An underestimated negative cloud feedback from cloud lifetime changes?

Abstract:

As the atmosphere warms, cloud populations at sensitive latitudes shift from ice and mixed-phase to entirely liquid. This phase change leads to a negative shortwave cloud feedback because liquid clouds, for a constant condensate path and area coverage, are more reflective than ice clouds. Recent work has emphasized this cloud phase feedback for climate change simulations, and the excessive negative feedback that results from excessive ice in the mean state of many models. Yet, the influence of precipitation on this phase change feedback is often ignored. Here, we use both observations and models to quantify not only this phase change influence on condensate but also on precipitation. In particular, we identify the influence of an expected decrease in precipitation efficiency by warm rain processes relative to cold rain processes. Most models with parameterized precipitation overestimate the propensity of warm clouds to rain. Thus, we expect these models to underestimate the condensate lifetime increase and with it the magnitude of the cloud feedback contribution. To test this hypothesis, we retune the warm rain processes in one climate model to better approximate the satellite-observed probability of warm rain in the present-day climate (Muelmenstaedt, Salzmann, Kay, Zelinka, Ma, Hornig, Quaas et al. submitted). In accordance with our hypothesis, lower (more realistic) warm rain probability leads to a stronger cloud feedback via the condensate lifetime. Moving beyond this study in one model -- Can the influence of cloud phase on feedbacks in this one model be checked with observations? Do observations confirm that warm clouds precipitate less efficiently than cold clouds? Will another model simulation confirm our provocative results? We’ll review the constraints as best they are known in late 2020.