Comprehensive Analysis of the Calculating Methods for Estimating Anthropogenic Aerosol Radiative Effects with CMIP6 Aerosol Forcing

Abstract:

The calculation methods for estimating anthropogenic aerosol (hereafter antA) radiative effects were analyzed using an atmospheric model with the simplified anthropogenic aerosol forcing provided by CMIP6. There are several calculation methods that can break the antA effective radiative forcing (ERF) into the instantaneous radiative forcing (RF) from aerosol–radiation interactions (RFari), the RF from aerosol–cloud interactions (RFaci), and the rapid adjustments. Both antA RFari and antA RFaci that calculated by double radiation calls in one present-day (PD) simulation are very robust, and can be used as offline simulation results. Thus, using this straightforward calculating method, the annual mean and seasonal variability of antA RF can be estimated from one-year PD simulation. The antA RFaci determined by this straightforward method is the exact antA Twomey effect (without its subsequent rapid adjustments), while the difference in total (anthropogenic and natural) aerosol RFaci between pre-industrial (PI) and PD simulations indicates the antA Twomey effect and the clouds forcing change induced by its rapid adjustments. The antA RFari determined by the difference between PD and PI simulations is more sensitive to model internal year-to-year variability compared with the antA RFari determined from one PD simulation. Furthermore, the antA RFari calculated with and without natural aerosol influence (i.e., different methods) is significantly different. In short, the choice of calculation method might impact the magnitude and reliability of modeled antA RF, and we also should pay attention to the difference between the antA RF we want and the antA RF we calculate.