Dynamic Pressure Drag on Rising Buoyant Thermals in a Neutrally Stable Environment

Journal Article · 01 November 2022 · Journal of the Atmospheric Sciences

DOI: https://doi.org/10.1175/jas-d-21-0274.1 · OSTI ID:2421424

Morrison, Hugh ^[1]; Jeevanjee, Nadir ^[2]; Yano, Jun-Ichi ^[3]

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1. a National Center for Atmospheric Research, Boulder, Colorado; OSTI
2. b Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey
3. c CNRM, UMR3589 (CNRS), Météo-France, Toulouse, France

Abstract This study examines dynamic pressure drag on rising dry buoyant thermals. A theoretical expression for drag coefficientC_das a function of several other nondimensional parameters governing thermal dynamics is derived based on combining the thermal momentum budget with the similarity theory of Scorer. Using values for these nondimensional parameters from previous studies, the theory suggests drag on thermals is small relative to that on solid spheres in laminar or turbulent flow. Two sets of numerical simulations of thermals in an unstratified, neutrally stable environment using an LES configuration of the Cloud Model 1 (CM1) are analyzed. One set has a relatively low effective Reynolds numberR_eand the other has an order-of-magnitude-higherR_e; these produce laminar and turbulent resolved-scale flows, respectively. Consistent with the theoreticalC_d, the magnitude of drag is small in all simulations. However, whereas the laminar thermals haveC_d≈ 0.01, the turbulent thermals have weaklynegativedrag (C_d≈ −0.1). This difference is explained by the laminar thermals having near vertical symmetry but the turbulent thermals exhibiting considerable vertical asymmetry of their azimuthally averaged flows. In the laminar thermals, buoyancy rapidly becomes concentrated around the main centers of rotation located along the horizontal central axis, leading to expansion of thermals via baroclinic vorticity generation but doing little to break vertical symmetry of the flow. Vertical asymmetry of the azimuthally averaged flow of turbulent thermals is attributed mainly to small-scale resolved eddies that are concentrated in the upper part of the thermals.

• Research Organization: University Corporation for Atmospheric Research (UCAR), Boulder, CO (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0020104
• OSTI ID: 2421424
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Journal Issue: 11 Vol. 79; ISSN 0022-4928
• Publisher: American Meteorological SocietyCopyright Statement
• Country of Publication: United States
• Language: English