Sticky Thermals: Evidence for a Dominant Balance between Buoyancy and Drag in Cloud Updrafts

Abstract The vertical velocities of convective clouds are of great practical interest because of their influence on many phenomena, including severe weather and stratospheric moistening. However, the magnitudes of forces giving rise to these vertical velocities are poorly understood, and the dominant balance is in dispute. Here, an algorithm is used to extract thousands of cloud thermals from a large-eddy simulation of deep and tropical maritime convection. Using a streamfunction to define natural boundaries for these thermals, the dominant balance in the vertical momentum equation is revealed. Cloud thermals rise with a nearly constant speed determined by their buoyancy and the standard drag law with a drag coefficient of 0.6. Contrary to suggestions that cloud thermals might be slippery, with a dominant balance between buoyancy and acceleration, cloud thermals are found here to be sticky, with a dominant balance between buoyancy and drag.

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Flow around a Complex Building: Experimental and Large-Eddy Simulation Comparisons

Journal of Applied Meteorology ◽

10.1175/jam2219.1 ◽

2005 ◽

Vol 44 (5) ◽

pp. 571-590 ◽

Cited By ~ 10

Author(s):

Ronald Calhoun ◽

Frank Gouveia ◽

Joseph Shinn ◽

Stevens Chan ◽

Dave Stevens ◽

...

Keyword(s):

Large Eddy Simulation ◽

Initial Conditions ◽

Sonic Anemometer ◽

Momentum Equation ◽

Full Potential ◽

Reynolds Stresses ◽

Additional Insight ◽

Eddy Simulation ◽

Large Eddy ◽

Complex Building

Abstract A field program to study atmospheric releases around a complex building was performed in the summers of 1999 and 2000. The focus of this paper is to compare field data with a large-eddy simulation (LES) code to assess the ability of the LES approach to yield additional insight into atmospheric release scenarios. In particular, transient aspects of the velocity and concentration signals are studied. The simulation utilized the finite-element method with a high-fidelity representation of the complex building. Trees were represented with a canopy term in the momentum equation. Inflow and outflow conditions were used. The upwind velocity was constructed from a logarithmic law fitted to velocities obtained on two levels from a tower equipped with a 2D sonic anemometer. A number of different kinds of comparisons of the transient velocity and concentration signals are presented—direct signal versus time, spectral, Reynolds stresses, turbulent kinetic energy signals, and autocorrelations. It is concluded that the LES approach does provide additional insight, but the authors argue that the proper use of LES should include consideration of cost and may require an increased connection to field sensors; that is, higher-resolution boundary and initial conditions need to be provided to realize the full potential of LES.

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Large Eddy Simulation of Microphysics and Influencing Factors in Shallow Convective Clouds

Atmosphere ◽

10.3390/atmos12040485 ◽

2021 ◽

Vol 12 (4) ◽

pp. 485

Author(s):

Zhuangzhuang Zhou ◽

Chongzhi Yin ◽

Chunsong Lu ◽

Xingcan Jia ◽

Fang Ye ◽

...

Keyword(s):

Large Eddy Simulation ◽

Influencing Factors ◽

Vertical Velocity ◽

Large Scale ◽

Cloud Droplet ◽

Relative Dispersion ◽

Cloud Base ◽

Convective Clouds ◽

Eddy Simulation ◽

Large Eddy

A flight of shallow convective clouds during the SCMS95 (Small Cumulus Microphysics Study 1995) observation project is simulated by the large eddy simulation (LES) version of the Weather Research and Forecasting Model (WRF-LES) with spectral bin microphysics (SBM). This study focuses on relative dispersion of cloud droplet size distributions, since its influencing factors are still unclear. After validation of the simulation by aircraft observations, the factors affecting relative dispersion are analyzed. It is found that the relationships between relative dispersion and vertical velocity, and between relative dispersion and adiabatic fraction are both negative. Furthermore, the negative relationships are relatively weak near the cloud base, strengthen with the increasing height first and then weaken again, which is related to the interplays among activation, condensation and evaporation for different vertical velocity and entrainment conditions. The results will be helpful to improve parameterizations related to relative dispersion (e.g., autoconversion and effective radius) in large-scale models.

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