Wind shear and buoyancy reversal at the stratocumulus top

  • A numerical experiment is designed to study the interaction at the stratocumulus top between a mean vertical shear and the buoyancy reversal due to evaporative cooling, without radiative cooling. Direct numerical simulation is used to eliminate the uncertainty introduced by turbulence models. It is found that the enhancement by shear-induced mixing of the turbulence caused by buoyancy reversal can render buoyancy reversal comparable to other forcing mechanisms. However, it is also found that (i) the velocity jump across the capping inversion Du needs to be relatively large and values of about 1ms21 that are typically associated with the convective motions inside the boundary layer are generally too small and (ii) there is no indication of cloud-top entrainment instability. To obtain these results,parameterizations of the mean entrainment velocity and the relevant time scales are derived from the study of the cloud-top vertical structure. Two overlapping layers can be identified: a background shear layer with a thicknessA numerical experiment is designed to study the interaction at the stratocumulus top between a mean vertical shear and the buoyancy reversal due to evaporative cooling, without radiative cooling. Direct numerical simulation is used to eliminate the uncertainty introduced by turbulence models. It is found that the enhancement by shear-induced mixing of the turbulence caused by buoyancy reversal can render buoyancy reversal comparable to other forcing mechanisms. However, it is also found that (i) the velocity jump across the capping inversion Du needs to be relatively large and values of about 1ms21 that are typically associated with the convective motions inside the boundary layer are generally too small and (ii) there is no indication of cloud-top entrainment instability. To obtain these results,parameterizations of the mean entrainment velocity and the relevant time scales are derived from the study of the cloud-top vertical structure. Two overlapping layers can be identified: a background shear layer with a thickness (1/3)(Du)2/Db, where Db is the buoyancy increment across the capping inversion and a turbulence layer dominated by free convection inside the cloud and by shear production inside the relatively thin overlap region. As turbulence intensifies, the turbulence layer encroaches into the background shear layer and defines thereby the entrainment velocity. Particularized to the first research flight of the Second Dynamics andChemistry of theMarine Stratocumulus (DYCOMS II) field campaign, the analysis predicts an entrainment velocity of about 3mms21 after 5–10 min—a velocity comparable to the measurements and thus indicative of the relevance of mean shear in that case.show moreshow less

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Metadaten
Author: Juan Pedro Mellado, Bjorn Stevens, Heiko SchmidtORCiD
URL:http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-13-0189.1
ISSN:1520-0469
Title of the source (English):Journal of the Atmospheric Sciences
Document Type:Scientific journal article not peer-reviewed
Language:English
Year of publication:2013
Volume/Year:71
Issue number:3
First Page:1040
Last Page:1057
Faculty/Chair:Fakultät 3 Maschinenbau, Elektro- und Energiesysteme / FG Numerische Strömungs- und Gasdynamik
Institution name at the time of publication:Fakultät für Maschinenbau, Elektrotechnik und Wirtschaftsingenieurwesen (eBTU) / JP Strömungsmodellierung
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