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**Harvard**

Fredriksson, S., Arneborg, L., Nilsson, H. och Handler, R. (2016) *Surface shear stress dependence of gas transfer velocity parameterizations using DNS*.

** BibTeX **

@article{

Fredriksson2016,

author={Fredriksson, Sam and Arneborg, Lars and Nilsson, Håkan and Handler, R.A.},

title={Surface shear stress dependence of gas transfer velocity parameterizations using DNS},

journal={Journal of Geophysical Research: Oceans},

issn={2169-9275},

volume={121},

issue={10},

pages={7369-7389},

abstract={Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, ν, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0<Ri<∞ ranging from convection-dominated to shear-dominated cases. The results are used to: (i) evaluate parameterizations of the air-water gas-exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, and (iii) find an expression for the gas-transfer velocity for flows driven by both convection and shear. The evaluated gas transfer-velocity parametrizations are based on either the rate of turbulent kinetic energy dissipation, the surface flow-divergence, the surface heat-flux, or the wind-speed. The parametrizations based on dissipation or divergence show an unfavorable Ri dependence for flows with combined forcing whereas the parametrization based on heat-flux only shows a limited Ri dependence. The two parametrizations based on wind speed give reasonable estimates for the transfer-velocity, depending however on the surface heat-flux. The transition from convection- to shear-dominated gas-transfer-velocity is shown to be at Ri≈0.004. Furthermore, the gas-transfer is shown to be well represented by two different approaches: (i) additive forcing expressed as kg,sum=AShearu*Ri/Ric+11/4Sc-n where Ric=AShear/ABuoy4, and (ii) either buoyancy or shear dominated expressed as, kg=ABuoyBν1/4Sc-n, Ri>Ric or kg=Ashearu*Sc-n, Ri<Ric. Here ABuoy=0.4 and AShear=0.1 are constants, and n is an exponent that depends on the water surface-characteristics.},

year={2016},

keywords={Air-sea gas exchange , Direct numerical simulations , Gas transfer velocity , Heat flux , Natural convection , Surface cooling , Turbulence , Wind},

}

** RefWorks **

RT Journal Article

SR Electronic

ID 244447

A1 Fredriksson, Sam

A1 Arneborg, Lars

A1 Nilsson, Håkan

A1 Handler, R.A.

T1 Surface shear stress dependence of gas transfer velocity parameterizations using DNS

YR 2016

JF Journal of Geophysical Research: Oceans

SN 2169-9275

VO 121

IS 10

SP 7369

OP 7389

AB Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, ν, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0<Ri<∞ ranging from convection-dominated to shear-dominated cases. The results are used to: (i) evaluate parameterizations of the air-water gas-exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, and (iii) find an expression for the gas-transfer velocity for flows driven by both convection and shear. The evaluated gas transfer-velocity parametrizations are based on either the rate of turbulent kinetic energy dissipation, the surface flow-divergence, the surface heat-flux, or the wind-speed. The parametrizations based on dissipation or divergence show an unfavorable Ri dependence for flows with combined forcing whereas the parametrization based on heat-flux only shows a limited Ri dependence. The two parametrizations based on wind speed give reasonable estimates for the transfer-velocity, depending however on the surface heat-flux. The transition from convection- to shear-dominated gas-transfer-velocity is shown to be at Ri≈0.004. Furthermore, the gas-transfer is shown to be well represented by two different approaches: (i) additive forcing expressed as kg,sum=AShearu*Ri/Ric+11/4Sc-n where Ric=AShear/ABuoy4, and (ii) either buoyancy or shear dominated expressed as, kg=ABuoyBν1/4Sc-n, Ri>Ric or kg=Ashearu*Sc-n, Ri<Ric. Here ABuoy=0.4 and AShear=0.1 are constants, and n is an exponent that depends on the water surface-characteristics.

LA eng

DO 10.1002/2016JC011852

LK http://dx.doi.org/10.1002/2016JC011852

LK http://publications.lib.chalmers.se/records/fulltext/244447/local_244447.pdf

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