Characterizing Uncertainties in Estimates of the Direct and Indirect Aerosol Radiative Forcings

Abstract:

Quantitative estimates of aerosol’s direct and indirect radiative forcings still suffer from large uncertainties due, at least in part, to a lack of rigorous and systematic understanding as to how the forcings are determined from perturbations to aerosol and cloud properties. This study examines this linkage of the forcing values to key uncertainties in fundamental processes of aerosols and clouds based on the estimates with different global models, including traditional types of climate models and an aerosol-coupled global cloud-resolving model. The model output are analyzed to investigate how the uncertainties in forcing stem from those in model representations of aerosol and cloud processes in an attempt to identify key factors that control the forcing values. The key findings include: i) the direct forcing systematically depends on how aerosol and cloud layers are stratified, ii) the forcing varies in both magnitude and sign with optical depth of clouds underlying the aerosol layer in a manner depending on the single-scattering albedo, and iii) the indirect forcing substantially depends on how the precipitation responds to perturbed aerosols as represented in different parameterizations of key cloud microphysical processes (e.g. the coalescence process). The analysis is also aided with simplified one-dimensional models of radiative transfer and cloud microphysics to better understand the relationship between the radiative forcing values and fundamental characters of aerosol and cloud processes as a basis for theoretically interpreting the global model statistics. Such a theoretical analysis provides a useful way to attribute the source of uncertainty in the radiative forcing to process-level uncertainties in aerosols, clouds and their interrelationship.