Global Line-by-Line Calculations of Aerosol Radiative Forcing: Understanding Sources of Error in GCM Radiation Codes

Traditionally, errors in GCM aerosol radiative forcing have been estimated using a small number of idealized cases. However, these few cases are not sufficient to capture the wide range of aerosol optical properties seen in a GCM. The protocol for RFMIP A-IRF solves this problem by performing benchmark 16 stream line-by-line calculations over the entire globe on each participating model's native grid using the actual meteorological and aerosol optical property fields seen by the GCM radiation code. Resulting in up to 2.6 million single column calculations. Therefore, for each GCM participating in RFMIP we will provide a benchmark aerosol forcing that can be directly compared to the forcing calculated by the GCM at every gridpoint.

For this study we apply the RFMIP Aerosol-IRF protocol to both GFDL’s AM4 and NCAR’s CESM 1.2.2. Our results demonstrate errors in fluxes as high as 30%. We find very different spatial patterns of error between the two models. This can partially be attributed to the different aerosol properties of the two GCMs. However, we also found choices made in the radiative transfer solution to have a significant impact. The usage of a two stream solver in both models resulted in an underestimate of atmospheric absorption and attenuation of flux at the surface. Perhaps more surprisingly, we found that the width of the spectral bands also played an important role. At visible and UV wavelengths CESM, which uses the RRTMG radiation scheme, has far fewer bands than the GFDL radiation scheme (5 vs. 13). We show that these broader bands of RRTMG result in an overestimate of absorption by aerosol. By interpolating the CESM aerosol properties to the narrower GFDL band structure it was possible to almost completely remove this error. This implies that a finer band structure in the UV and visible can greatly improve the representation of aerosol radiative transfer in GCMs.

The two stream and band width errors have a notable geographic variation and exhibit a dependence on both the aerosol specie type and background environment. Therefore, highlighting that errors in aerosol radiative transfer for a particular GCM are uniquely tied to its aerosol distribution and spectral properties.