Evaluations of atmospheric downward longwave radiation from 44 coupled general circulation models of CMIP5

Abstract

Atmospheric downward longwave radiation at the surface (Ld) quantifies the atmospheric greenhouse effect. This study evaluated Ld simulations from 44 general circulation models (GCMs) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) with a comprehensive data set of Ld observations at 156 global-distributed sites from 1992 to 2005. Compared with the Baseline Surface Radiation Network data that are of the highest quality among the available Ld data sets, CMIP5 GCM Ld has a negligible bias, much better than CMIP3 GCMs, likely because of the improvement of low cloud simulations in CMIP5 models. However, the selection of validation data has an important impact on the evaluation results. The global mean Ld inferred from different bias-removing methods are nearly the same, approximately 341 W m−2 globally averaged from 1992 to 2005. CMIP5 GCMs showed that global Ld increased at a rate of 1.54 W m−2 per decade (p < 0.01) from 1979 to 2005, which is consistent with available reanalyses. This good agreement in long-term trends of Ld is likely because both reanalyses and CMIP5 models reproduced the observed warming and the associated increase of water vapor content in the lower atmosphere. However, CMIP5 GCMs are still poor in producing monthly anomalies of Ld.