A193-03
Cloud source and sink process as a path toward constraining climate sensitivity
Tuesday, 15 December 2020: 07:09
Virtual
Daniel McCoy, University of Wyoming, Laramie, United States, Gregory Elsaesser, Columbia University/NASA GISS, Dept. of Applied Physics and Applied Mathematics, New York, NY, United States, Paul Field, United Kingdom Met Office, Exeter, United Kingdom, Mark Zelinka, Lawrence Livermore National Laboratory, Livermore, CA, United States, Johannes Mulmenstadt, Pacific Northwest National Laboratory, Richland, WA, United States, Michelle Elizabeth Frazer, Princeton University, Princeton, NJ, United States, Zachary J Lebo, University of Wyoming, Atmospheric Science, Laramie, WY, United States, Ivy Tan, McGill University, Montreal, Canada and Casey Wall, Scripps Institution of Oceanography, La Jolla, CA, United States
Abstract:
The leading contributor to GCM diversity in equilibrium climate sensitivity (ECS) is spread in shortwave (SW) cloud feedback. This is because clouds exist on finer temporal and spatial scales than even the most computationally expensive GCMs can resolve and they must be parameterized. In the last decade, confidence has grown in a positive subtropical SW cloud feedback, with progress on quantifying the feedback magnitude and underlying mechanisms. On the other hand, GCMs continue to simulate a diversity of both negative and positive extratropical SW cloud feedbacks. Between CMIP5 and CMIP6, the multimodel mean extratropical SW cloud feedback has become more positive. The multimodel mean in CMIP6 is nearly one standard deviation above the CMIP5 models. This has resulted in a dramatic increase in the climate sensitivity in CMIP6. It is critical to investigate the extratropical SW cloud feedback in order to understand and interpret results emerging from the GCMs participating in CMIP6.
We show that the response of extratropical liquid cloud to warming in GCMs, and by extension the extratropical SW cloud feedback that they project is a function of how clouds respond to converging extratropical moisture. Increased convergence of moisture into the extratropics in response to warming is a robust feature in GCM predictions. Extratropical cloud is in approximate steady-state with the environment. For a given perturbation in water vapor GCMs with more efficient precipitation sinks will experience minimal changes in cloud as they reach a new steady-state, but GCMs with more efficient cloud sources will experience substantial changes in cloud. We analyze of a suite of GCMs where cloud source and sink parameterizations that are relevant to the extratropics have been systematically perturbed. This demonstrates causality flowing from source and sink parameterizations to the extratropical cloud response to warming. These results are used to interpret the behavior of a wide range of CMIP5 and CMIP6 models. Model behavior is contrasted with satellite observations and reanalysis to identify GCMs that agree with observed extratropical cloud, precipitation and meteorological variability. Conclusions as to the most realistic range of extratropical SW cloud feedback (and by extension ECS) are drawn.
