Competition between distributed and localized buoyancy fluxes in a confined volume

1999 ◽  
Vol 391 ◽  
pp. 319-336 ◽  
Author(s):  
M. G. WELLS ◽  
R. W. GRIFFITHS ◽  
J. S. TURNER
Keyword(s):  
Heat Flux ◽  
Mixed Layer ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Flux Ratio ◽  
Density Stratification ◽  
Uniform Heat Flux ◽  
Uniform Heat ◽  
Boundary Flux

We investigate the convection and density stratification that form when buoyancy fluxes are simultaneously applied to a finite volume in both a turbulent buoyant plume from a small source and as a uniform heat flux from a horizontal boundary. The turbulent plume tends to produce a stable density stratification, whereas the distributed flux from a boundary tends to force vigorous overturning and vertical mixing. Experiments show that steady, partially mixed and partially stratified states can exist when the plume buoyancy flux is greater than the distributed flux.When the two fluxes originate from the same boundary, the steady state involves a balance between the rate at which the mixed layer deepens due to encroachment and vertical advection of the stratified water far from the plume due to the plume volume flux acquired by entrainment. There is a monotonic relationship between the normalized mixed layer depth and flux ratio R (boundary flux/plume flux) for 0<R<1, and the whole tank overturns for R>1. The stable density gradient in the stratified region is primarily due to the buoyancy from the plume but is strengthened by a stabilizing temperature gradient resulting from entrainment of heat into the plume from the mixed layer. This result may be relevant to the upper oceans of high latitude where there is commonly a destabilizing heat flux from the sea surface as well as more localized and intense deep convection from the surface.For the case of fluxes from a plume on one boundary and a uniform heat flux from the opposite boundary the shape of the density profile is that given by the Baines & Turner (1969) ‘filling-box’ mechanism, with the gradient reduced by a factor (1 + R) due to the heating. Thus, when R<−1 there is no stratified region and the whole water column overturns. When 0>R>−1, the constant depth of the convecting layer is determined by a balance between buoyancy and turbulent kinetic energy in the outflow layer from the plume.

scholarly journals Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO3.4

2015 ◽  
Vol 8 (10) ◽  
pp. 3119-3130 ◽  
Author(s):  
C. Heuzé ◽  
J. K. Ridley ◽  
D. Calvert ◽  
D. P. Stevens ◽  
K. J. Heywood
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Surface Waters ◽  
Bottom Water ◽  
Climate Model ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Weddell Sea ◽  
Antarctic Bottom Water

Abstract. Most CMIP5 (Coupled Model Intercomparison Project Phase 5) models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross seas. To identify the mechanisms triggering Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO3.4 forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing give warmer surface waters, open larger polynyas causing more saline surface waters and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

scholarly journals Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO

2015 ◽  
Vol 8 (3) ◽  
pp. 2949-2972 ◽  
Author(s):  
C. Heuzé ◽  
J. K. Ridley ◽  
D. Calvert ◽  
D. P. Stevens ◽  
K. J. Heywood
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Surface Waters ◽  
Bottom Water ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Weddell Sea ◽  
Cmip5 Models ◽  
Antarctic Bottom Water

Abstract. Most CMIP5 models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross Seas. To identify the triggering mechanisms leading to Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing are warmer, open larger polynyas and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

scholarly journals Lateral Heat Exchange after the Labrador Sea Deep Convection in 2008

2014 ◽  
Vol 44 (12) ◽  
pp. 2991-3007 ◽  
Author(s):  
Weiwei Zhang ◽  
Xiao-Hai Yan
Keyword(s):  
Heat Flux ◽  
Mixed Layer ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Labrador Sea ◽  
Boundary Current ◽  
Strong Fluctuation ◽  
Layer Depth ◽  
Ensemble Mean ◽  
Lateral Heat

Abstract The mechanisms through which convected water restratifies in the Labrador Sea are still under debate. The Labrador Sea restratification after deep convection in the 2007/08 winter is studied with an eddy-resolving numerical model. The modeled mixed layer depth during wintertime resembles the Argo observed mixed layer very well, and the lateral heat flux during the subsequent restratification is in line with observations. The Irminger rings (IRs) are reproduced with fresher caps above the 300-m depths, and they are identified and tracked automatically. The model underestimates both the number of IRs in the convection area and the heat they carry. The underestimation is most likely caused by the errors in the direction of the west Greenland currents in the model, which causes more IRs propagating westward, and only the IRs originating south of 61.5°N are able to propagate southward, yet with speed much slower than observed speed. The model still observed three eddies propagating into the convection area during the restratification phase in 2008, and their thermal contribution ranges from 1% to 4% if the estimation is made at the time when they enter the convection area. If all newly generated eddies are considered, then the ensemble-mean contributions by the IRs become 5.3%. The more detailed and direct heat flux by IRs is difficult to derive because of the strong fluctuation of the identified eddy radius. Nevertheless, the modeled lateral heat flux is largely composed of the boundary current eddies and convective eddies, thus it is possible for the model to maintain an acceptable thermal balance.

A Nondimensional Analysis of Boiling Dry a Vertical Channel with a Uniform Heat Flux

1981 ◽  
Vol 103 (4) ◽  
pp. 667-672 ◽  
Author(s):  
K. H. Sun ◽  
R. B. Duffey ◽  
C. Lin
Keyword(s):  
Heat Flux ◽  
Nuclear Power ◽  
Closed Form Solution ◽  
Vertical Channel ◽  
Form Solution ◽  
Uniform Heat Flux ◽  
Two Phase ◽  
Drift Flux ◽  
Rod Bundle ◽  
Uniform Heat

A thermal-hydraulic model has been developed for describing the phenomenon of hydrodynamically-controlled dryout, or the boil-off phenomenon, in a vertical channel with a spatially-averaged or uniform heat flux. The use of the drift flux correlation for the void fraction profile, along with mass and energy balances for the system, leads to a dimensionless closed-form solution for the predictions of two-phase mixture levels and collapsed liquid levels. The physical significance of the governing dimensionless parameters are discussed. Comparisons with data from single-tube experiments, a 3 × 3 rod bundle experiment, and the Three Mile Island nuclear power plant show good agreement.

scholarly journals Class of dilute granular Couette flows with uniform heat flux

2011 ◽  
Vol 83 (2) ◽  
Author(s):  
Francisco Vega Reyes ◽  
Vicente Garzó ◽  
Andrés Santos
Keyword(s):  
Heat Flux ◽  
Uniform Heat Flux ◽  
Uniform Heat
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