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Abstract

Direct numerical simulations resolving meter and submeter scales in the cloud-top region of stratocumulus are used to investigate the interactions between a mean vertical wind shear and in-cloud turbulence driven by evaporative and radiative cooling. There are three major results. First, a critical velocity jump exists, above which shear significantly broadens the entrainment interfacial layer (EIL), enhances cloud-top cooling, and increases the mean entrainment velocity; shear effects are negligible when the velocity jump is below . Second, a depletion velocity jump exists, above which shear-enhanced mixing reduces cloud-top radiative cooling, thereby weakening the large convective motions; shear effects remain localized within the EIL when the velocity jump is below . The critical velocity jump and depletion velocity jump are provided as a function of in-cloud and free-tropospheric conditions, and one finds and for typical subtropical conditions. Third, the individual contributions to the mean entrainment velocity from mixing, radiative cooling, and evaporative cooling strongly depend on the choice of the reference height where the entrainment velocity is calculated. This result implies that the individual contributions to the mean entrainment velocity should be estimated at a comparable height while deriving entrainment-rate parameterizations. A strong shear alters substantially the magnitude and the height where these individual contributions reach their maxima, which further demonstrates the importance of shear on the dynamics of stratocumulus clouds.