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What physical mechanisms cause positive subtropical low cloud feedbacks in climate models?
  • Mark J Webb,
  • Adrian Lock,
  • Tomoo Ogura
Mark J Webb
UK Met Office Hadley Centre

Corresponding Author:mark.webb@metoffice.gov.uk

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Adrian Lock
Met Office
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Tomoo Ogura
National Institute for Environmental Studies
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We investigate positive subtropical low cloud feedback mechanisms in climate models which have performed the CMIP6/CFMIP-3 AMIP and AMIP uniform +4K experiments while saving CFMIP-3 process diagnostics on model levels. Our analysis focuses on the trade cumulus/stratocumulus transition region between California and Hawaii, where positive low cloud feedbacks are present in the JJA season. We introduce a methodology to} test various positive cloud feedback mechanisms proposed in the literature as primary explanations for the low cloud responses in the models. Causal hypotheses are tested by comparing their predictions with the models’ responses of clouds, cloud controlling factors, boundary layer depth and temperature/humidity tendencies to climate warming. Changes in boundary layer depth, relative humidity in the cloud layer and humidity advection at the top of the boundary layer are shown to distinguish among the hypotheses considered. For the cases examined, our approach rules out 4/5 of the mechanisms considered in half of the models and 3/5 in the remainder. We argue that unambiguously identifying the positive feedback mechanisms operating in models will in some cases require intervention experiments designed to test specific hypotheses.
03 Apr 2023Submitted to ESS Open Archive
04 Apr 2023Published in ESS Open Archive