Variations in water vapor continuum radiative transfer with atmospheric conditions

A newly formulated empirical water vapor continuum (the BPS continuum) is employed, in conjunction with ERA-40 data, to advance the understanding of how variations in the water vapor profile can alter the impact of the continuum on the Earth's clear-sky radiation budget. Three metrics are investigated: outgoing longwave radiation (OLR), Longwave surface downwelling radiation (SDR) and shortwave absorption (SWA). We have also performed a detailed geographical analysis on the impact of the BPS continuum upon these metrics and compared the results to those predicted by the commonly used MT CKD model. The globally averaged differences in these metrics when calculated with MT CKD 2.5 versus BPS were found to be 0.1%, 0.4% and 0.8% for OLR, SDR and SWA respectively. Furthermore, the impact of uncertainty upon these calculations is explored using the uncertainty estimates of the BPS model. The radiative response of the continuum to global changes in atmospheric temperature and water vapor content are also investigated. For the latter, the continuum accounts for up to 35% of the change in OLR and 65% of the change in SDR, brought about by an increase in water vapor in the tropics.