Recent Revision of the Water Vapour Continuum Model and Impact on Clear-Sky Shortwave Broadband Radiative Transfer Calculations

Abstract

The water vapour continuum needs to be accurately accounted for in atmospheric radiative transfer calculations. The offline ECMWF radiation scheme was used to assess the impact of the current disagreements in shortwave continuum absorption on the accuracy of clear-sky calculations of solar fluxes and heating rates from 2000 to 20,000 cm⁻¹ for three standard atmospheres: tropical, mid-latitude summer, and sub-arctic winter. These calculations were carried out at a solar zenith angle of 45°, surface albedo of 0.2, and total solar irradiance of 1361.0 W m⁻². The MT_CKD 2.5, MT_CKD 4.2, and CAVIAR continuum models were compared. These continuum models were each used to train the gas-optics tables required by the radiation scheme. Differences in the shortwave continuum have a modest impact on shortwave absorption and heating rates. The largest differences are found in the tropical atmosphere due to its higher water vapour content. Absorbed fluxes computed with MT_CKD 2.5 and MT_CKD 4.2 differ by up to ~1.3 W m⁻² (0.6%) while those with CAVIAR and MT_CKD 4.2 differ by up to ~1.7 W m⁻² (0.8%). The heating rate calculated with MT_CKD 2.5 is up to ~0.018 K d⁻¹ (0.8%) less than that obtained with MT_CKD 4.2. Compared to the heating rate computed with MT_CKD 4.2, the heating rate with CAVIAR is up to ~0.035 K d⁻¹ (1.5%) higher.