Evaluation of Surface Upward Longwave Radiation in the CMIP6 Models with Ground and Satellite Observations

Surface upward longwave radiation (SULR) is an indicator of thermal conditions over the Earth's surface. In this study, we validated the simulated SULR from 51 Coupled Model Intercomparison Project (CMIP6) general circulation models (GCMs) through a comparison with ground measurements and satellite-retrieved SULR from the Clouds and the Earth's Radiant Energy System, Energy Balanced and Filled (CERES EBAF). Moreover, we improved the SULR estimations by a fusion of multiple CMIP6 GCMs using multimodel ensemble (MME) methods. Large variations were found in the monthly mean SULR among the 51 CMIP6 GCMs; the bias and root mean squared error (RMSE) of the individual CMIP6 GCMs at 133 sites ranged from -3 to 24 W m(-2) and 22 to 38 W m(-2), respectively, which were higher than those found between the CERES EBAF and GCMs. The CMIP6 GCMs did not improve the overestimation of SULR compared to the CMIP5 GCMs. The Bayesian model averaging (BMA) method showed better performance in simulating SULR than the individual GCMs and simple model averaging (SMA) method, with a bias of 0 W m(-2) and an RMSE of 19.29 W m(-2) for the 133 sites. In terms of the global annual mean SULR, our best estimation for the CMIP6 GCMs using the BMA method was 392 W m(-2) during 2000-2014. We found that the SULR varied between 386 and 393 W m(-2) from 1850 to 2014, exhibiting an increasing tendency of 0.2 W m(-2) per decade (p < 0.05).

Automated summary

This study validated simulated SULR from 51 CMIP6 GCMs and improved estimations by fusing multiple models. Large variations were found in individual CMIP6 GCMs, with higher biases and RMSE compared to CERES EBAF and GCMs. The Bayesian model averaging method showed better performance in simulating SULR.