Understanding Controls of the Extratropical Liquid Water Path Feedback in Mixed-Phase Clouds through an Idealized GCM

Abstract:

A negative shortwave cloud feedback in high latitudes associated with an increase in liquid water path (LWP) in mixed-phase clouds is a robust feature of global warming simulations, but multiple mechanisms for the LWP increase have been hypothesized. Here, we use an idealized general circulation model (GCM) to diagnose physical mechanisms of this extratropical LWP feedback, the sensitivity of that feedback, and controls of LWP and ice water path (IWP). Perturbation experiments are designed to direct a 2K warming to isolated portions of the large-scale cloud scheme as well as the calculation of saturation specific humidity. Thus, we test two proposed mechanisms for the LWP response: phase changes in mixed-phase clouds and adiabatic cloud water content increase. We find the increase in extratropical LWP to be driven principally by the weakening of microphysical liquid-to-ice conversions, mainly the Bergeron-Findeisen process. While ice is lost through the strengthening of microphysical melting processes, an increase in saturation specific humidity results in a net increase in IWP. The sensitivity of the LWP feedback is tested through a suite of warming experiments with varied macrophysical and microphysical parameters. The results demonstrate a strong dependence of LWP feedback on the climatological LWP and independence from the climatological IWP. With this idealized setup that allows for a clean isolation of mechanisms, we refine understanding of the extratropical LWP feedback as predictable by the amount of climatological liquid, not susceptible ice, and suggest a more nuanced physical re-conceptualization.