Comprehensive tool for calculation of radiative fluxes: illustration of shortwave aerosol radiative effect sensitivities to the details in aerosol and underlying surface characteristics
The evaluation of aerosol radiative effect on broadband
hemispherical solar flux is often performed using simplified spectral and
directional scattering characteristics of atmospheric aerosol and underlying
surface reflectance. In this study we present a rigorous yet fast
computational tool that accurately accounts for detailed variability of both
spectral and angular scattering properties of aerosol and surface
reflectance in calculation of direct aerosol radiative effect. The tool is
developed as part of the GRASP (Generalized Retrieval of Aerosol and Surface
Properties) project. We use the tool to evaluate instantaneous and daily
average radiative efficiencies (radiative effect per unit aerosol optical
thickness) of several key atmospheric aerosol models over different surface
types. We then examine the differences due to neglect of surface reflectance
anisotropy, nonsphericity of aerosol particle shape and accounting only for
aerosol angular scattering asymmetry instead of using full phase function.
For example, it is shown that neglecting aerosol particle nonsphericity
causes mainly overestimation of the aerosol cooling effect and that
magnitude of this overestimate changes significantly as a function of solar
zenith angle (SZA) if the asymmetry parameter is used instead of detailed
phase function. It was also found that the nonspherical–spherical
differences in the calculated aerosol radiative effect are not modified
significantly if detailed BRDF (bidirectional reflectance distribution
function) is used instead of Lambertian approximation of surface
reflectance. Additionally, calculations show that usage of only angular
scattering asymmetry, even for the case of spherical aerosols, modifies the
dependence of instantaneous aerosol radiative effect on SZA. This effect can
be canceled for daily average values, but only if sun reaches the zenith;
otherwise a systematic bias remains. Since the daily average radiative
effect is obtained by integration over a range of SZAs, the errors vary with
latitude and season. In summary, the present analysis showed that use of
simplified assumptions causes systematic biases, rather than random
uncertainties, in calculation of both instantaneous and daily average
aerosol radiative effect. Finally, we illustrate application of the rigorous
aerosol radiative effect calculations performed as part of GRASP aerosol
retrieval from real POLDER/PARASOL satellite observations
