The effect of the differences in near-infrared water vapour continuum models on the absorption of solar radiation

There are currently significant disagreements in the strength of the water vapour continuum in the near-infrared region. To understand the effects of these disagreements on the absorption of solar radiation, line-by-line radiative transfer calculations were performed from 2000 to 10,000 cm⁻¹ (1–5 μm) for three standard atmospheres; tropical, mid-latitude summer and sub-arctic winter atmospheres. These calculations were carried out at a solar zenith angle of 60° using line parameters from HITRAN (HIgh-resolution TRANsmission). Three currently available water vapour continuum models were selected for this study; versions 2.5 and 3.2 of the semi-empirical MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) model and the laboratory-measured CAVIAR (Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance) model. The differences between the contributions of both MT_CKD models to near-infrared absorption and heating are modest for all three atmospheres. The additional absorption due the CAVIAR model more than doubles those due to both MT_CKD models for the tropical and mid-latitude summer atmospheres. For both atmospheres, the extra heating of the CAVIAR model is up to a factor of 5 more than those of the MT_CKD models. For the sub-arctic winter atmosphere, the differences between the extra absorption and heating of the CAVIAR and those of both MT_CKD models are relatively less. Thus, an update of the MT_CKD model from version 2.5 to 3.2 has a relatively small impact on near-infrared spectrally integrated absorbed solar fluxes and heating rates. But their contributions to the calculations of these quantities differ significantly from that of the much stronger CAVIAR model.