NOTES AND CORRESPONDENCE On Aerosol Direct Shortwave Forcing and the Henyey–Greenstein Phase Function

This technical note extends previous Mie calculations to show that there are complex relationships between the asymmetry parameter g and the upscatter fractions for monodirectional incident radiation b(m0). Except for intermediate zenith angles and for the upscatter fraction for diffuse radiation, there are significant differences between b(m0) predicted by the Mie theory and that approximated by a Henyey–Greenstein phase function. While the Henyey–Greenstein phase function is widely used in radiative transfer calculations to characterize aerosol or cloud droplet scattering, it may cause important discrepancies in the computation of the aerosol direct radiative forcing, depending on solar zenith angle, aerosol size, and refractive index. The implications of this work for aerosol and climate-related studies are also discussed.