Observational Relationship Between Entrainment Rate and Environmental Relative Humidity and Implications for Convection Parameterization

Lu, Chen; Sun, Cheng; Liu, Yangang; Zhang, Guang J.; Wang, Danyang; Niu, Shengjie; Yin, Yan; Qiu, Yujun; Jin, Lina

doi:10.1029/2018gl080264

Cited by 28 publications

(37 citation statements)

References 76 publications

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“…Mean values (over the cloud depth) of ε , δ , and M are varied as a function of the environmental RH of different simulations and are presented in Figure . It is found that ε , δ , and M could be approximated linearly related to the RH where a positive correlation between ε and RH is observed, which was suggested in some previous studies (Lu et al, ; Stirling & Stratton, ). However, a negative correlation is seen between δ and RH as observed previously by Böing et al ().…”

Section: Resultssupporting

confidence: 84%

“…Some LES studies have found that the entrainment rate becomes higher as the environmental RH is higher (Axelsen, ; Stirling & Stratton, ). Similarly, Lu et al () also found from the aircraft observations that the entrainment rate is positively correlated with the RH. However, Bechtold et al () and Zhao et al () using a climate model argued that the entrainment rate and RH are negatively correlated in deep convection.…”

Section: Introductionmentioning

confidence: 99%

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Parameterization of Entrainment Rate and Mass Flux in Continental Cumulus Clouds: Inference From Large Eddy Simulation

Bera

Prabha

2019

JGR Atmospheres

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Large eddy simulation with contrasting environmental humidity is used to study entrainment and detrainment rates and convective mass flux during the development stages of continental deep cumulus clouds. The cumulus clouds studied here are deeper than marine shallow cumulus or stratocumulus in the published literature. The main objective of the study is to parameterize the entrainment and detrainment rates in monsoon clouds during the break monsoon periods over land, useful for the large‐scale atmospheric models. A systematic decrease in cloud liquid water path is noted in drier environments as clouds become shallower due to the reduction of positive buoyancy in the cloud core. Updraft and downdraft velocities in the subcloud layers are strengthened in the drier environments, which are found to affect in‐cloud updraft strength. A most important finding of the present study is the effect of environmental humidity on the entrainment (and detrainment) rates and the convective mass flux. Many previous studies on this topic reported opposite results. The present study found that decreasing environmental humidity leads to a decrease in both the entrainment rate and the convective mass flux but an increase in the detrainment rate. The relationship of the entrainment parameters with the environmental relative humidity can be used for parameterization in the large‐scale models. The physical mechanism responsible for such response is identified as the strengthening of the subcloud updrafts compensated by the enhanced downdraft into the boundary layer in the drier environments, and this mechanism subsequently led to higher in‐cloud updraft velocity, resulting in lower entrainment rate.

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Section: Resultssupporting

confidence: 84%

Section: Introductionmentioning

confidence: 99%

Parameterization of Entrainment Rate and Mass Flux in Continental Cumulus Clouds: Inference From Large Eddy Simulation

Bera

Prabha

2019

JGR Atmospheres

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“…4): in drier environments, a larger fraction of entrained air becomes negatively buoyant and detrains, leaving a smaller but less dilute cloud core. This trend is consistent with the observations of Lu et al (2018) and the simulations of Stirling and Stratton (2012) and Bera and Prabha (2019), but inconsistent with Derbyshire et al (2004) Moreover, cloud dilution was found to be strongly sensitive to continentality, with fractional dilution rates in maritime clouds about twice those in continental clouds. To explain this sensitivity, a similarity theory of shallow-cumulus transports based on the turbulent-kinetic-energy (TKE) budget was used (Grant and Brown, 1999;Grant and Lock, 2004;Kirshbaum and Grant, 2012).…”

Section: Discussionsupporting

confidence: 86%

“…The buoyancy-sorting concept provides a useful physical explanation of the sensitivity of cloud dilution to cloud-layer RH found herein, as well as that in the numerical simulations of Stirling and Stratton (2012) and Bera and Prabha (2019) and the observations of Lu et al (2018). For a given amount of cloud-environmental mixing, a smaller fraction of entrained air becomes negatively buoyant and detrains in moister environments, resulting in more diluted cloud cores.…”

Section: Discussionsupporting

confidence: 60%

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

Drueke¹,

Kirshbaum²,

Kollias³

2020

Preprint

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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.

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“…The above discussion indicates that, while the controls on shallow-cumulus entrainment and dilution are an active area of research, at least some of the environmental sensitivities and corresponding physical explanations of this process are either poorly understood or differ between different studies. Improving this understanding is essential given that the representation of cumulus convection, and particularly the entrainment process, is a major source of global climate sensitivity in those models (e.g., Murphy et al, 2004;Knight et al, 2007;Sanderson et al, 2008;Rougier et al, 2009;Klocke et al, 2011). In both global climate and many weather forecast models, shallow cumuli are parameterized due to insufficient grid resolution and will continue to be parameterized for some time.…”

Section: Introductionmentioning

confidence: 99%

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

2020

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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 increases, 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 a 3 % decrease for a doubling for the former and a 4 % reduction in dilution for a 50 % deeper subcloud layer. 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.

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Observational Relationship Between Entrainment Rate and Environmental Relative Humidity and Implications for Convection Parameterization

Cited by 28 publications

References 76 publications

Parameterization of Entrainment Rate and Mass Flux in Continental Cumulus Clouds: Inference From Large Eddy Simulation

Parameterization of Entrainment Rate and Mass Flux in Continental Cumulus Clouds: Inference From Large Eddy Simulation

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

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

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