Entrainment and Detrainment in Numerically Simulated Cumulus Congestus Clouds. Part II: Cloud Budgets

Abstract Lagrangian particle tracking is used in a large-eddy simulation to study an individual cumulus congestus. This allows for the direct measurement of the convective entrainment rate and of the residence times of entrained parcels within the cloud. The entrainment rate obtained by Lagrangian direct measurement is found to be higher than that obtained using the recently introduced method of Eulerian direct measurement. This discrepancy is explained by the fast recirculation of air in and out of cloudy updrafts, which Eulerian direct measurement is unable to resolve. By filtering these fast recirculations, the Lagrangian calculation produces a result in very good agreement with the Eulerian calculation. The Lagrangian method can also quantify some aspects of entrainment that cannot be probed with Eulerian methods. For instance, it is found that more than half of the air that is entrained by the cloud during its lifetime is air that was previously detrained by the cloud. Nevertheless, the cloud is highly diluted by entrained air: for cloudy air above 2 km, its mean height of origin is well above the cloud base. This paints a picture of a cloud that rapidly entrains both environmental air and its own detritus.

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Cumulus congestus over the Indian Ocean

Weather ◽

10.1002/wea.2331 ◽

2014 ◽

Vol 69 (8) ◽

pp. E1-E1

Keyword(s):

Indian Ocean ◽

The Indian Ocean ◽

Cumulus Congestus

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Lagrangian Investigation of the Precipitation Efficiency of Convective Clouds

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0159.1 ◽

2015 ◽

Vol 72 (3) ◽

pp. 1045-1062 ◽

Cited By ~ 15

Author(s):

Wolfgang Langhans ◽

Kyongmin Yeo ◽

David M. Romps

Keyword(s):

Large Eddy Simulations ◽

Cloud Base ◽

Water Molecules ◽

Lagrangian Particle ◽

Surface Precipitation ◽

Convective Clouds ◽

Precipitation Efficiency ◽

Surface Rainfall ◽

Large Eddy ◽

Cumulus Congestus

Abstract The precipitation efficiency of cumulus congestus clouds is investigated with a new Lagrangian particle framework for large-eddy simulations. The framework is designed to track particles representative of individual water molecules. A Monte Carlo approach facilitates the transition of particles between the different water classes (e.g., vapor, rain, or graupel). With this framework, it is possible to reconstruct the pathways of water as it moves from vapor at a particular altitude to rain at the surface. By tracking water molecules through both physical and microphysical space, the precipitation efficiency can be studied in detail as a function of height. Large-eddy simulations of individual cumulus congestus clouds show that the clouds convert entrained vapor to surface precipitation with an efficiency of around 10%. About two-thirds of all vapor that enters the cloud does so by entrainment in the free troposphere, but free-tropospheric vapor accounts for only one-third to one-half of the surface rainfall, with the remaining surface rainfall originating as vapor entrained through the cloud base. The smaller efficiency with which that laterally entrained water is converted into surface precipitation results from the smaller efficiencies with which it condenses, forms precipitating hydrometeors, and reaches the surface.

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A PDF-Based Formulation of Microphysical Variability in Cumulus Congestus Clouds*

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0129.1 ◽

2015 ◽

Vol 73 (1) ◽

pp. 167-184 ◽

Cited By ~ 3

Author(s):

Yefim L. Kogan ◽

David B. Mechem

Keyword(s):

Mesoscale Model ◽

Distribution Functions ◽

Cumulus Clouds ◽

Atmospheric Sciences ◽

Microphysical Process ◽

Eddy Simulation ◽

Shallow Cumulus ◽

Toga Coare ◽

Accretion Rates ◽

Cumulus Congestus

Abstract Calculating unbiased microphysical process rates over mesoscale model grid volumes necessitates knowledge of the subgrid-scale (SGS) distribution of variables, typically represented as probability distribution functions (PDFs) of the prognostic variables. In the 2014 Journal of the Atmospheric Sciences paper by Kogan and Mechem, they employed large-eddy simulation of Rain in Cumulus over the Ocean (RICO) trade cumulus to develop PDFs and joint PDFs of cloud water, rainwater, and droplet concentration. In this paper, the approach of Kogan and Mechem is extended to deeper, precipitating cumulus congestus clouds as represented by a simulation based on conditions from the TOGA COARE field campaign. The fidelity of various PDF approximations was assessed by evaluating errors in estimating autoconversion and accretion rates. The dependence of the PDF shape on grid-mean variables is much stronger in congestus clouds than in shallow cumulus. The PDFs obtained from the TOGA COARE simulations for the calculation of accretion rates may be applied to both shallow and congestus cumulus clouds. However, applying the TOGA COARE PDFs to calculate autoconversion rates introduces unacceptably large errors in shallow cumulus clouds, thus precluding the use of a “universal” PDF formulation for both cloud types.

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Cloud-Spacing Effects upon Entrainment and Rainfall along a Convective Line

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-17-0363.1 ◽

2018 ◽

Vol 57 (8) ◽

pp. 1865-1882 ◽

Cited By ~ 1

Author(s):

Daniel H. Moser ◽

Sonia Lasher-Trapp

Keyword(s):

Thermodynamic Characteristics ◽

Local Environment ◽

Separation Distance ◽

Convective Rainfall ◽

Cumulus Clouds ◽

Spacing Effects ◽

Cloud Development ◽

Cumulus Congestus ◽

Entrained Air ◽

New Generation

AbstractCumulus clouds modify their immediate surroundings by detraining their warm, humid updrafts. When clouds are closely spaced, this conditioning of the local environment may alter the properties of the air entrained by neighboring clouds and slow their dilution. This effect has not been quantified, nor has its importance been determined for influencing the amount of convective rainfall from a system of neighboring clouds. Here, a series of idealized numerical simulations, which are based on an observed line of precipitating cumulus congestus clouds, is performed using increasingly smaller cloud spacing to investigate how cloud proximity may alter entrainment, cloud development, and convective rainfall. For clouds of radius R, which is approximately 1 km in these simulations, distances between updraft centers from 4R through 9R are tested. Over this range, the initial clouds all exhibit negligible differences in the directly calculated entrainment rates and in the thermodynamic characteristics of the entrained air. Instead, for cloud separation distances of less than 6R, the subcloud inflow is increasingly disturbed, limiting initial cloud depths and slowing updraft speeds and precipitation onset. Ultimately, however, these same cases produce a new generation of clouds that are stronger and produce more rainfall than for all other cases. The smaller cloud separation distance allows precipitation outflows from the initial clouds to meet and force new, stronger cloud updrafts. For this second generation of clouds, their entrained air is slightly more humid, but the stronger updrafts and ingestion of residual ice and precipitation from earlier clouds appear to be most important for enhancing their rainfall.

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