Abstract
Abstract. 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.
Highlights
Direct atmospheric aerosol radiative forcing remains one of the most uncertain components in evaluation of Earth’s climate change (Andreae et al, 2005; Hansen et al, 2011)
Strong dependence of instantaneous aerosol radiative effect on solar zenith angle (SZA) implies importance of (i) the proper intercomparison of instantaneous values assessed in different time and location and (ii) the evaluation of the daily average radiative effect, which is obtained by integration over corresponding range of SZAs in a given day and location
The results show that, while employing the detailed phase function (Fig. 9a, b), the spherical aerosol model leads to overestimation of cooling at top of atmosphere (TOA) and bottom of atmosphere (BOA) over dark surfaces; the relative differences in the instantaneous values range between ∼ 1 and 9.5 % and depend on the SZA (Fig. 9c, d)
Summary
Direct atmospheric aerosol radiative forcing remains one of the most uncertain components in evaluation of Earth’s climate change (Andreae et al, 2005; Hansen et al, 2011). This SZA dependence of aerosol radiative effect at the top of atmosphere (TOA) appeared in an earlier simple expression developed for calculations of Earth–atmosphere albedo perturbation (Lenoble et al, 1982) Later it was confirmed by exact radiative transfer computations (e.g., Bellouin et al, 2004), taken into account in space instrument forcing studies using POLDER (Boucher and Tanré, 2000) and MODIS (Remer and Kaufman, 2006) and using AERONET retrievals (e.g., Derimian et al, 2008, 2012; García et al, 2012). We apply our simulation for (i) evaluating radiative effect of several key aerosol models; (ii) we stress importance of diurnal dependence (dependence on SZA) of the aerosol radiative effect and (iii) examine the effects of assumptions using simplified representations of aerosol phase function, particle shape and directional properties of surface reflectance.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
