The sensitivity of OLR/DLR to atmospheric temperature and water vapor

Although the temperature dependence of gas absorption has long been known and included in radiative transfer computation, its quantitative effect on climate feedback has not been adequately highlighted and distinguished from the Planck temperature effect. By using radiative Jacobians, which quantify the sensitivity of radiative flux to atmospheric temperature perturbations, the influence of the gas absorptivity's temperature dependence upon the outgoing longwave radiation (OLR) at the top of the atmosphere and the downward longwave radiation (DLR) at the surface are analyzed using a line-by-line radiative transfer model. The results show that the temperature dependence of gas absorptivity strongly influences the radiation energy transfer within the atmosphere. In terms of OLR, although the temperature-dependent gas absorptivity effect induces only a minor change compared to the Planck effect on the overall sensitivity of the outgoing irradiance flux to a +1K temperature perturbation, it allows more of the radiation, mainly in the atmospheric window region, emitted by the surface to leave the atmosphere-surface system than if the absorptivity is not temperature-dependent. In terms of DLR, this effect reduces the downward radiation from the atmosphere to the surface. Generally speaking, the temperature dependence of gas absorptivity reduces the climate sensitivity of the surface temperature, with the effect becoming more prominent in a warmer climate.