24-10-2012, 05:22 PM
Defining Top-of-Atmosphere Flux Reference Level for Earth Radiation Budget Studies
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 TOAflux 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 8W/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.