On the Non-monotonic Variation of the Entrainment Buoyancy Flux with Wind Shear

The magnitude of the entrainment buoyancy flux, and hence the growth rate of the convective boundary layer, does not increase monotonically with wind shear. Explanations for this have previously been based on wind-shear effects on the turbulence kinetic energy. By distinguishing between turbulent an...

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Bibliographic Details
Published in:Boundary-layer meteorology 2022-09, Vol.184 (3), p.463-477
Main Authors: Fodor, Katherine, Mellado, Juan Pedro, Haghshenas, Armin
Format: Article
Language:English
Subjects:
Atmospheric Protection/Air Quality Control/Air Pollution
Atmospheric Sciences
Boundary layers
Buoyancy
Buoyancy flux
Correlation
Dynamic meteorology
Earth and Environmental Science
Earth Sciences
Energy
Entrainment
Fluctuations
Force and energy
Froude number
Growth rate
Kinetic energy
Meteorology
Mixed layer
Research Article
Statistical methods
Turbulence
Velocity
Vertical velocities
Wind
Wind effects
Wind shear
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Summary:The magnitude of the entrainment buoyancy flux, and hence the growth rate of the convective boundary layer, does not increase monotonically with wind shear. Explanations for this have previously been based on wind-shear effects on the turbulence kinetic energy. By distinguishing between turbulent and non-turbulent regions, we provide an alternative explanation based on two competing wind-shear effects: the initial decrease in the correlation between buoyancy and vertical velocity fluctuations, and the increase in the turbulent area fraction. The former is determined by the change in the dominant forcing; without wind shear, buoyancy fluctuations drive vertical velocity fluctuations and the two are thus highly correlated; with wind shear, vertical velocity fluctuations are partly determined by horizontal velocity fluctuations via the transfer of kinetic energy through the pressure–strain correlation, thus reducing their correlation with the buoyancy field. The increasing turbulent area fraction, on the other hand, is determined by the increasing shear production of turbulence kinetic energy inside the entrainment zone. We also show that the dependence of these conditional statistics on the boundary-layer depth and on the magnitude of the wind shear can be captured by a single non-dimensional variable, which can be interpreted as an entrainment-zone Froude number.
ISSN:0006-8314
1573-1472
DOI:10.1007/s10546-022-00712-x