The global and tropical means of clear-sky outgoing longwave radiation (hereafter, OLRc) simulated by the new GFDL atmospheric general circulation model, AM2, tend to be systematically lower than satellite observations by about 4 Wm-2, even though the AM2 total-sky radiation budget is tuned to be consistent with these observations. Here we quantify the source of errors in AM2-simulated OLRc over the tropical oceans by comparing the synthetic outgoing IR spectra at the top of the atmosphere based on AM2 simulations to observed IRIS spectra. After the sampling disparity between IRIS and AM2 is minimized, AM2 still shows considerable negative bias in the simulated monthly mean OLRc over the tropical oceans, indicating that the sampling disparity does not account for the majority of the bias. A good agreement between the OLRc from the ECMWF ERA-40 reanalysis and from IRIS observations also supports this point. Decomposition of OLRc shows that the negative bias comes mainly from the H2O bands and can be explained by a too humid layer around 6-9 km in the model. Meanwhile, a positive bias exists in channels sensitive to near-surface humidity and temperature, which implies that the boundary layer in the model might be too dry. These facts suggest that the negative bias in the simulated OLRc can be attributed to model deficiencies, especially the large-scale water vapor transport. We also find that AM2-simulated OLRc has ~1 Wm-2 positive bias originating from the stratosphere; this positive bias should exist in simulated total-sky OLR as well.

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