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Using stability composites to analyse cloud feedbacks in the CMIP3/CFMIP-1 slab models.Mark Webb (Met Office)

CFMIP-GCSS BLWG Meeting, Vancouver, June 2009

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Acknowledgements

Sandrine BonyChris Bretherton

Hugo Lambert Adrian Lock Mark Ringer Steve Sherwood Keith Williams Rob Wood

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Is low cloud feedback spread due to a particular cloud regime?

Bony and Dufresne, 2005 (GRL) - analysed 15 AOGCMs within tropics 30N/S

- weakly subsiding regions showed largest spread

Medeiros et al, 2008 (J Climate) - analysed 2 AGCMs within tropics 35N/S

- shallow cumulus explained differences

Williams and Webb, 2009 (Climate Dynamics)- analysed 10 slab AOGCMs - globally

- stratocumulus explained largest part of spread- significant contributions from tropics and extra-tropics

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Is low cloud feedback spread driven by large-scale forcings?

Chris Bretherton proposed compositing tropical cloud responses and large-scale forcings in a number of models by lower tropospheric stability (LTS)

Are large scale forcings similar between models in LTS range where feedbacks differ?

If so, then differences are likely to be due to local (moist) physics.

This question is highly relevant to single column cloud feedback studies such as the current CFMIP-BLWG case.

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Approach

Apply method of Wyant et al, 2009 (Journal of Advances in Modeling Earth Systems, in press) to 11 slab models:

Sort monthly grid point values across the low latitude oceans (LLO) 30N/S by lower tropospheric stability (LTS)

Produce cloud feedback composites in equally sized percentile range bins of LTS

Also produce equivalent composites of the climate responses of LTS, local SST and subsidence rate

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unstable stable unstable stable

unstable stable

unstable stable

Net cloud feedback Wm-2/K LW cloud feedback Wm-2/K

SW cloud feedback Wm-2/K

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unstable stable unstable stable

unstable stable unstable stable

SW cloud feedback Wm-2/K

LTS response K/K

1.5m T response K/K

700 response K/K

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unstable stable unstable stable

unstable stable unstable stable

Precip response mm/day/K

w500 response hPa/day/KSW cloud feedback Wm-2/K

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Spread in sub-tropical feedback is largest in strongly stable regions, mainly due to strong positive feedbacks in IPSL/MRI.

Remaining models show roughly even spread across the tropics, suggesting shallow convection is also playing a role. Negative shortwave cloud feedbacks in stable areas are seen in CAM3, INM and HadSM3 only. Will SCM case study give mostly positive shortwave cloud feedbacks?

LTS / w500 responses do vary within LTS bins, so we can’t rule out the possibility that they contribute to feedback differences.

However they are uncorrelated with feedbacks across the ensemble, suggesting neither is the leading cause of spread.

Conclusions.