Direct Aerosol Radiative Forcing Diagnostics in the Radiative Forcing Model Intercomparison Project

The Radiative Forcing Model Intercomparison Project (RFMIP) will assess sources of error and spread in the radiative forcing calculated by state-of-the-art climate models for historical and future climate studies. RFMIP consists of three linked components: one assessing the forcing by greenhouse gases, one the forcing by natural and anthropogenic aerosols, and one linking these with estimates of effective forcing inferred from global model integrations via careful diagnosis.
Each component relies on intensive calculations and analysis. These include very large-scale reference calculations for forcing by greenhouse gases and aerosols using computationally-intensive line-by- line radiative transfer models, careful and consistent diagnosis of aerosol radiative properties, and the systematic construction of a set of radiative kernels useful for diagnosing forcing, including one for each model and one based on observations. The results of these calculations aim to provide benchmarks for the two largest sources of direct forcing against which model calculations can be compared, forming the basis for metrics of climate model performance to ensure that CMIP6 models are subject to uniform forcing.

A proof-of-concept for aerosol forcing diagnostics with the CESM and CM3 climate models will be presented, using massively-parallel, extensively-validated line-by-line radiative transfer calculations with scattering. We show how these diagnostics identify both how and to what extent burdens, optical properties, and radiative transfer routines contribute to error in aerosol forcing. In CESM and CM3, we find that the number of streams used in the radiative transfer affects the accuracy with which these models compute aerosol forcing, especially under heavy aerosol loading, and we also find significant differences in the aerosol optical properties between the two models for different aerosol types. This proof-of-concept demonstrates that the small amount of data requested by RFMIP from the contributing CMIP6 models will lead to rigorous diagnostics of aerosol direct radiative forcing across the multi-model ensemble.