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https://doi.org/10.5194/acp-2020-336
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-336
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 Apr 2020

29 Apr 2020

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A revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Environmental sensitivities of shallow-cumulus dilution. Part I: Selected thermodynamic conditions

Sonja Drueke1, Daniel J. Kirshbaum1, and Pavlos Kollias2 Sonja Drueke et al.
  • 1Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, QC, Canada
  • 2School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA

Abstract. Cumulus entrainment, and its consequent dilution of buoyant cloud cores, strongly regulates the life cycle of shallow cumuli yet remains poorly understood. Herein, new insights into this problem are obtained through large-eddy simulations that systematically investigate the sensitivity of shallow-cumulus dilution to cloud-layer relative humidity (RH), cloud- and subcloud-layer depths, and continentality (i.e., the land-ocean contrast). The simulated cloud-core dilution is found to be strongly sensitive to continentality, with fractional dilution rates twice as large over the ocean as over land. Using a similarity theory based on the turbulent-kinetic-energy (TKE) budget, the reduced cloud-core dilution over land is attributed to larger cloud-base mass flux (mb), driven by stronger surface heating and subcloud turbulence. As mb increasies, the fractional dilution rate must decrease to maintain energetic equilibrium. A positive sensitivity is also found to cloud-layer RH, with the core dilution increasing by 25–50 % for a 10 % enhancement in RH. This sensitivity is interpreted using the buoyancy-sorting hypothesis, in that mixtures of cloud and environmental air are more likely to become negatively buoyant and detrain (rather than diluting the cloud core) in drier cloud layers. By contrast, the sensitivities of (marine) shallow-cumulus dilution to cloud- and subcloud-layer depths are weak, with virtually no sensitivity to the former and only a 15 % reduction in dilution for a tripling of the latter (from 250 m to 750 m). These surprisingly weak sensitivities are readily explained by offsetting effects in the TKE similarity theory. Altogether, these experimental findings provide useful, though still incomplete, guidance for flow-dependent shallow-cumulus entrainment formulations in large-scale models.

Sonja Drueke et al.

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Sonja Drueke et al.

Sonja Drueke et al.

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Latest update: 24 Oct 2020
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Short summary
This numerical study provides insights into selected environmental sensitivities of shallow-cumulus dilution. Among the parameters under consideration, the dilution of the cloud cores is strongly sensitive to continentality and cloud-layer relative humidity and weakly sensitive to subcloud- and cloud-layer depths. The impacts of all four parameters are interpreted using a similarity theory of shallow cumulus and buoyancy-sorting arguments.
This numerical study provides insights into selected environmental sensitivities of...
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