RELATIONSHIP BETWEEN ASYMMETRY PARAMETER AND HEMISPHERIC BACKSCATTER RATIO - IMPLICATIONS FOR CLIMATE FORCING BY AEROSOLS

Calculations of direct climate forcing by anthropogenic aerosols commo nly use radiative transfer parameters, including asymmetry parameter g . One method of obtaining the asymmetry parameter of a particle popula tion is to convert measured values of the hemispheric-to-total-scatter ratio (backscatter ratio b) into their corresponding g values. We com pare a conversion derived from Mie calculations with one derived from the Henyey-Greenstein (HG) phase function to show that the HG method s ystematically overestimates g for typical size distributions of accumu lation-mode aerosols. A delta-Eddington radiative transfer calculation is used to show that a 10% overestimation of g can systematically red uce climate forcing as a result of aerosols by 12% or more. Mie comput ations are used to derive an empirical relationship between backscatte r ratio and asymmetry parameter for log-normal accumulation-mode aeros ols. This relationship can be used to convert the backscatter ratio to the asymmetry parameter, independent of geometric mean diameter D-gv or complex refractive index m, but the conversion requires knowledge o f the breadth sigma(g) of the size distribution.