On The Impact of Cloud Phase Partitioning on Climate Feedbacks and Climate Sensitivity

Mixed-phase clouds, comprised of both liquid droplets and ice crystals, are ubiquitous in the Earth's mid- and high-troposphere. Depending on their relative proportion of liquid droplets to ice crystals, which have inherently different optical properties, their influence on various major climate feedbacks and hence the Earth's radiative budget and climate sensitivity will change. Here, we perturb four microphysical parameters in the atmospheric model, CAM5, of NCAR's global coupled climate model, CESM, to reproduce global observations of average cloud phase and average dust aerosol frequencies obtained by NASA's spaceborne dual-wavelength polarization lidar, CALIOP, over the time period extending from December 2007 to June 2014 at the -10°C, -15°C, -20°C and -25°C isotherms. The four cloud microphysical parameters include i) the Wegener-Bergeron-Findeisen timescale, ii) the fraction of dust aerosols active as IN, iii) the number of monolayers required to deactivate ice nuclei, and iv) the solubility parameter that determines wet deposition rates. The combination of cloud microphysical parameters that best reproduced cloud phase and aerosol distributions obtained by CALIOP, were incorporated into CESM and full climate simulations with present-day and doubled carbon dioxide concentrations were run to equilibrium. The cloud, water vapour, Planck, lapse rate and albedo feedback parameters as well as the climate sensitivity were calculated to determine the relative strength of the influence of cloud phase partitioning on the climate system. Thus, the aim of this study is to determine the relative importance of various cloud microphysical parameters controlling mixed-phase cloud thermodynamic phase for reproducing realistic present-day cloud phase distributions. This information is subsequently used to predict how the climate system will respond to increased concentrations of carbon dioxide.