Variation in shortwave water vapour continuum and impact on clear-sky shortwave radiative feedback

Abstract. This work assesses the impact of the current differences in the strength of the shortwave water vapour continuum on clear-sky calculations of shortwave radiative feedback. Four continuum models were used: the MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies; versions 2.5, 3.2 and 4.1.1) and CAVIAR (Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance) models. Radiative transfer calculations were performed with the ECMWF radiation scheme (‘ecRad’). The correlated k-distribution gas-optics tables required for ecRad computations were trained with each of these continuum models using the ECMWF software tool. The gas-optics tables trained with the different continuum models were used alternatively in the shortwave. The atmosphere configuration was; fixed surface temperatures (T_S) between 270–330 K, fixed relative humidity at 80 %, a moist adiabatic lapse rate for the tropospheric temperature and an isothermal stratosphere with the tropopause temperature fixed at 175 K. At T_S =288 K, it was found that the revisions of the MT_CKD model in the shortwave over the last decade have a modest effect (~0.3 %) on the estimated shortwave feedback. Compared to MT_CKD 4.1.1, the stronger CAVIAR model has a relatively greater impact; the shortwave feedback is ~0.006 W m^-2 K^-1(~1.6 %) more positive. The uncertainty in the shortwave feedback increases up to 0.008 W m^-2 K^-1(~2.0 %) between the MT_CKD models and 0.018 W m^-2 K^-1(~4.6 %) between CAVIAR and MT_CKD 4.1.1 models at T_S≈300 K. Constraining the shortwave continuum will contribute to reducing the non-negligible shortwave feedback uncertainties at higher T_S.