Comprehensive tool for calculation of radiative fluxes: illustration of shortwave aerosol radiative effect sensitivities to the details in aerosol and underlying surface characteristics

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.

Radiative absorption enhancement of dust mixed with anthropogenic pollution over East Asia

“…The direct aerosol radiative efficiency strongly depends on solar zenith angle (e.g., Derimian et al, 2016). To investigate the influence of solar zenith angle on the result of the present study, the aerosol radiative efficiency as a function of solar zenith angle were calculated for various aerosol types (Fig.…”

“…SSA curvature of greater than 0.1, which has been suggested for "moderate mixing" , was applied for the mixed-type aerosol. We employed an additional criterion, Ångström exponent, which is related to the aerosol size and the widely used in aerosol classification (Eck et al, 2010;Derimian et al, 2016), to further constrain the aerosol classification. Results show that Ångström exponent values are smaller for coarse mode dust aerosols (lower than 0.7 in the present study), larger for fine mode anthropogenic aerosols (greater than 1.4 in this research), and range from 0.8 and 1.3 for the mixed-type aerosols, respectively.…”

Effect of sea breeze circulation on aerosol mixing state and radiative properties in a desert setting

“…The air mass backward trajectories are obtained using the 3-D HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model of the US National Oceanic and Atmospheric Administration (NOAA) (Draxler and Hess, 1998). The runs for backward trajectories are performed using the global data assimilation system.…”

“…The discontinuity in the slope implies a loss of solar energy received at the surface presumably due to changes in the aerosol properties or atmospheric water vapor content. To evaluate the sea-breeze-induced radiative effect, we calculated the solar fluxes and the net instantaneous direct aerosol radiative effect using a computational tool described in Derimian et al (2016). Note that the calculated solar flux is for the wavelength range of 0.2-4.0 µm, while the measured is for 0.3-2.8 µm, which implies about 3 % bias due to the cut-off of the spectral range (the accuracy of the measurements themselves is about 3-5 % as well).…”

“…At the top of the atmosphere, a negative value signifies additionally reflected radiation due to aerosol presence and therefore a radiative cooling of the whole surface-atmosphere system. More details about the calculation tool and approach used can be found in Derimian et al (2016). Figure 13c and d present the calculated instantaneous net aerosol radiative effect at the bottom and the top of the atmosphere before and during sea breeze.…”