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Infrared “Iris Effect” Leads to Strong Positive Low Cloud Feedback

Presentation Date
Thursday, December 16, 2021 at 10:50am
Location
Convention Center - Room 286-287
Authors

Author

Abstract

Equilibrium climate sensitivity (ECS) has a large spread among climate models. In the Coupled Model Inter-comparison Project Phase 6 (CMIP6), the inter-model spread in ECS grew even wider than before. Again, the shortwave cloud feedback associated with low cloud fraction change contributes significantly to the ECS diversity. We find that the inter-model spread in subsidence strength response to surface warming drives ~40% of the spread in tropical low cloud fraction change in CMIP6. The model difference in subsidence strength response is strongly correlated with the tropical-mean outgoing longwave radiation change, which is dominated by the high cloud fraction change. The models with a stronger decrease of high cloud fraction under warming leads to less weakening of large-scale subsidence rate and a greater reduction of low cloud fraction. The decrease of tropical-mean high cloud fraction is highly correlated with the tightening of tropical ascent area, which could be associated with a greater aggregation of convection in a warmer climate. Compared to the observations, most CMIP6 models underestimate the infrared “iris effect” and only three models with ECS of 4.5 K or highly approximately capture the magnitude of the decrease in high cloud fraction per unit surface warming.

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