Toward Understanding the Shortwave Cloud Feedback in Climate Change: Exploring the Mechanisms of Extratropical Liquid Water Path Increase in Mixed-Phase Clouds in a Warming Climate

A negative shortwave cloud feedback associated with an increase in extratropical 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. Discerning the physical mechanisms of this key cloud feedback is critical to ensuring that complex general circulation models (GCMs) ground climate change predictions in not only consensus but also mechanistic understanding, especially with the potential for unrealistic interactions. Here, we use an idealized GCM—a simple tool whose workings can be entirely grasped—to diagnose physical mechanisms of this extratropical LWP feedback. In this GCM, with completely passive water and cloud tracers, cloud effects are decoupled from dynamics allowing cloud processes to be explored in isolation.

Perturbation experiments are designed to direct a 2K warming to isolated components of the stratiform cloud module 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 with warming to be driven principally by the suppression 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 ice water path (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 model’s climatological LWP and independence from climatological IWP, contrary to suggestions in the literature of a deterministic power of the amount of susceptible ice. An analysis of budgetary tendency terms reveals the differing microphysical controls of LWP and IWP. While cloud liquid tendencies balance locally, cloud ice tendencies balance remotely due to a slower fall speed and dependence of key terms on the cloud ice gradient and fluxes. Thus, when liquid-to-ice conversion tendencies (such as the Bergeron-Findeisen process) adjust, the resulting changes in steady-state liquid and ice may not be offsetting.

These results suggest a re-conceptualization of the physics of the extratropical LWP increase: the mixed-phase cloud feedback is more nuanced than a simple shift from ice to liquid cloud content. Here, using an 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.