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Effects of Entrainment on the Tracer Budgets of Convective Thermals.

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Title: Effects of Entrainment on the Tracer Budgets of Convective Thermals.
Authors: Watanabe, Shun-ichi I.1 (AUTHOR) swatanabe@mri-jma.go.jp, Sueki, Kenta1 (AUTHOR)
Source: Journal of the Atmospheric Sciences. Aug2026, Vol. 83 Issue 8, p1505-1522. 18p.
Subjects: Buoyancy, Large eddy simulation models, Buoyancy-driven flow, Dilution, Plumes (Fluid dynamics), Derivatives (Mathematics), Natural heat convection
Abstract: A thermal, a discrete structure that rises due to positive buoyancy, is a fundamental element of cumulus convection. The entrainment processes of convective thermals are examined using large-eddy simulations of idealized deep convection. A diagnostic framework is employed to quantify exchanges between the thermal and its environment, accurately capturing the evolution of passive tracer concentration. The bulk entrainment rate, representing the dilution of the thermal due to mixing with the surrounding environment, is evaluated based on the tracer concentration. This rate is theoretically linked to the resolved entrainment rate defined based on the mass flux passing through the thermal surface and the net entrainment rate defined based on the volume change of the thermal. An inverse relationship is found between the resolved entrainment rate and the thermal vertical velocity W, consistent with its definition. In contrast, the dependence of bulk entrainment rate on the thermal radius R exhibits more complexity: It increases with R for small R and decreases with R for larger R. These behaviors originate from the opposing dependencies of the resolved entrainment rate, which decreases with R, and the tracer mixing ratio difference between entraining and detraining air, which increases with R. Bulk entrainment rates for other physical variables are also evaluated. Differences in the bulk entrainment rates among variables are attributed to variations in anomaly distributions around the thermal, which influence the difference between entraining and detraining air and thus the degree of dilution. Significance Statement: Deep convection is composed of thermals, which are discrete structures that rise due to positive buoyancy generated by latent heat release. As thermals ascend, they exchange momentum, heat, and moisture with the surrounding environment through entrainment. This process strongly influences their buoyancy and vertical velocity. In this study, we investigated the entrainment process using a large-eddy simulation of idealized deep convection. Our results show that the extent of this exchange depends on the thermal's vertical velocity, size, and the spatial distribution of the exchanged quantities. These findings contribute to a better understanding of deep convection. [ABSTRACT FROM AUTHOR]
Copyright of Journal of the Atmospheric Sciences is the property of American Meteorological Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Label: Title
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  Data: Effects of Entrainment on the Tracer Budgets of Convective Thermals.
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  Data: <searchLink fieldCode="AR" term="%22Watanabe%2C+Shun-ichi+I%2E%22">Watanabe, Shun-ichi I.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> swatanabe@mri-jma.go.jp</i><br /><searchLink fieldCode="AR" term="%22Sueki%2C+Kenta%22">Sueki, Kenta</searchLink><relatesTo>1</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+the+Atmospheric+Sciences%22">Journal of the Atmospheric Sciences</searchLink>. Aug2026, Vol. 83 Issue 8, p1505-1522. 18p.
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  Data: <searchLink fieldCode="DE" term="%22Buoyancy%22">Buoyancy</searchLink><br /><searchLink fieldCode="DE" term="%22Large+eddy+simulation+models%22">Large eddy simulation models</searchLink><br /><searchLink fieldCode="DE" term="%22Buoyancy-driven+flow%22">Buoyancy-driven flow</searchLink><br /><searchLink fieldCode="DE" term="%22Dilution%22">Dilution</searchLink><br /><searchLink fieldCode="DE" term="%22Plumes+%28Fluid+dynamics%29%22">Plumes (Fluid dynamics)</searchLink><br /><searchLink fieldCode="DE" term="%22Derivatives+%28Mathematics%29%22">Derivatives (Mathematics)</searchLink><br /><searchLink fieldCode="DE" term="%22Natural+heat+convection%22">Natural heat convection</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: A thermal, a discrete structure that rises due to positive buoyancy, is a fundamental element of cumulus convection. The entrainment processes of convective thermals are examined using large-eddy simulations of idealized deep convection. A diagnostic framework is employed to quantify exchanges between the thermal and its environment, accurately capturing the evolution of passive tracer concentration. The bulk entrainment rate, representing the dilution of the thermal due to mixing with the surrounding environment, is evaluated based on the tracer concentration. This rate is theoretically linked to the resolved entrainment rate defined based on the mass flux passing through the thermal surface and the net entrainment rate defined based on the volume change of the thermal. An inverse relationship is found between the resolved entrainment rate and the thermal vertical velocity W, consistent with its definition. In contrast, the dependence of bulk entrainment rate on the thermal radius R exhibits more complexity: It increases with R for small R and decreases with R for larger R. These behaviors originate from the opposing dependencies of the resolved entrainment rate, which decreases with R, and the tracer mixing ratio difference between entraining and detraining air, which increases with R. Bulk entrainment rates for other physical variables are also evaluated. Differences in the bulk entrainment rates among variables are attributed to variations in anomaly distributions around the thermal, which influence the difference between entraining and detraining air and thus the degree of dilution. Significance Statement: Deep convection is composed of thermals, which are discrete structures that rise due to positive buoyancy generated by latent heat release. As thermals ascend, they exchange momentum, heat, and moisture with the surrounding environment through entrainment. This process strongly influences their buoyancy and vertical velocity. In this study, we investigated the entrainment process using a large-eddy simulation of idealized deep convection. Our results show that the extent of this exchange depends on the thermal's vertical velocity, size, and the spatial distribution of the exchanged quantities. These findings contribute to a better understanding of deep convection. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Journal of the Atmospheric Sciences is the property of American Meteorological Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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      – Type: doi
        Value: 10.1175/JAS-D-25-0177.1
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 18
        StartPage: 1505
    Subjects:
      – SubjectFull: Buoyancy
        Type: general
      – SubjectFull: Large eddy simulation models
        Type: general
      – SubjectFull: Buoyancy-driven flow
        Type: general
      – SubjectFull: Dilution
        Type: general
      – SubjectFull: Plumes (Fluid dynamics)
        Type: general
      – SubjectFull: Derivatives (Mathematics)
        Type: general
      – SubjectFull: Natural heat convection
        Type: general
    Titles:
      – TitleFull: Effects of Entrainment on the Tracer Budgets of Convective Thermals.
        Type: main
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          Name:
            NameFull: Watanabe, Shun-ichi I.
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            NameFull: Sueki, Kenta
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          Dates:
            – D: 01
              M: 08
              Text: Aug2026
              Type: published
              Y: 2026
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              Value: 83
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              Value: 8
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            – TitleFull: Journal of the Atmospheric Sciences
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