Subtropical stratocumulus and its effect on climate

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Subtropical stratocumulus and its effect on climate

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Title: Subtropical stratocumulus and its effect on climate
Author: Caldwell, Peter M
Abstract: Despite widespread recognition of the importance of subtropical stratocumulus clouds to the planetary energy budget, the strength and even the sign of stratocumulus response to increased levels of greenhouse gases is still unknown. In this study, we attempt to address some of the causes for this uncertainty.One challenge to assessing stratocumulus response to warming is that stratocumulus behavior is determined both by small-scale local processes and by the large-scale circulation. We present a new method which sidesteps this range-of-scales difficulty by explicitly resolving fine-scale detail in the stratocumulus region while implicitly including the effects of the large-scale circulation. The results of this model suggest that stratocumulus feedback acts to cool the planet.Another reason why stratocumulus response to climate change is uncertain is that processes in the stratocumulus-topped boundary layer are not well understood or parameterized due to a dearth in measurements of key quantities. We improve this situation by deriving estimates of the diurnal cycle of entrainment from shipboard measurements taken during the East Pacific Investigation of Climate (EPIC) 2001 cruise. Comparison between these observations and values derived from several existing entrainment parameterizations is better than expected, suggesting that entrainment should be relatively well captured by models. The EPIC data is also used to construct a diurnal cycle of buoyancy flux profiles under the assumption of well-mixed conditions. These profiles, in conjunction with a comparison of the difference between cloudbase and lifting condensation level, suggest that drizzle is acting during EPIC to limit early-morning turbulent mixing.We also perform a more in-depth comparison between a large-eddy simulation (LES) and the EPIC data. The LES was found to reproduce many features of the EPIC observations. In particular, budgets of boundary-layer moisture and energy, the diurnal cycle of subsidence, boundary-layer stratification, and the relation between buoyancy forcing and entrainment were well-simulated. The model was unable to reproduce the observed precipitation rates or diurnal cycle of boundary-layer depth. Sensitivity studies found subsidence to have little effect on liquid water path, which was significantly increased in simulations where drizzle was artificially decreased.

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