Entrainment and detrainment statistics of a stationary shallow cumulus cloud

Jingzi Huang, Henry C. Burridge, Stephan R. de Roode, Maarten van Reeuwijk

Quarterly Journal of the Royal Meteorological Society · 2025

DOI: 10.1002/qj.4920
CloudsEntrainmentConvectionDNSTNTIMixingSPARKLE

Abstract

Afullyresolvedshallowcumuluscloudsimulation(albeitatalowerReynolds number than in the atmosphere) is performed in a quiescent environment to investigate instantaneous entrainment and detrainment. Despite constant boundary conditions and environment, the cloud displays cyclic puff-like behaviour,indicatingthatpuffsareanintrinsicfeatureofthesystemnotlinked to thermals in the sub-cloud region. The cloud dynamics are examined via both Reynolds-averaged statistics and conditionally averaged statistics. The Reynolds-averagedstatisticsrevealthatthemajorityofentrainmentoccursover the upper part of cloud and that the cloud creates an intrusion at about its halfheight(about1,000m)throughwhichitdetrains.Usinganoveltechnique that enables direct evaluation of instantaneous entrainment and detrainment fluxes, we examine instantaneous entrainment and detrainment at two inter- faces:(1)thecloudboundary,whichseparatesthecloudfromtheenvironment; and (2) the updraught boundary, which separates the rising flow (updraught) fromthedescendingflow(subsidingshell).Thedatashowthattheentrainment anddetrainmentratesareafactorof2largerthanthoseestimatedfromthebulk assumption,consistentwithotherstudies.Furthermore,entrainmentisstrongly correlatedtotherisingpuffs,asevidencedbyconditioningthestatisticsonthe instantaneousbuoyancy.

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