Wind-shear effects on radiatively and evaporatively driven stratocumulus tops
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hdl:2117/179805
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Data publicació2018-09-01
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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 (Du)crit
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 (Du)crit. Second, a depletion velocity jump (Du)dep 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 (Du)dep. The critical velocity jump and depletion velocity jump are provided as a function of in-cloud and free-tropospheric conditions, and one finds (Du)crit
’ 124ms21 and (Du)dep’ 3210ms21 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.Astrong 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.
CitacióSchulz, B.; Mellado, J. P. Wind-shear effects on radiatively and evaporatively driven stratocumulus tops. "Journal of the atmospheric sciences", 1 Setembre 2018, vol. 75, p. 3245-3263.
ISSN0022-4928
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SchulzMellado2018.pdf | 2,368Mb | Visualitza/Obre |