Implications of Lorenz-Mie scattering by cloud droplets in an absorbing atmosphere for cloud feedbacks

There is still considerable uncertainty as to the magnitude of cloud feedbacks to anthropogenic climate change . The magnitude is determined by changes in the bulk cloud radiative effects (CREs) on both solar and terrestrial radiation in response to changes in the Earth's average surface temperature. The range of CRE responses of low-altitude liquid clouds is one of the dominant sources of uncertainty. To date, the CREs for these clouds have been computed using variants of classical far-field Mie theory applied to spherical particles (i.e., water droplets) embedded in a non-absorbing medium. At many wavelengths where water vapor is the predominant radiatively active gas, the assumption of a non-absorbing medium is manifestly violated, for example in the near-infrared near the primary and overtone absorption bands of H₂O. For this reason, it is important to redo the calculation of CREs using new, generalized Lorenz-Mie scattering in an absorbing atmosphere (Mishchenko et al, 2017 and 2018). We quantify differences in reflection, transmission, and absorption of sunlight and terrestrial radiation by liquid clouds and the impacts of these differences on low-cloud feedbacks to increasing surface temperatures.