Environmental sensitivities of shallow-cumulus dilution – Part 1: Selected thermodynamic conditions

Drueke, Sonja; Kirshbaum, Daniel J.; Kollias, Pavlos

doi:10.5194/acp-20-13217-2020

Cited by 13 publications

(24 citation statements)

References 75 publications

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“…As in Part I of this study, we conduct LES of shallow cumulus ensembles using the Bryan Cloud Model version 17 (CM1; Bryan and Fritsch, 2002). In LES mode, CM1 accurately reproduces the findings from past LES inter-comparison studies of shallow cumuli (Drueke et al, 2019(Drueke et al, , 2020. To examine diverse cloud fields, we consider one maritime case and one continental case, the former based on the LES inter-comparison study of the Barbados Oceanographic and Meteorological Experiment (BOMEX) by Siebesma et al (2003), and the latter based on the LES inter-comparison of shallow cumuli at the US Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) observatory in Oklahoma (Brown et al, 2002).…”

Section: Model Configurationmentioning

confidence: 73%

“…This finding can be explained by a simple buoyancy-sorting argument. Drueke et al (2020) also found a strong sensitivity of shallowcumulus dilution to continentality, in that simulated maritime cumuli experienced about twice the dilution of corresponding continental cumuli. The sensitivity was linked to larger cloud-base mass fluxes over land, driven by stronger sensible heat fluxes and subcloud turbulence.…”

Section: Introductionmentioning

confidence: 79%

“…The concept of cloud radius (R) regulating cloud dilution has prevailed for decades (e.g., Morton, 1957), and can be explained by the notion that, as R increases, the entrainment flux into the cloud, which depends on the cloud circumference, cannot keep pace with the increasing cloud cross-sectional area. While Drueke et al (2020) also found a generally strong correlation between cloud width and cloud dilution, it was not universal. Thus, while R is an important controlling parameter, its effects may be overwhelmed by other factors.…”

Section: Introductionmentioning

confidence: 89%

“…The sensitivity was linked to larger cloud-base mass fluxes over land, driven by stronger sensible heat fluxes and subcloud turbulence. Additionally, Drueke et al (2020) found the cloud dilution to be relatively insensitive to cloudand subcloud-layer depths. A doubling of the former resulted in only a 2-3 % change of the dilution rate and a 50 % increase in the latter resulted in only a 4 % decrease in dilution.…”

Section: Introductionmentioning

confidence: 98%

“…The dilution experienced by shallow cumuli is partially controlled by environmental conditions. In the first part of this study, we used LES to investigate the impacts of selected thermodynamics conditions on the cloud-core dilution (Drueke et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

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Environmental sensitivities of shallow-cumulus dilution – Part 2: Vertical wind profile

Drueke¹,

Kirshbaum²,

Kollias³

2021

Preprint

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Abstract. This second part of a numerical study on shallow-cumulus dilution focuses on the sensitivity of cloud dilution to changes in the vertical wind profile. Insights are obtained through large-eddy simulations of maritime and continental cloud fields. In these simulations, the speed of the initially uniform geostrophic wind, and the strength of geostrophic vertical wind shear in the cloud and subcloud layer are varied. Increases in the cloud-layer vertical wind shear (up to 9 m/s/km) lead to 40–50 % larger cloud-core dilution rates compared to their respective unsheared counterparts. When the background wind speed, on the other hand, is enhanced by up to 10 m/s and subcloud-layer vertical wind shear develops or is initially prescribed, the dilution rate decreases by up to 25 %. The sensitivities of the dilution rate are linked to the updraft strength and the properties of the entrained air. Increases in the wind speed or vertical wind shear result in lower vertical velocities across all sets of experiments with stronger reductions in the cloud-layer wind shear simulation (27–47 %). Weaker updrafts are exposed to mixing with the drier surrounding air for a longer time period, allowing more entrainment to occur (i.e., the "core exposure effect"). However, reduced vertical velocities, in concert with increased cloud-layer turbulence, also assist in widening the humid shell surrounding the cloud cores, leading to entrainment of more humid air (i.e., the "core-shell dilution effect"). In the experiments with cloud-layer vertical wind shear, the core exposure effect dominates and the cloud-core dilution increases with increasing shear. Conversely, when the wind speed is increased and subcloud-layer vertical wind shear develops or is imposed, the core-shell dilution effect dominates to induce a purifying effect. The sensitivities are generally stronger in the maritime simulations, where weaker sensible heat fluxes lead to narrower, more tilted, and, therefore, more suppressed cumuli when cloud-layer shear is imposed. Moreover, in the experiments with subcloud wind shear, the weaker baseline turbulence in the maritime case allows for a larger turbulence enhancement, resulting in a widening of the transition zones between the cores and their environment, leading to the entrainment of more humid air.

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