Investigating the impact of the shortwave water vapor continuum upon climate simulations using GFDL global models

We have added the BPS-MTCKD 2.0 parameterization for the shortwave water vapor continuum to the Geophysical Fluid Dynamics Laboratory (GFDL) global model. We find that inclusion of the shortwave continuum in the fixed sea surface temperature case (AM3) results in a similar increase in shortwave absorption and heating rates to that seen for the “benchmark” line-by-line radiative transfer calculations. The surface energy budget adjusts to the inclusion of the shortwave continuum predominantly through a decrease in both surface latent and sensible heat. This leads to a decrease in tropical convection and a subsequent 1% reduction in tropical rainfall. The inclusion of the shortwave continuum in the fully coupled atmosphere-ocean model (CM3) yields similar results, but a smaller overall reduction of 0.5% in tropical rainfall due to global warming of ~0.1 K linked to enhanced near-infrared absorption. We also investigated the impact of adding a stronger version of BPS-MTCKD (version 1.1) to the global climate model (GCM). In most cases we found that the GCM responds in a similar manner to both continua but that the strength of the response scales with the level of absorbed shortwave radiation. Global warming experiments were run in both AM3 and CM3. The shortwave continuum was found to cause a 7 to 15% increase in clear-sky global dimming depending upon whether the stronger or weaker continuum version was used. Neither version resulted in a significant change to the climate sensitivity.