is included as a secondary parameter. Surface-absorbed fluxes deduced from this simple parameterized model generally agree to within 10 W m−2 with the absorbed fluxes determined from detailed radiative transfer calculations, without including information on the presence or absence of cloud, cloud type, optical thickness, or surface type.

Abstract

Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known.

Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends strongly on SZA and moderately on precipitable water and cloud type. The linear relationship provides a simple parameterization to estimate surface-absorbed flux from satellite-measured reflected flux at the TOA. Unlike other models, the present model makes explicit use of the SZA. Precipitable water is included as a secondary parameter. Surface-absorbed fluxes deduced from this simple parameterized model generally agree to within 10 W m−2 with the absorbed fluxes determined from detailed radiative transfer calculations, without including information on the presence or absence of cloud, cloud type, optical thickness, or surface type.

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