A critical examination of the observed first aerosol indirect effect

Abstract

The relative change in cloud droplet number concentration with respect to the relative change in aerosol number concentration, α, is an indicator of the strength of the aerosol indirect effect and is commonly used in models to parameterize this effect. Based on Twomey's analytical expression, the values of α derived from measurements of an individual cloud (i.e., αT) can be as large as 0.60-0.90. In contrast, the values of α derived from direct measurements of polluted and clean clouds (i.e., α Δ) typically range from 0.25 to 0.85, corresponding to a weaker but more uncertain cooling effect. Clearly, reconciling αΔ with αT is necessary to properly calculate the indirect aerosol forcing. In this study, the terms that are involved in determining αT and αΔ are first analytically examined. Then, by analyzing satellite data over subtropical oceans, the satellite-observed αΔ can be successfully related to Twomey's analytical solution. It is found that except for the dust-influenced region of the northeastern Atlantic Ocean, injecting continental aerosols into a marine background may significantly reduce the average aerosols' ability to act as cloud condensation nuclei. Taking this competing effect into account may reduce the cooling effect proposed by Twomey from 0.76 to 0.28. It is also found that the variability of the adiabaticity (i.e., the cloud dilution state with respect to adiabatic cloud) among different clouds accounts for ∼50% uncertainty in αΔ. Based on these results, the authors explain the claimed discrepancies in the first aerosol indirect effect (AIE) from different methods and on different scales and present an improved parameterization of the first AIE that can be used in global climate models. © 2009 American Meteorological Society.