Stratocumdus instability reconsidered: a search for physical mechanisms

Conditions under which stratocumulus cloud decks become unstable and break apart into patchy trade cumulus are investigated using observations made over the Pacific and a theoretical analysis with a mixed-layer model. That the criterion that cloud-top entrainment fluxes be turbulence-generating is sufficient for instability is found to be overstated: it appears from the observations either that solid cloud decks can support positive entrainment buoyancy fluxes or that the criterion is simply impossible to measure due to inherent uncertainties about cloud-top structure. The former conclusion is also supported by the model analysis. Other criteria for stratocumulus instability are sought, and it is concluded that further numerical experiments are necessary to establish certainty about the process.