Absorption of Solar Radiation by the Water Vapor Continuum (Invited Presentation)

The amount of solar radiation absorbed by the near infra-red water vapor continuum remains uncertain and a topic of controversy. For example, recent measurements of the water vapor continuum spectra around 7000 cm-1 differ by almost two orders of magnitude. The contribution of the continuum to global shortwave absorption has been estimated to be between 1.1 and 3.2 Wm-2. Despite these uncertainties, recent measurements are starting to point towards a more likely version of the near-infrared continuum. We have combined the latest measurements to create an empirical estimate of the water vapor continuum (BPS 2.0) in the near-infrared windows. A line by line radiation code has been used to predict the global and regional changes in solar heating rates and absorbed radiation due to the continuum. The values are compared to the commonly used MT_CKD model and earlier versions of BPS.

The BPS 2.0 shortwave continuum has also been parameterized into the GFDL radiation code. We have run both the atmospheric only (AM3) and fully coupled (CM3) versions of the GFDL GCM. In the atmosphere only case the shortwave continuum reduces tropical precipitation by 1%. This is reduced by half in the fully coupled CM3 case due to the warming induced by the shortwave continuum. We discuss some of the physical mechanisms by which the continuum alters the climate. The inclusion of the water vapor continuum in the GCM does not appear to significantly impact the model climate sensitivity. However, the continuum does enhance the clear-sky global dimming due to water vapor by ~15% in a warming climate.