Shallowness of Tropical Low Clouds as a Predictor of Climate Models' Response to Warming

TitleShallowness of Tropical Low Clouds as a Predictor of Climate Models' Response to Warming
Publication TypeJournal Article
Year of Publication2015
AuthorsBrient, Florent, Schneider Tapio, Tan Zhihong, Bony Sandrine, Qu Xin, and Hall Alex
JournalClimate Dynamics
Date Published10/2015
Abstract / Summary

How tropical low clouds change with climate remains the dominant source of uncertainty in global warming projections. An analysis of an ensemble of CMIP5 climate models reveals that a significant part of the spread in the models’ climate sensitivity can be accounted by differences in the climatological shallowness of tropical low clouds in weak-subsidence regimes: models with shallower low clouds in weak-subsidence regimes tend to have a higher climate sensitivity than models with deeper low clouds. The dynamical mechanisms responsible for the model differences are analyzed. Competing effects of parameterized boundary-layer turbulence and shallow convection are found to be essential. Boundary-layer turbulence and shallow convection are typically represented by distinct parameterization schemes in current models—parameterization schemes that often produce opposing effects on low clouds. Convective drying of the boundary layer tends to deepen low clouds and reduce the cloud fraction at the lowest levels; turbulent moistening tends to make low clouds more shallow but affects the low-cloud fraction less. The relative importance different models assign to these opposing mechanisms contributes to the spread of the climatological shallowness of low clouds and thus to the spread of low-cloud changes under global warming.

DOI10.1007/s00382-015-2846-0
Journal: Climate Dynamics
Year of Publication: 2015
Date Published: 10/2015

How tropical low clouds change with climate remains the dominant source of uncertainty in global warming projections. An analysis of an ensemble of CMIP5 climate models reveals that a significant part of the spread in the models’ climate sensitivity can be accounted by differences in the climatological shallowness of tropical low clouds in weak-subsidence regimes: models with shallower low clouds in weak-subsidence regimes tend to have a higher climate sensitivity than models with deeper low clouds. The dynamical mechanisms responsible for the model differences are analyzed. Competing effects of parameterized boundary-layer turbulence and shallow convection are found to be essential. Boundary-layer turbulence and shallow convection are typically represented by distinct parameterization schemes in current models—parameterization schemes that often produce opposing effects on low clouds. Convective drying of the boundary layer tends to deepen low clouds and reduce the cloud fraction at the lowest levels; turbulent moistening tends to make low clouds more shallow but affects the low-cloud fraction less. The relative importance different models assign to these opposing mechanisms contributes to the spread of the climatological shallowness of low clouds and thus to the spread of low-cloud changes under global warming.

DOI: 10.1007/s00382-015-2846-0
Citation:
Brient, F, T Schneider, Z Tan, S Bony, X Qu, and A Hall.  2015.  "Shallowness of Tropical Low Clouds as a Predictor of Climate Models' Response to Warming."  Climate Dynamics.  https://doi.org/10.1007/s00382-015-2846-0.