Transcript Slide 1
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 © Crown copyright Met Office Acknowledgements Sandrine Bony Chris Bretherton Hugo Lambert Adrian Lock Mark Ringer Steve Sherwood Keith Williams Rob Wood © Crown copyright Met Office 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 © Crown copyright Met Office 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. © Crown copyright Met Office 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 © Crown copyright Met Office Net cloud feedback Wm-2/K unstable stable LW cloud feedback Wm-2/K unstable stable unstable stable SW cloud feedback Wm-2/K unstable © Crown copyright Met Office stable SW cloud feedback Wm-2/K unstable stable LTS response K/K unstable © Crown copyright Met Office stable 1.5m T response K/K unstable stable q700 response K/K unstable stable SW cloud feedback Wm-2/K unstable stable w500 response hPa/day/K unstable stable Precip response mm/day/K unstable © Crown copyright Met Office stable unstable stable Conclusions. 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. © Crown copyright Met Office