Using an Idealized Climate Model to Elucidate Physical Mechanisms and Improve Model Representation of Extratropical Cloud Feedbacks

Abstract

Despite recent advances, the extratropical cloud feedback is at the heart of uncertainty and intermodel spread in climate sensitivity among comprehensive General Circulation Models (GCMs). Here, to shed light on the underlying mechanisms, we use an idealized GCM---the Held-Suarez dynamical core with the addition of passive water vapor and cloud tracers and full cloud microphysics---to cleanly decompose the effects of individual cloud processes. A set of process-level experiments are designed to elucidate the mechanisms of an increase in extratropical liquid water path (LWP) in mixed-phase clouds with warming: higher adiabatic cloud water content, weaker liquid-to-ice conversion through the Bergeron-Findeisen process, and faster melting of ice and snow to rain. Perturbed parameter experiments demonstrate a strong dependence of the LWP feedback on the climatological LWP and independence from the climatological susceptible ice and supercooled liquid fraction. To examine the broader physical controls of extratropical humidity and clouds, separate saturation adjustment and full cloud scheme controls are used to consider the strength of advection-condensation theory. It is found that sub-grid-scale relative humidity distribution assumed within the cloud macrophysics limits isentropic transport of tropical moisture to the polar troposphere. Also, within the model's explicit treatment of cloud microphysics, re-evaporation of hydrometeors moistens and increases clouds in the lower troposphere. To investigate the direct influence of dynamical changes on humidity and clouds, we explore the impacts of altered meridional temperature gradients. We show that predicted extratropical jet shifts with warming have little effect on shortwave cloud radiative effects because they mainly affect cloud fraction, not cloud thickness which is principally connected to absolute temperature change. Overall, the results substantiate the utility of such idealized models for elucidating cloud processes in a systematic manner and highlight key cloud processes to constrain. Additionally, a set of realistic experiments are designed in a comprehensive GCM to study geoengineering using stratospheric aerosols and compare efficacy and impacts of reflecting and absorbing aerosols (sulfate) versus only reflecting aerosols (calcite). We evaluate which regions are better or worse off with geoengineering in a 2xCO2 world in terms of various climate metrics and consider the policy implications.