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1
KAY, ANTHONY. "Warm discharges in cold fresh water. Part 1. Line plumes in a uniform ambient." Journal of Fluid Mechanics 574 (February 15, 2007): 239–71. http://dx.doi.org/10.1017/s0022112006004101.
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Turbulent buoyant plumes in cold fresh water are analysed, assuming a quadratic dependence of density on temperature. The model is based on the assumption that entrainment velocity is proportional to vertical velocity in the plume. Numerical and asymptotic solutions are obtained for both rising and descending plumes from virtual sources with all possible combinations of buoyancy, volume and momentum fluxes. Physical sources can be identified as points on trajectories of plumes from virtual sources.The zero-buoyancy condition, at which the plume and the ambient have equal densities but their temperatures are on opposite sides of the temperature of maximum density, is of particular importance. If an upwardly buoyant plume rising through a body of water reaches the surface before passing through its zero-buoyancy level, it will form a surface gravity current; otherwise, the plume water will return to the source as a fountain. The height at which zero buoyancy is attained generally decreases as the source momentum flux increases: greater plume velocity produces greater entrainment and hence more rapid temperature change. Descending plumes, if ejected downwards against upward buoyancy, may be classified as strongly or weakly forced according to whether they reach the zero-buoyancy condition before being brought to rest. If they do, they continue to descend with favourable buoyancy; otherwise, they may form an inverted fountain. Once a descending plume has attained downward buoyancy, it can continue to descend indefinitely, ultimately behaving like a plume in a fluid with a linear equation of state. In contrast, a rising plume will eventually come to rest, however large its initial upward buoyancy and momentum fluxes are.
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WELLS, M. G., R. W. GRIFFITHS, and J. S. TURNER. "Competition between distributed and localized buoyancy fluxes in a confined volume." Journal of Fluid Mechanics 391 (July 25, 1999): 319–36. http://dx.doi.org/10.1017/s0022112099005248.
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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.
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DIEZ, FRANCISCO J., and WERNER J. A. DAHM. "Effects of heat release on turbulent shear flows. Part 3. Buoyancy effects due to heat release in jets and plumes." Journal of Fluid Mechanics 575 (March 2007): 221–55. http://dx.doi.org/10.1017/s0022112006004277.
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An integral method is presented for determining effects of buoyancy due to heat release on the properties of reacting jets and plumes. This method avoids the Morton entrainment hypothesis entirely, and thus removes the ad hoc ‘entrainment modelling’ required in most other integral approaches. We develop the integral equation for the local centreline velocity uc(x), which allows modelling in terms of the local flow width δ (x). In both the momentum-dominated jet limit and buoyancy-dominated plume limit, dimensional arguments show δ (x) ≈ x, and experimental data show the proportionality factor cδ to remain constant between these limits. The entrainment modelling required in traditional integral methods is thus replaced by the observed constant cδ value in the present method. In non-reacting buoyant jets, this new integral approach provides an exact solution for uc(x) that shows excellent agreement with experimental data, and gives simple expressions for the virtual origins of jets, plumes and buoyant jets. In the exothermically reacting case, the constant cδ value gives an expression for the buoyancy flux B(x) that allows the integral equation for uc(x) to be solved for arbitrary exit conditions. The resulting uc(x) determines the local mass, momentum and buoyancy fluxes throughout the flow, as well as the centreline mixture fraction ζc(x) and thus the flame length L. The latter provides the proper parameters Ω andΛ that determine buoyancy effects on the flame, and provides power-law scalings in the momentum-dominated and buoyancy-dominated limits. Comparisons with buoyant flame data show excellent agreement over a wide range of conditions.
4
Cessi, Paola, and Christopher L. Wolfe. "Adiabatic Eastern Boundary Currents." Journal of Physical Oceanography 43, no. 6 (June 1, 2013): 1127–49. http://dx.doi.org/10.1175/jpo-d-12-0211.1.
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Abstract The dynamics of the eastern boundary current of a high-resolution, idealized model of oceanic circulation are analyzed and interpreted in terms of residual mean theory. In this framework, it is clear that the eastern boundary current is adiabatic and inviscid. Nevertheless, the time-averaged potential vorticity is not conserved along averaged streamlines because of the divergence of Eliassen–Palm fluxes, associated with buoyancy and momentum eddy fluxes. In particular, eddy fluxes of buoyancy completely cancel the mean downwelling or upwelling, so that there is no net diapycnal residual transport. The eddy momentum flux acts like a drag on the mean velocity, opposing the acceleration from the eddy buoyancy flux: in the potential vorticity budget this results in a balance between the divergences of eddy relative vorticity and buoyancy fluxes, which leads to a baroclinic eastern boundary current whose horizontal scale is the Rossby deformation radius and whose vertical extent depends on the eddy buoyancy transport, the Coriolis parameter, and the mean surface buoyancy distribution.
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Hieronymus, Magnus, and Jonas Nycander. "The Buoyancy Budget with a Nonlinear Equation of State." Journal of Physical Oceanography 43, no. 1 (January 1, 2013): 176–86. http://dx.doi.org/10.1175/jpo-d-12-063.1.
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Abstract The nonlinear equation of state of seawater introduces a sink or source of buoyancy when water parcels of unequal salinities and temperatures are mixed. This article contains quantitative estimates of these nonlinear effects on the buoyancy budget of the global ocean. It is shown that the interior buoyancy sink can be determined from surface buoyancy fluxes. These surface buoyancy fluxes are calculated using two surface heat flux climatologies, one based on in situ measurements and the other on a reanalysis, in both cases using a nonlinear equation of state. It is also found that the buoyancy budget in the ocean general circulation model Nucleus for European Modeling of the Ocean (NEMO) is in good agreement with the buoyancy budgets based on the heat flux climatologies. Moreover, an examination of the vertically resolved buoyancy budget in NEMO shows that in large parts of the ocean the nonlinear buoyancy sink gives the largest contribution to this budget.
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Nuijens, Louise, and Bjorn Stevens. "The Influence of Wind Speed on Shallow Marine Cumulus Convection." Journal of the Atmospheric Sciences 69, no. 1 (January 1, 2012): 168–84. http://dx.doi.org/10.1175/jas-d-11-02.1.
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Abstract The role of wind speed on shallow marine cumulus convection is explored using large-eddy simulations and concepts from bulk theory. Focusing on cases characteristic of the trades, the equilibrium trade wind layer is found to be deeper at stronger winds, with larger surface moisture fluxes and smaller surface heat fluxes. The opposing behavior of the surface fluxes is caused by more warm and dry air being mixed to the surface as the cloud layer deepens. This leads to little difference in equilibrium surface buoyancy fluxes and cloud-base mass fluxes. Shallow cumuli are deeper, but not more numerous or more energetic. The deepening response is necessary to resolve an inconsistency in the subcloud layer. This argument follows from bulk concepts and assumes that the lapse rate and flux divergence of moist-conserved variables do not change, based on simulation results. With that assumption, stronger winds and a fixed inversion height imply larger surface moisture and buoyancy fluxes (heat fluxes are small initially). The consequent moistening tends to decrease cloud-base height, which is inconsistent with a larger surface buoyancy flux that tends to increase cloud-base height, in order to maintain the buoyancy flux at cloud base at a fixed fraction of its surface value (entrainment closure). Deepening the cloud layer by increasing the inversion height resolves this inconsistency by allowing the surface buoyancy flux to remain constant without further moistening the subcloud layer. Because this explanation follows from simple bulk concepts, it is suggested that the internal dynamics (mixing) of clouds is only secondary to the deepening response.
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Deremble, Bruno, and W. K. Dewar. "First-Order Scaling Law for Potential Vorticity Extraction due to Wind." Journal of Physical Oceanography 42, no. 8 (August 1, 2012): 1303–12. http://dx.doi.org/10.1175/jpo-d-11-0136.1.
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Abstract Surface sources and sinks of potential vorticity (PV) have been examined recently in various publications. These are normally identified as the mechanical and buoyant PV fluxes with the former scaled according to wind stress and the latter from buoyancy flux. The authors here examine a PV source that is often overlooked: namely, the diabatically forced source due to wind-driven deepening. Based on an idealized model of the mixed layer, the rate of deepening of the mixed layer due to wind is translated into PV extraction. The authors propose the first-order scaling law as an estimate of the net PV flux due to diabatic wind effects in the absence of other buoyancy effects. This law is verified and calibrated in several numerical experiments. Then, the authors compare the magnitude of the PV extraction due to wind to the other factors responsible for PV input/output: namely, air–sea heat flux, freshwater flux, and Ekman wind-driven currents. Finally, to illustrate the impact of the mixing induced by wind, the authors conclude with a global air–sea PV budget in the North Atlantic basin. The wind-driven diabatic PV flux is found to be comparable to all other sources in all cases and is distinguished by acting only to extract PV.
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Mirajkar, Harish N., Partho Mukherjee, and Sridhar Balasubramanian. "On the dynamics of buoyant jets in a linearly stratified ambient." Physics of Fluids 35, no. 1 (January 2023): 016609. http://dx.doi.org/10.1063/5.0136231.
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We report mean flow and turbulence characteristics of a buoyant jet evolving in a linearly stratified ambient with stratification strength [Formula: see text]. The velocity and density fields are captured experimentally using simultaneous particle image velocimetry and planar laser-induced fluorescence technique. We report our findings by strategically choosing four axial locations such that it covers different flow regimes; namely, momentum-dominated region, buoyancy-dominated region, neutral buoyant layer, and plume cap region. The results at these axial locations are presented as a function of the radial co-ordinate to provide a whole field picture of the flow dynamics. From the mean axial velocity and density fields, it is seen that the velocity and the scalar (density) widths are of the same magnitude in the momentum-dominated region but show significant difference in the buoyancy-dominated region and beyond. It is also seen that the axial velocity for the buoyant jet is consistently higher than pure jet at different axial locations due to buoyancy-aided momentum. With the help of turbulent kinetic energy (TKE) budget analysis, it is seen that the shear production ( P) and TKE dissipation ([Formula: see text]) for a buoyant jet are higher compared to the case of pure jet at different axial locations, cementing the role of buoyancy and stratification on the flow dynamics. Further, it is observed that the buoyancy flux ( B) aids and destroys TKE intermittently in the radial direction, and it is at least [Formula: see text] smaller than P, [Formula: see text], and the mean flow buoyancy flux ( F). Finally, the relative strength of the turbulent transport of momentum to that of scalar in the radial direction is quantified using the turbulent Prandtl number, [Formula: see text]. It is seen that [Formula: see text] [Formula: see text] upto the neutral buoyant layer and [Formula: see text] 0.6 in the plume cap region. The current set of results obtained from experiments are first of its kind and elucidates various aspects of the flow which hitherto remained unknown and will also prove to be useful in testing numerical simulations for buoyancy-driven flows.
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Kunze, Eric, John B. Mickett, and James B. Girton. "Destratification and Restratification of the Spring Surface Boundary Layer in a Subtropical Front." Journal of Physical Oceanography 51, no. 9 (September 2021): 2861–82. http://dx.doi.org/10.1175/jpo-d-21-0003.1.
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AbstractDestratification and restratification of a ~50-m-thick surface boundary layer in the North Pacific Subtropical Front are examined during 24–31 March 2017 in the wake of a storm using a ~5-km array of 23 chi-augmented EM-APEX profiling floats (u, υ, T, S, χT), as well as towyo and ADCP ship surveys, shipboard air-sea surface fluxes, and parameterized shortwave penetrative radiation. During the first four days, nocturnal destabilizing buoyancy fluxes mixed the surface layer over almost its full depth every night followed by restratification to N ~ 2 × 10−3 rad s−1 during daylight. Starting on 28 March, nocturnal destabilizing buoyancy fluxes weakened because weakening winds reduced latent heat flux. Shallow mixing and stratified transition layers formed above ~20-m depth. A remnant layer in the lower part of the surface layer was insulated from destabilizing surface forcing. Penetrative radiation, turbulent buoyancy fluxes, and horizontal buoyancy advection all contribute to its restratification, closing the budget to within measurement uncertainties. Buoyancy advective restratification (slumping) plays a minor role. Before 28 March, measured advective restratification is confined to daytime; is often destratifying; and is much stronger than predictions of geostrophic adjustment, mixed-layer eddy instability, and Ekman buoyancy flux because of storm-forced inertial shear. Starting on 28 March, while small, the subinertial envelope of measured buoyancy advective restratification in the remnant layer proceeds as predicted by mixed-layer eddy parameterizations.
10
Hogg, Andrew J., Edward J. Goldsmith, and Mark J. Woodhouse. "Unsteady turbulent line plumes." Journal of Fluid Mechanics 856 (September 28, 2018): 103–34. http://dx.doi.org/10.1017/jfm.2018.698.
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The unsteady ascent of a buoyant, turbulent line plume through a quiescent, uniform environment is modelled in terms of the width-averaged vertical velocity and density deficit. It is demonstrated that for a well-posed, linearly stable model, account must be made for the horizontal variation of the velocity and the density deficit; in particular the variance of the velocity field and the covariance of the density deficit and velocity fields, represented through shape factors, must exceed threshold values, and that models based upon ‘top-hat’ distributions in which the dependent fields are piecewise constant are ill-posed. Numerical solutions of the nonlinear governing equations are computed to reveal that the transient response of the system to an instantaneous change in buoyancy flux at the source may be captured through new similarity solutions, the form of which depend upon both the ratio of the old to new buoyancy fluxes and the shape factors.
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Barkan, Roy, Kraig B. Winters, and Stefan G. Llewellyn Smith. "Rotating horizontal convection." Journal of Fluid Mechanics 723 (April 16, 2013): 556–86. http://dx.doi.org/10.1017/jfm.2013.136.
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Abstract‘Horizontal convection’ (HC) is the generic name for the flow resulting from a buoyancy variation imposed along a horizontal boundary of a fluid. We study the effects of rotation on three-dimensional HC numerically in two stages: first, when baroclinic instability is suppressed and, second, when it ensues and baroclinic eddies are formed. We concentrate on changes to the thickness of the near-surface boundary layer, the stratification at depth, the overturning circulation and the flow energetics during each of these stages. Our results show that, for moderate flux Rayleigh numbers ($O(1{0}^{11} )$), rapid rotation greatly alters the steady-state solution of HC. When the flow is constrained to be uniform in the transverse direction, rapidly rotating solutions do not support a boundary layer, exhibit weaker overturning circulation and greater stratification at all depths. In this case, diffusion is the dominant mechanism for lateral buoyancy flux and the consequent buildup of available potential energy leads to baroclinically unstable solutions. When these rapidly rotating flows are perturbed, baroclinic instability develops and baroclinic eddies dominate both the lateral and vertical buoyancy fluxes. The resulting statistically steady solution supports a boundary layer, larger values of deep stratification and multiple overturning cells compared with non-rotating HC. A transformed Eulerian-mean approach shows that the residual circulation is dominated by the quasi-geostrophic eddy streamfunction and that the eddy buoyancy flux has a non-negligible interior diabatic component. The kinetic and available potential energies are greater than in the non-rotating case and the mixing efficiency drops from ${\sim }0. 7$ to ${\sim }0. 17$. The eddies play an important role in the formation of the thermal boundary layer and, together with the negatively buoyant plume, help establish deep stratification. These baroclinically active solutions have characteristics of geostrophic turbulence.
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Imhoff, Paul T., and Theodore Green. "Experimental investigation of double-diffusive groundwater fingers." Journal of Fluid Mechanics 188 (March 1988): 363–82. http://dx.doi.org/10.1017/s002211208800076x.
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Using a sand-tank model and the salt-sugar system, double-diffusive fingers formed in a saturated porous medium. In contrast to the quasi-steady fingering typically observed in a viscous fluid, the fingering here was quite unsteady. The fingers’ structure was observed, and measurements of the sugar flux indicate that double-diffusive groundwater fingers can transport solutes at rates as much as two orders of magnitude larger than those associated with molecular diffusion in motionless groundwater. The buoyancy-flux ratio, r = αFT/βFS, increased from r = 0.65 ± 0.02 (at Rρ = 1.02) to r = 0.81 ± 0.06 (at Rρ = 1.50), where Rρ is the density-anomaly ratio. (Using the salt-sugar system in a viscous fluid, r was previously shown to decrease with increasing Rρ.) The buoyancy flux due to sugar varied approximately as R−5.6ρ, which is almost identical with the variation found for salt-sugar fingers in a viscous fluid. The model of Green (1984) was applied to the experiments and predicted buoyancy-flux ratios and finger widths that were in fairly good agreement with the measured values, although the predicted buoyancy fluxes due to sugar were significantly larger than the measured fluxes.
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Colas, François, Xavier Capet, James C. McWilliams, and Zhijin Li. "Mesoscale Eddy Buoyancy Flux and Eddy-Induced Circulation in Eastern Boundary Currents." Journal of Physical Oceanography 43, no. 6 (June 1, 2013): 1073–95. http://dx.doi.org/10.1175/jpo-d-11-0241.1.
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Abstract A dynamical interpretation is made of the mesoscale eddy buoyancy fluxes in the Eastern Boundary Currents off California and Peru–Chile, based on regional equilibrium simulations. The eddy fluxes are primarily shoreward and upward across a swath several hundred kilometers wide in the upper ocean; as such they serve to balance mean offshore air–sea heating and coastal upwelling. In the stratified interior the eddy fluxes are consistent with the adiabatic hypothesis associated with a mean eddy-induced velocity advecting mean buoyancy and tracers. Furthermore, with a suitable gauge choice, the horizontal fluxes are almost entirely aligned with the mean horizontal buoyancy gradient, consistent with the advective parameterization scheme of Gent and McWilliams. The associated diffusivity κ is surface intensified, matching the vertical stratification profile. The fluxes span the across-shore band of high eddy energy, but their alongshore structure is unresolved because of sampling limitations. In the surface layer the eddy flux is significantly diabatic with a shallow eddy-induced circulation cell and downgradient lateral diapycnal flux. The dominant eddy generation process is baroclinic instability, but there are significant regional differences between the upwelling systems in the flux and κ that are not consistent with simple instability theory.
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Zhao, Ming, and Philip H. Austin. "Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part I: Transport." Journal of the Atmospheric Sciences 62, no. 5 (May 1, 2005): 1269–90. http://dx.doi.org/10.1175/jas3414.1.
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Abstract This paper is the first in a two-part series in which the life cycles of numerically simulated shallow cumulus clouds are systematically examined. The life cycle data for six clouds with a range of cloud-top heights are isolated from an equilibrium trade cumulus field generated by a large-eddy simulation (LES) with a uniform resolution of 25 m. A passive subcloud tracer is used to partition the cloud life cycle transport into saturated and unsaturated components; the tracer shows that on average cumulus convection occurs in a region with time-integrated volume roughly 2 to 3 times that of the liquid-water-containing volume. All six clouds exhibit qualitatively similar vertical mass flux profiles with net downward mass transport at upper levels and net upward mass flux at lower levels. This downward mass flux comes primarily from the unsaturated cloud-mixed convective region during the dissipation stage and is evaporatively driven. Unsaturated negatively buoyant cloud mixtures dominate the buoyancy and mass fluxes in the upper portion of all clouds while saturated positively buoyant cloud mixtures dominate the fluxes at lower levels. Small and large clouds have distinct vertical profiles of heating/cooling and drying/moistening, with small clouds cooling and moistening throughout their depth, while larger clouds cool and moisten at upper levels and heat and dry at lower levels. The simulation results are compared to the predictions of conceptual models commonly used in shallow cumulus parameterizations.
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Cessi, Paola, and Christopher L. Wolfe. "Eddy-Driven Buoyancy Gradients on Eastern Boundaries and Their Role in the Thermocline." Journal of Physical Oceanography 39, no. 7 (July 1, 2009): 1595–614. http://dx.doi.org/10.1175/2009jpo4063.1.
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Abstract It is demonstrated that eddy fluxes of buoyancy at the eastern and western boundaries maintain alongshore buoyancy gradients along the coast. Eddy fluxes arise near the eastern and western boundaries because on both coasts buoyancy gradients normal to the boundary are strong. The eddy fluxes are accompanied by mean vertical flows that take place in narrow boundary layers next to the coast where the geostrophic constraint is broken. These ageostrophic cells have a velocity component normal to the coast that balances the geostrophic mean velocity. It is shown that the dynamics in these thin ageostrophic boundary layers can be replaced by effective boundary conditions for the interior flow, relating the eddy flux of buoyancy at the seaward edge of the boundary layers to the buoyancy gradient along the coast. These effective boundary conditions are applied to a model of the thermocline linearized around a mean stratification and a state of rest. The linear model parameterizes the eddy fluxes of buoyancy as isopycnal diffusion. The linear model produces horizontal gradients of buoyancy along the eastern coast on a vertical scale that depends on both the vertical diffusivity and the eddy diffusivity. The buoyancy field of the linear model agrees very well with the mean state of an eddy-resolving computation. Because the east–west difference in buoyancy is related to the zonally integrated meridional velocity, the linear model successfully predicts the meridional overturning circulation.
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Wagner, Patrick, Markus Scheinert, and Claus W. Böning. "Contribution of buoyancy fluxes to tropical Pacific sea level variability." Ocean Science 17, no. 4 (August 20, 2021): 1103–13. http://dx.doi.org/10.5194/os-17-1103-2021.
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Abstract. Regional anomalies of steric sea level are either due to redistribution of heat and freshwater anomalies or due to ocean–atmosphere buoyancy fluxes. Interannual to decadal variability in sea level across the tropical Pacific is mainly due to steric variations driven by wind stress anomalies. The importance of air–sea buoyancy fluxes is less clear. We use a global, eddy-permitting ocean model and a series of sensitivity experiments with quasi-climatological momentum and buoyancy fluxes to identify the contribution of buoyancy fluxes for interannual to decadal sea level variability in the tropical Pacific. We find their contribution on interannual timescales to be strongest in the central tropical Pacific at around a 10∘ latitude in both hemispheres and also relevant in the very east of the tropical domain. Buoyancy-flux-forced anomalies are correlated with variations driven by wind stress changes, but their effect on the prevailing anomalies and the importance of heat and freshwater fluxes vary locally. In the eastern tropical basin, interannual sea level variability is amplified by anomalous heat fluxes, while the importance of freshwater fluxes is small, and neither has any impact on decadal timescales. In the western tropical Pacific, the variability on interannual and decadal timescales is dampened by both heat and freshwater fluxes. The mechanism involves westward-propagating Rossby waves that are triggered during El Niño–Southern Oscillation (ENSO) events by anomalous buoyancy fluxes in the central tropical Pacific and counteract the prevailing sea level anomalies once they reach the western part of the basin.
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Kim, Min-Seong, Byung-Hyuk Kwon, Tae-Young Goo, and Sueng-Pil Jung. "Dropsonde-Based Heat Fluxes and Mixed Layer Height over the Sea Surface near the Korean Peninsula." Remote Sensing 15, no. 1 (December 21, 2022): 25. http://dx.doi.org/10.3390/rs15010025.
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Dropsonde-based sensible heat flux, latent heat flux, and buoyancy flux were estimated over the sea around the Korean Peninsula in 2021. During a preceding severe weather (SW) mission, a total of 243 dropsondes were released from a National Institute of Meteorological Sciences (NIMS) Atmospheric Research Aircraft (NARA). The heat fluxes were indirectly validated by comparison with model-based heat fluxes. The sensible heat flux calculated by the bulk transfer method depended entirely on the temperature difference between the sea level and atmosphere, whereas the latent heat flux was mainly affected by wind speed. Boundary layer heights above 800 m are closely related to buoyancy flux, which is greater in regions with higher sea surface temperatures. Furthermore, the utility of the dropsonde was confirmed in the marine atmospheric boundary layer (MABL) growth, which is difficult to observe in situ and, a relationship was proposed for estimating MABL based on mean meteorological data over the sea level.
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Ramaprian, B. R., and H. Haniu. "Measurements in Two-Dimensional Plumes in Crossflow." Journal of Fluids Engineering 111, no. 2 (June 1, 1989): 130–38. http://dx.doi.org/10.1115/1.3243613.
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The mean-flow and turbulent properties of two-dimensional buoyant jets discharged vertically upward into a crossflowing ambient have been measured in a hydraulic flume, using laser velocimetry and microresistance thermometry. The trajectory of the resulting inclined plume is found to be nearly straight, beyond a short distance from the source. The flow is essentially characterized by the presence of buoyancy forces along (s-direction) and perpendicular (n-direction) to the trajectory. While the s-component buoyancy tends to destabilize the flow and hence raise the overall level of turbulence in the flow, the n-component buoyancy tends to augment turbulence on the upper part of the flow and inhibit turbulence on the lower part. The experimental data are used to examine these effects quantitatively.
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BAINES, PETER G. "Two-dimensional plumes in stratified environments." Journal of Fluid Mechanics 471 (November 5, 2002): 315–37. http://dx.doi.org/10.1017/s0022112002002215.
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Laboratory experiments on the flow of negatively buoyant two-dimensional plumes adjacent to a wall in a density-stratified environment are described. The flow passes through several stages, from an inertial jet to a buoyant plume, to a neutrally buoyant jet, and then a negatively buoyant plume when it overshoots its equilibrium density. This fluid then ‘springs back’ and eventually occupies an intermediate range of heights. The flow is primarily characterized by the initial value of the buoyancy number, B0 = Q0N3/g′02, where Q0 is the initial volume flux per unit width, g′0 is the initial buoyancy and N is the buoyancy frequency of the environment. Scaled with the initial equilibrium depth D of the in flowing fluid, the maximum depth of penetration increases with B0, as does the width of the initial down flow, which is observed to increase very slowly with distance downward. Observations are made of the profiles of flow into and away from the plume as a function of height. Various properties of the flow are compared with predictions from the ‘standard’ two-dimensional entraining plume model, and this shows generally consistent agreement, although there are differences in magnitudes and in details. This flow constrasts with flows down gentle slopes into stratified environments, where two-way exchange of fluid occurs.
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CAULFIELD, C. P., and ANDREW W. WOODS. "The mixing in a room by a localized finite-mass-flux source of buoyancy." Journal of Fluid Mechanics 471 (November 5, 2002): 33–50. http://dx.doi.org/10.1017/s0022112002002082.
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The mixing produced by a turbulent buoyant plume with finite mass flux in a room is examined analytically and numerically. The entrainment of ambient fluid into the ascending buoyant plume leads to a return flow in the room which carries fluid downwards from the top of the room. The cycling of ambient fluid through the buoyant plume and the return flow causes the density to become uniform and gradually evolve towards that of the source fluid. As a result the buoyancy flux associated with the input fluid decreases and the plume motion becomes dominated by the source momentum flux. We develop an asymptotic model of the mixing using buoyant plume theory for a momentum-dominated flow. This provides an analytical description of the evolution of the density in the room which is in excellent accord with a full numerical simulation, and provides an improved description of the experimental filling-box data originally presented by Baines & Turner (1969).
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Hirabara, Mikitoshi, Hiroshi Ishizaki, and Ichiro Ishikawa. "Effects of the Westerly Wind Stress over the Southern Ocean on the Meridional Overturning." Journal of Physical Oceanography 37, no. 8 (August 1, 2007): 2114–32. http://dx.doi.org/10.1175/jpo3112.1.
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Abstract Numerical experiments were conducted to clarify the processes through which the Southern Ocean wind affects the meridional overturning (NA cell) associated with North Atlantic Deep Water production. These were based on idealized single- and twin-basin (idealized Atlantic and Pacific Ocean) models with a periodically connected passage under various forcings at the surface. Relationships among the wind stresses, the NA cell, and the buoyancy fluxes were investigated. Increased westerly wind stresses increase the surface buoyancy gains in the Southern Ocean under the density-restoring boundary condition. The buoyancy anomalies excited in the Southern Ocean propagate as baroclinic waves into the northern North Atlantic, modify the density field, and enhance the NA cell, which increases buoyancy losses there until the global buoyancy flux budget balances. The results from experiments using a realistically configured global ocean model confirm that the Southern Ocean wind effects on the NA cell can be understood consistently through thermodynamics and that the wind stresses outside the channel latitudes, as well as those at the Cape Horn latitude, affect the global buoyancy fluxes and the NA cell.
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Jeevanjee, Nadir, and David M. Romps. "Effective Buoyancy, Inertial Pressure, and the Mechanical Generation of Boundary Layer Mass Flux by Cold Pools." Journal of the Atmospheric Sciences 72, no. 8 (August 1, 2015): 3199–213. http://dx.doi.org/10.1175/jas-d-14-0349.1.
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Abstract The Davies-Jones formulation of effective buoyancy is used to define inertial and buoyant components of vertical force and to develop an intuition for these components by considering simple cases. This decomposition is applied to the triggering of new boundary layer mass flux by cold pools in a cloud-resolving simulation of radiative–convective equilibrium (RCE). The triggering is found to be dominated by inertial forces, and this is explained by estimating the ratio of the inertial forcing to the buoyancy forcing, which scales as H/h, where H is the characteristic height of the initial downdraft and h is the characteristic height of the mature cold pool’s gust front. In a simulation of the transition from shallow to deep convection, the buoyancy forcing plays a dominant role in triggering mass flux in the shallow regime, but the force balance tips in favor of inertial forcing just as precipitation sets in, consistent with the RCE results.
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Shakespeare, Callum J., and Andrew McC. Hogg. "An Analytical Model of the Response of the Meridional Overturning Circulation to Changes in Wind and Buoyancy Forcing." Journal of Physical Oceanography 42, no. 8 (August 1, 2012): 1270–87. http://dx.doi.org/10.1175/jpo-d-11-0198.1.
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Abstract An analytical model of the full-depth ocean stratification and meridional overturning circulation for an idealized Atlantic basin with a circumpolar channel is presented. The model explicitly describes the ocean response to both Southern Ocean winds and the global pattern and strength of prescribed surface buoyancy fluxes. The construction of three layers, defined by the two isopycnals of overturning extrema, allows the description of circulation and stratification in both the upper and abyssal ocean. The system is fully solved in the adiabatic limit to yield scales for the surface layer thickness, buoyancies of each layer, and overturning magnitudes. The analytical model also allows scaling of the Antarctic Circumpolar Current (ACC) transport. The veracity of the three-layer framework and derived scales is confirmed by applying the analytical model to an idealized geometry, eddy-permitting ocean general circulation model. Consistent with previous results, the abyssal overturning is found to scale inversely with wind stress, whereas the North Atlantic overturning and surface-layer thickness scale linearly with wind stress. In terms of the prescribed surface buoyancy fluxes, increased negative fluxes (buoyancy removal) in the North Atlantic increase the North Atlantic overturning and surface-layer thickness, whereas increased positive fluxes in the middle and low latitudes lead to a decrease in both parameters. Increased negative surface buoyancy fluxes to the south of Drake Passage increase the abyssal overturning and reduce the abyssal buoyancy. The ACC transport scales to first order with the sum of the Ekman transport and the abyssal overturning and thus increases with both wind stress and southern surface buoyancy flux magnitude.
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Wong, Sin Wei, M. A. Omar Awang, and Anuar Ishak. "Stagnation-Point Flow toward a Vertical, Nonlinearly Stretching Sheet with Prescribed Surface Heat Flux." Journal of Applied Mathematics 2013 (2013): 1–6. http://dx.doi.org/10.1155/2013/528717.
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An analysis is carried out to study the steady two-dimensional stagnation-point flow of an incompressible viscous fluid towards a stretching vertical sheet. It is assumed that the sheet is stretched nonlinearly, with prescribed surface heat flux. This problem is governed by three parameters: buoyancy, velocity exponent, and velocity ratio. Both assisting and opposing buoyant flows are considered. The governing partial differential equations are transformed into a system of ordinary differential equations and solved numerically by finite difference Keller-box method. The flow and heat transfer characteristics for different values of the governing parameters are analyzed and discussed. Dual solutions are found in the opposing buoyant flows, while the solution is unique for the assisting buoyant flows.
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Ruiz, Irene, Anna Rubio, Ana J. Abascal, and Oihane C. Basurko. "Modelling floating riverine litter in the south-eastern Bay of Biscay: a regional distribution from a seasonal perspective." Ocean Science 18, no. 6 (December 5, 2022): 1703–24. http://dx.doi.org/10.5194/os-18-1703-2022.
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Abstract. Although rivers contribute to the flux of litter to the marine environment, estimates of riverine litter amounts and detailed studies on floating riverine litter behaviour once it has reached the sea are still scarce. This paper provides an analysis of the seasonal behaviour of floating marine litter released by rivers within the south-eastern Bay of Biscay based on riverine litter characterizations, drifters, and high-frequency radar observations and Lagrangian simulations. Virtual particles were released in the coastal area as a proxy of the floating fraction of riverine litter entering from rivers and reaching the open waters. Particles were parameterized with a wind drag coefficient (Cd) to represent their trajectories and fate according to the buoyancy of the litter items. They were forced with numerical winds and measured currents provided by high-frequency radars covering selected seasonal week-long periods between 2009 and 2021. To gain a better insight into the type and buoyancy of the items, samples collected from a barrier placed at the Deba River (Spain) were characterized at the laboratory. Items were grouped into two categories: low-buoyancy items (objects not exposed to wind forcing, e.g. plastic bags) and highly buoyant items (objects highly exposed to wind forcing, e.g. bottles). Overall, low-buoyancy items encompassed almost 90 % by number and 68 % by weight. Weakly buoyant items were parameterized with Cd = 0 % and highly buoyant items with Cd = 4 %; this latter value is the result of the joint analysis of modelled and observed trajectories of four satellite drifting buoys released at the Adour (France), Deba (Spain), and Oria (Spain) river mouths. Particles parameterized with Cd = 4 % drifted faster towards the coast through the wind, notably during the first 24 h. In summer, over 97 % of particles beached after 1 week of simulation. In autumn this value fell to 54 %. In contrast, low-buoyancy items took longer to arrive at the shoreline, particularly during spring with fewer than 25 % of particles beached by the end of the simulations. The highest concentrations (>200 particles km−1) were recorded during summer for Cd = 4 % in the French region of Pyrénées-Atlantiques. Results showed that the regions in the study area were highly affected by rivers within or nearby the region itself. These results couple observations and a river-by-river modelling approach and can assist decision-makers on setting emergency responses to high fluxes of floating riverine litter and on defining future monitoring strategies for heavily polluted regions within the south-eastern Bay of Biscay.
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McWilliams, James C., and M. Jeroen Molemaker. "Baroclinic Frontal Arrest: A Sequel to Unstable Frontogenesis." Journal of Physical Oceanography 41, no. 3 (March 1, 2011): 601–19. http://dx.doi.org/10.1175/2010jpo4493.1.
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Abstract In a large-scale deformation flow, lateral and vertical buoyancy gradients sharpen through baroclinic frontogenesis near the surface boundary. A “thermally direct” ageostrophic secondary circulation cell arises during frontogenesis to maintain geostrophic, hydrostatic (thermal wind) momentum balance for the alongfront flow. Unstable three-dimensional fluctuations can grow during frontogenesis by baroclinic instability of the alongfront shear flow that converts frontal potential energy to fluctuation energy. At finite amplitude, the fluctuations provide alongfront-averaged eddy momentum and buoyancy fluxes that arrest the frontal sharpening even while the deformation flow persists. The frontal ageostrophic secondary circulation reverses to become a “thermally indirect” cell in the center of the front. This allows an approximate opposition between ageostrophic advection and eddy-flux divergence in the frontal buoyancy gradient variance (i.e., frontal strength) balance equation, implying frontal equilibration. During the approximately equilibrated phase, the energy exchange rates among the deformation flow, front, and fluctuations are all reduced in comparison with a solution without eddy-flux feedback on the frontal evolution. The mean stratification is enhanced by both frontogenesis and eddy vertical buoyancy flux. The thermally indirect secondary circulation arises from eddy fluxes acting to force a departure in thermal-wind balance for the alongfront flow, overwhelming the single-cell thermally direct circulation induced by the deformation flow. The equilibrated thermal-wind imbalance in the frontal flow is appreciable, and its magnitude is set by the cross-front eddy flux of alongfront vorticity. This demonstrates an essentially inviscid, baroclinic, dynamical process for frontogenetic arrest through frontal instability.
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CARDOSO, SILVANA S. S., and SEAN T. MCHUGH. "Turbulent plumes with heterogeneous chemical reaction on the surface of small buoyant droplets." Journal of Fluid Mechanics 642 (November 30, 2009): 49–77. http://dx.doi.org/10.1017/s0022112009991674.
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A model is developed for a turbulent plume with heterogeneous chemical reaction rising in an unbounded environment. The chemical reaction, which may generate or deplete buoyancy in the plume, occurs at the interface between two phases, a continuous phase and a dispersed one. We study the case in which a buoyant reactant is released at the source and forms the dispersed phase, consisting of very small bubbles, droplets or particles. Once in contact with the ambient fluid, a first-order irreversible reaction takes place at the surface of the, for example, droplets. The behaviour of this plume in a uniform and stratified environment is examined. We show that the dynamics of a pure plume with such heterogeneous reaction is completely determined by the ratio of the environmental buoyancy frequency N and a frequency parameter associated with the chemical reaction, G. The group G is a measure of the ability of the reaction to generate buoyancy in the plume. In a uniform environment, the sign of parameter G fully determines the plume motion. When the reaction generates buoyancy (positive G) the motion is unbounded, whilst when reaction depletes buoyancy (negative G) the plume reaches a level of neutral buoyancy. A relation for this neutral buoyancy level as a function of the initial buoyancy flux of the plume and G is calculated. Our theoretical predictions compared well with experimental results using a plume of calcium carbonate particles descending in an acidic aqueous solution. In a stratified environment, the motion of the plume is always bounded, irrespective of the magnitude of G, and we determine the level of maximum buoyancy flux, as well as those of zero buoyancy and zero momentum as a function of N/G. Finally, our model is applied to study the dynamics of a localized release of carbon dioxide in the ocean.
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Chen, Yen-Cho, and J. N. Chung. "The linear stability of mixed convection in a vertical channel flow." Journal of Fluid Mechanics 325 (October 25, 1996): 29–51. http://dx.doi.org/10.1017/s0022112096008026.
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In this study, the linear stability of mixed-convection flow in a vertical channel is investigated for both buoyancy-assisted and -opposed conditions. The disturbance momentum and energy equations were solved by the Galerkin method. In addition to the case with a zero heat flux perturbation boundary condition, we also examined the zero temperature perturbation boundary condition. In general, the mixed-convection flow is strongly destabilized by the heat transfer and therefore the fully developed heated flow is very unstable and very difficult to maintain in nature. For buoyancy-assisted flow, the two-dimensional disturbances dominate, while for buoyancy-opposed flow, the Rayleigh–Taylor instability prevails for zero heat flux perturbation boundary condition, and for the zero temperature perturbation on the boundaries the two-dimensional disturbances dominate except at lower Reynolds numbers where the Rayleigh–Taylor instability dominates again. The instability characteristics of buoyancy-assisted flow are found to be strongly dependent on the Prandtl number whereas the Prandtl number is a weak parameter for buoyancy-opposed flow. Also the least-stable disturbances are nearly one-dimensional for liquids and heavy oils at high Reynolds numbers in buoyancy-assisted flows.From an energy budget analysis, we found that the thermal–buoyant instability is the dominant type for buoyancy-assisted flow. In buoyancy-opposed flow, under the zero temperature perturbation boundary condition the Rayleigh–Taylor instability dominates for low-Reynolds-number flow and then the thermal–shear instability takes over for the higher Reynolds numbers whereas the Rayleigh–Taylor instability dominates solely for the zero heat flux perturbation boundary condition. It is found that the instability characteristics for some cases of channel flow in this study are significantly different from previous results for heated annulus and pipe flows. Based on the distinctly different wave speed characteristics and disturbance amplification rates, we offer some suggestions regarding the totally different laminar–turbulent transition patterns for buoyancy-assisted and -opposed flows.
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Gladstone, Charlotte, and Andrew W. Woods. "Detrainment from a turbulent plume produced by a vertical line source of buoyancy in a confined, ventilated space." Journal of Fluid Mechanics 742 (February 21, 2014): 35–49. http://dx.doi.org/10.1017/jfm.2013.640.
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AbstractNew experiments are presented which explore the dynamics of a turbulent buoyant plume produced by a vertically distributed linear source of buoyancy of strength $f$ per unit height. In a uniform environment, the plume volume flux increases with height from the base of the source, $z$, as $q(z) = {2^{-1/3}} {\pi }^{2/3} \alpha ^{4/3} f^{1/3} z^2$ where the entrainment coefficient, $\alpha = 0.09\pm 0.01$. In an enclosed space, with a net upward vertical ventilation flow $Q_V$, the buoyant plume generates a steady ambient stratification. The lowest part of the space, $z<h_i$, where $q(h_i)=Q_V$, is filled with fluid supplied by the ventilation flow and there is a net upflow in the ambient. Above this, $z>h_i$, the ambient fluid is linearly stratified with a reduced gravity gradient $f/Q_V$, and has no net vertical motion. Instead, for $z>h_i$, the time-averaged volume flux in the plume equals the ventilation flow. The intermittent entrainment of ambient fluid into the plume is now matched by intermittent detrainment from the plume, and the mean buoyancy in the plume relative to the ambient remains constant. The supply of fresh ventilation fluid to the ambient in the linearly stratified zone only occurs through the local detrainment and consequent horizontal intrusion of fluid from the plume. This has key implications for design of ventilation systems, in which there may be vertically distributed sources of buoyancy.
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Thurnherr, A. M., L. Clément, L. St. Laurent, R. Ferrari, and T. Ijichi. "Transformation and Upwelling of Bottom Water in Fracture Zone Valleys." Journal of Physical Oceanography 50, no. 3 (March 2020): 715–26. http://dx.doi.org/10.1175/jpo-d-19-0021.1.
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AbstractClosing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.
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STURMAN, J. J., and G. N. IVEY. "Unsteady convective exchange flows in cavities." Journal of Fluid Mechanics 368 (August 10, 1998): 127–53. http://dx.doi.org/10.1017/s002211209800175x.
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Horizontal exchange flows driven by spatial variation of buoyancy fluxes through the water surface are found in a variety of geophysical situations. In all examples of such flows the timescale characterizing the variability of the buoyancy fluxes is important and it can vary greatly in magnitude. In this laboratory study we focus on the effects of this unsteadiness of the buoyancy forcing and its influence on the resulting flushing and circulation processes in a cavity. The experiments described all start with destabilizing forcing of the flows, but the buoyancy fluxes are switched to stabilizing forcing at three different times spanning the major timescales characterizing the resulting cavity-scale flows. For destabilizing forcing, these timescales are the flushing time of the region of forcing, and the filling-box timescale, the time for the cavity-scale flow to reach steady state. When the forcing is stabilizing, the major timescale is the time for the fluid in the exchange flow to pass once through the forcing boundary layer. This too is a measure of the time to reach steady state, but it is generally distinct from the filling-box time. When a switch is made from destabilizing to stabilizing buoyancy flux, inertia is important and affects the approach to steady state of the subsequent flow. Velocities of the discharges from the end regions, whether forced in destabilizing or stabilizing ways, scaled as u∼(Bl)1/3 (where B is the forcing buoyancy flux and l is the length of the forcing region) in accordance with Phillips' (1966) results. Discharges with destabilizing and stabilizing forcing were, respectively, Q−∼(Bl)1/3H and Q+∼(Bl)1/3δ (where H is the depth below or above the forcing plate and δ is the boundary layer thickness). Thus Q−/Q+>O(1) provided H>O(δ), as was certainly the case in the experiments reported, demonstrating the overall importance of the flushing processes occurring during periods of cooling or destabilizing forcing.
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Strauss, Clément, Didier Ricard, and Christine Lac. "Dynamics of the Cloud–Environment Interface and Turbulence Effects in an LES of a Growing Cumulus Congestus." Journal of the Atmospheric Sciences 79, no. 3 (March 2022): 593–619. http://dx.doi.org/10.1175/jas-d-20-0386.1.
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Abstract A giga-large-eddy simulation of a cumulus congestus has been performed with a 5-m resolution to examine the fine-scale dynamics and mixing on its edges. At 5-m resolution, the dynamical production of subgrid turbulence clearly dominates over the thermal production, whereas the situation is reversed for resolved turbulence, the tipping point occurring near the 250-m scale. For cloud dynamics, the toroidal circulation already obtained in previous observational and numerical studies remains, with a strong signature on the resolved turbulent fluxes, the most important feature for the exchanges between the cloud and its environment even though numerous smaller eddies are also well resolved. The environment compensates for the upward mass flux through a large-scale compensating subsidence and the so-called subsiding shell composed of cloud-edge downdrafts, both having a significant contribution. A partition is used to characterize the dynamics, buoyancy, and turbulence of the inner and outer edges of the cloud, the cloud interior, and the far environment. On the edges of the cloud, downdrafts caused by the eddies and by evaporative cooling effects coexist with a buoyancy reversal while the cloud interior is mostly rising and positively buoyant. An alternative simulation in which evaporative cooling is suppressed indicates that this process reinforces the downdrafts near the edges of the cloud and causes a general decrease of the convective circulation. Evaporative cooling also has an impact on the buoyancy reversal and on the fate of the engulfed air inside the cloud.
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Yoshikawa, Yutaka. "Scaling Surface Mixing/Mixed Layer Depth under Stabilizing Buoyancy Flux." Journal of Physical Oceanography 45, no. 1 (January 2015): 247–58. http://dx.doi.org/10.1175/jpo-d-13-0190.1.
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AbstractThis study concerns the combined effects of Earth’s rotation and stabilizing surface buoyancy flux upon the wind-induced turbulent mixing in the surface layer. Two different length scales, the Garwood scale and Zilitinkevich scale, have been proposed for the stabilized mixing layer depth under Earth’s rotation. Here, this study analyzes observed mixed layer depth plus surface momentum and buoyancy fluxes obtained from Argo floats and satellites, finding that the Zilitinkevich scale is more suited for observed mixed layer depths than the Garwood scale. Large-eddy simulations (LESs) reproduce this observed feature, except under a weak stabilizing flux where the mixed layer depth could not be identified with the buoyancy threshold method (because of insufficient buoyancy difference across the mixed layer base). LESs, however, show that the mixed layer depth if defined with buoyancy ratio relative to its surface value follows the Zilitinkevich scale even under such a weak stabilizing flux. LESs also show that the mixing layer depth is in good agreement with the Zilitinkevich scale. These findings will contribute to better understanding of the response of stabilized mixing/mixed layer depth to surface forcings and hence better estimation/prediction of several processes related to stabilized mixing/mixed layer depth such as air–sea interaction, subduction of surface mixed layer water, and spring blooming of phytoplankton biomass.
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Nurser, A. J. George, and Stephen M. Griffies. "Relating the Diffusive Salt Flux just below the Ocean Surface to Boundary Freshwater and Salt Fluxes." Journal of Physical Oceanography 49, no. 9 (September 2019): 2365–76. http://dx.doi.org/10.1175/jpo-d-19-0037.1.
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AbstractWe detail the physical means whereby boundary transfers of freshwater and salt induce diffusive fluxes of salinity. Our considerations focus on the kinematic balance between the diffusive fluxes of salt and freshwater, with this balance imposed by mass conservation for an element of seawater. The flux balance leads to a specific balanced form for the diffusive salt flux immediately below the ocean surface and, in the Boussinesq approximation, to a specific form for the salinity flux. This balanced form should be used in specifying the surface boundary condition for the salinity equation and the contribution of freshwater to the buoyancy budget.
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Cerovečki, Ivana, Lynne D. Talley, and Matthew R. Mazloff. "A Comparison of Southern Ocean Air–Sea Buoyancy Flux from an Ocean State Estimate with Five Other Products." Journal of Climate 24, no. 24 (December 15, 2011): 6283–306. http://dx.doi.org/10.1175/2011jcli3858.1.
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Abstract The authors have intercompared the following six surface buoyancy flux estimates, averaged over the years 2005–07: two reanalyses [the recent ECMWF reanalysis (ERA-Interim; hereafter ERA), and the National Centers for Environmental Prediction (NCEP)–NCAR reanalysis 1 (hereafter NCEP1)], two recent flux products developed as an improvement of NCEP1 [the flux product by Large and Yeager and the Southern Ocean State Estimate (SOSE)], and two ad hoc air–sea flux estimates that are obtained by combining the NCEP1 or ERA net radiative fluxes with turbulent flux estimates using the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk formulas with NCEP1 or ERA input variables. The accuracy of SOSE adjustments of NCEP1 atmospheric fields (which SOSE uses as an initial guess and a constraint) was assessed by verification that SOSE reduces the biases in the NCEP1 fluxes as diagnosed by the Working Group on Air–Sea Fluxes (Taylor), suggesting that oceanic observations may be a valuable constraint to improve atmospheric variables. Compared with NCEP1, both SOSE and Large and Yeager increase the net ocean heat loss in high latitudes, decrease ocean heat loss in the subtropical Indian Ocean, decrease net evaporation in the subtropics, and decrease net precipitation in polar latitudes. The large-scale pattern of SOSE and Large and Yeager turbulent heat flux adjustment is similar, but the magnitude of SOSE adjustments is significantly larger. Their radiative heat flux adjustments patterns differ. Turbulent heat fluxes determined by combining COARE bulk formulas with NCEP1 or ERA should not be combined with unmodified NCEP1 or ERA radiative fluxes as the net ocean heat gain poleward of 25°S becomes unrealistically large. The other surface flux products (i.e., NCEP1, ERA, Large and Yeager, and SOSE) balance more closely. Overall, the statistical estimates of the differences between the various air–sea heat flux products tend to be largest in regions with strong ocean mesoscale activity such as the Antarctic Circumpolar Current and the western boundary currents.
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Large, William G., Edward G. Patton, and Peter P. Sullivan. "Nonlocal Transport and Implied Viscosity and Diffusivity throughout the Boundary Layer in LES of the Southern Ocean with Surface Waves." Journal of Physical Oceanography 49, no. 10 (October 2019): 2631–52. http://dx.doi.org/10.1175/jpo-d-18-0202.1.
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AbstractObservations from the Southern Ocean Flux Station provide a wide range of wind, buoyancy, and wave (Stokes) forcing for large-eddy simulation (LES) of deep Southern Ocean boundary layers. Almost everywhere there is a nonzero angle Ω between the shear and the stress vectors. Also, with unstable forcing there is usually a depth where there is stable stratification, but zero buoyancy flux and often a number of depths above where there is positive flux, but neutral stratification. These features allow nonlocal transports of buoyancy and of momentum to be diagnosed, using either the Eulerian or Lagrangian shear. The resulting profiles of nonlocal diffusivity and viscosity are quite similar when scaled according to Monin–Obukhov similarity theory in the surface layer, provided the Eulerian shear is used. Therefore, a composite shape function is constructed that may be generally applicable. In contrast, the deeper boundary layer appears to be too decoupled from the Stokes component of the Lagrangian shear. The nonlocal transports can be dominant. The diagnosed across-shear momentum flux is entirely nonlocal and is highly negatively correlated with the across-shear component of the wind stress, just as nonlocal and surface buoyancy fluxes are related. Furthermore, in the convective limit the scaling coefficients become essentially identical, with some consistency with atmospheric experience. The nonlocal contribution to the along-shear momentum flux is proportional to (1 − cosΩ) and is always countergradient, but is unrelated to the aligned wind stress component.
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Cerovečki, Ivana, Lynne D. Talley, Matthew R. Mazloff, and Guillaume Maze. "Subantarctic Mode Water Formation, Destruction, and Export in the Eddy-Permitting Southern Ocean State Estimate." Journal of Physical Oceanography 43, no. 7 (July 1, 2013): 1485–511. http://dx.doi.org/10.1175/jpo-d-12-0121.1.
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Abstract Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air–sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air–sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air–sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air–sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air–sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air–sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.
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Eigenmann, R., S. Metzger, and T. Foken. "Generation of free convection due to changes of the local circulation system." Atmospheric Chemistry and Physics Discussions 9, no. 3 (May 7, 2009): 11367–411. http://dx.doi.org/10.5194/acpd-9-11367-2009.
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Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection events in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed free convection to be induced in situations where high buoyancy fluxes and a simultaneously occurring wind speed collapse were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyant forces then dominate over shear forces within turbulence production. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) calculated from directly measured turbulent fluxes. An analysis of the scales of turbulent motions during the free convection event using wavelet transform confirms the large-eddy scale character of the detected plume-like coherent structures. Regarding the entire COPS measurement period, free convection events (FCEs) in the morning hours occur on about 50% of all days.
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Poulsen, Mads B., Markus Jochum, James R. Maddison, David P. Marshall, and Roman Nuterman. "A Geometric Interpretation of Southern Ocean Eddy Form Stress." Journal of Physical Oceanography 49, no. 10 (October 2019): 2553–70. http://dx.doi.org/10.1175/jpo-d-18-0220.1.
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AbstractAn interpretation of eddy form stress via the geometry described by the Eliassen–Palm flux tensor is explored. Complimentary to previous works on eddy Reynolds stress geometry, this study shows that eddy form stress is fully described by a vertical ellipse, whose size, shape, and orientation with respect to the mean flow shear determine the strength and direction of vertical momentum transfers. Following a recent proposal, this geometric framework is here used to form a Gent–McWilliams eddy transfer coefficient that depends on eddy energy and a nondimensional geometric parameter α, bounded in magnitude by unity. The parameter α expresses the efficiency by which eddies exchange energy with baroclinic mean flow via along-gradient eddy buoyancy flux—a flux equivalent to eddy form stress along mean buoyancy contours. An eddy-resolving ocean general circulation model is used to estimate the spatial structure of α in the Southern Ocean and assess its potential to form a basis for parameterization. The eddy efficiency α averages to a low but positive value of 0.043 within the Antarctic Circumpolar Current, consistent with an inefficient eddy field extracting energy from the mean flow. It is found that the low eddy efficiency is mainly the result of that eddy buoyancy fluxes are weakly anisotropic on average. The eddy efficiency is subject to pronounced vertical structure and is maximum at ~3-km depth, where eddy buoyancy fluxes tend to be directed most downgradient. Since α partly sets the eddy form stress in the Southern Ocean, a parameterization for α must reproduce its vertical structure to provide a faithful representation of vertical stress divergence and eddy forcing.
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Réchou, A., P. Durand, A. Druilhet, and B. Bénech. "Turbulence structure of the boundary layer below marine clouds in the SOFIA experiment." Annales Geophysicae 13, no. 10 (October 31, 1995): 1075–86. http://dx.doi.org/10.1007/s00585-995-1075-y.
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Abstract. The SOFIA (Surface of the Ocean: Flux and Interaction with the Atmosphere) experiment, included in the ASTEX (Atlantic Stratocumulus Transition Experiment) field program, was conducted in June 1992 in the Azores region in order to investigate air-sea exchanges, as well as the structure of the atmospheric boundary layer and its capping low-level cloud cover. We present an analysis of the vertical structure of the marine atmospheric boundary layer (MABL), and especially of its turbulence characteristics, deduced from the aircraft missions performed during SOFIA. The meteorological situations were characteristic of a temperate latitude under anticyclonic conditions, i.e., with weak to moderate winds, weak surface sensible heat flux, and broken capping low-altitude cloud cover topped by a strong trade inversion. We show that the mixed layer, driven by the surface fluxes, is decoupled from the above cloud layer. Although weak, the surface buoyancy flux, and the convective velocity scale deduced from it, are relevant for scaling the turbulence moments. The mixed layer then follows the behaviour of a continental convective boundary layer, with the exception of the entrainment process, which is weak in the SOFIA data. These results are confirmed by conditional sampling analysis, which shows that the major turbulence source lies in the buoyant moist updrafts at the surface.
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Mehaddi, Rabah, Fabien Candelier, and Olivier Vauquelin. "Naturally bounded plumes." Journal of Fluid Mechanics 717 (February 1, 2013): 472–83. http://dx.doi.org/10.1017/jfm.2012.587.
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AbstractThis paper investigates theoretically the vertical evolution of a turbulent plume into a linearly stratified ambient fluid, by regarding it as composed of two distinct regions. In the first region, called the positive buoyant region, the plume buoyancy and the plume momentum act in the same upward direction, whereas in the second region, called the negative buoyant region, they act in opposite directions. In a first step, analytical expressions for the plume variables at the transition height (i.e. between the two regions) are obtained from one-dimensional conservation equations, using the plume entrainment model and under the Boussinesq approximation. In a second step, these variables are used in order to determine analytically the buoyancy and volume fluxes as well as the density deficit of the plume at its top. In this investigation, the transition height (denoted ${z}_{t} $) and the total plume height (denoted ${z}_{p} $) are obtained in the form of two integrals. These integrals are evaluated asymptotically in three different cases associated with particular flow regimes. Finally, the limit of the Boussinesq assumption for such flows is discussed.
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van Sommeren, Daan D. J. A., C. P. Caulfield, and Andrew W. Woods. "Turbulent buoyant convection from a maintained source of buoyancy in a narrow vertical tank." Journal of Fluid Mechanics 701 (May 10, 2012): 278–303. http://dx.doi.org/10.1017/jfm.2012.158.
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AbstractWe describe new experiments to examine the buoyancy-induced mixing which results from the injection of a small constant volume flux of fluid of density ${\rho }_{s} $ at the top of a long narrow vertical tank with square cross-section which is filled with fluid of density ${\rho }_{0} \lt {\rho }_{s} $. The injected fluid vigorously mixes with the less dense fluid which initially occupies the tank, such that a dense mixed region of turbulent fluid propagates downwards during the initial mixing phase of the experiment. For an ideal source of constant buoyancy flux ${B}_{s} $, we show that the height of the mixed region grows as $h\ensuremath{\sim} { B}_{s}^{1/ 6} {d}^{1/ 3} {t}^{1/ 2} $ and that the horizontally averaged reduced gravity $ \overline{{g}^{\ensuremath{\prime} } } = g( \overline{\rho } \ensuremath{-} {\rho }_{0} )/ {\rho }_{0} $ at the top of tank increases as $ \overline{{g}^{\ensuremath{\prime} } } (0)\ensuremath{\sim} { B}_{s}^{5/ 6} {d}^{\ensuremath{-} 7/ 3} {t}^{1/ 2} $, where $d$ is the width of the tank. Once the mixed region reaches the bottom of the tank, the turbulent mixing continues in an intermediate mixing phase, and we demonstrate that the reduced gravity at each height increases approximately linearly with time. This suggests that the buoyancy flux is uniformly distributed over the full height of the tank. The overall density gradient between the top and bottom of the mixed region is hence time-independent for both the mixing phases before and after the mixed region has reached the bottom of the tank. Our results are consistent with previous models developed for the mixing of an unstable density gradient in a confined geometry, based on Prandtl’s mixing length theory, which suggest that the turbulent diffusion coefficient and the magnitude of the local turbulent flux are given by the nonlinear relations ${ \kappa }_{T}^{\mathit{nl}} = {\lambda }^{2} {d}^{2} \mathop{ (\partial \overline{{g}^{\ensuremath{\prime} } } / \partial z)}\nolimits ^{1/ 2} $ and ${J}^{\mathit{nl}} = {\lambda }^{2} {d}^{2} \mathop{ (\partial \overline{{g}^{\ensuremath{\prime} } } / \partial z)}\nolimits ^{3/ 2} $, respectively. The $O(1)$ constant $\lambda $ relates the width of the tank to the characteristic mixing length of the turbulent eddies. Since the mixed region is characterized by a time-independent overall density gradient, we also tested the predictions based on a linear model in which the turbulent diffusion coefficient is approximated by a constant ${ \kappa }_{T}^{l} $. We solve the corresponding nonlinear and linear turbulent diffusion equations for both mixing phases, and show a good agreement with experimental profiles measured by a dye attenuation technique, in particular for the solutions based on the nonlinear model.
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DEVENISH, B. J., G. G. ROONEY, and D. J. THOMSON. "Large-eddy simulation of a buoyant plume in uniform and stably stratified environments." Journal of Fluid Mechanics 652 (April 9, 2010): 75–103. http://dx.doi.org/10.1017/s0022112010000017.
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We consider large-eddy simulation (LES) of buoyant plumes in uniform and stably stratified environments. We show that in the former case the results agree well with the simple plume model of Morton, Taylor & Turner (Proc. R. Soc. Lond. A, vol. 234, 1956, p. 1). In particular, we calculate an entrainment constant which is consistent with laboratory and field measurements and find no significant difference between the radial spreading rates of vertical velocity and buoyancy. In a stably stratified environment, the LES plume shows better agreement with Morton et al. (1956) below the level at which the buoyancy first vanishes than above this level. Above the level of neutral buoyancy, the LES plume is characterized by an ascending core of negative buoyancy surrounded by a descending annulus of positive buoyancy. We compare the LES data with the model of Bloomfield & Kerr (J. Fluid Mech., vol. 424, 2000, p. 197), which explicitly accounts for these coherent motions. The model exhibits many qualitative aspects of the LES plume and quantitative agreement can be improved by adjusting the downward volume flux relative to the upward volume flux in a manner consistent with the LES plume. This simple adjustment, along with revised values of the entrainment constants, represents the combined effects of an overturning region at the top of the plume (where a fluid element reverses direction), ‘plume-top’ entrainment (whereby the plume entrains ambient fluid above the plume) as well as lateral entrainment and detrainment processes (both external and internal) occurring above the top of the model plume.
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Fan, Y. "Formation of Arching Flux Tubes at the Base of the Solar Convection Zone." Symposium - International Astronomical Union 203 (2001): 273–75. http://dx.doi.org/10.1017/s0074180900219281.
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Solar active regions are believed to correspond to the topmost portions of Ω-shaped arching flux tubes that have risen buoyantly from the base of the solar convection zone, where strong toroidal magnetic fields are being generated by the dynamo process. The development of such emerging Ω-loops is likely a result of the buoyant instability associated with the submerged toroidal magnetic field. Using an anelastic MHD code, we simulate the formation of buoyant, arching flux tube structures as a result of the non-linear growth of the undular instability of a neutrally buoyant layer of horizontal, unidirectional magnetic field at the base of the solar convection zone.
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Gong, Kaigang, Bingguo Zhu, Bin Peng, and Jixiang He. "Numerical Investigation of Heat Transfer Characteristics of scCO2 Flowing in a Vertically-Upward Tube with High Mass Flux." Entropy 24, no. 1 (January 1, 2022): 79. http://dx.doi.org/10.3390/e24010079.
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In this work, the heat transfer characteristics of supercritical pressure CO2 in vertical heating tube with 10 mm inner diameter under high mass flux were investigated by using an SST k-ω turbulent model. The influences of inlet temperature, heat flux, mass flux, buoyancy and flow acceleration on the heat transfer of supercritical pressure CO2 were discussed. Our results show that the buoyancy and flow acceleration effect based on single phase fluid assumption fail to explain the current simulation results. Here, supercritical pseudo-boiling theory is introduced to deal with heat transfer of scCO2. scCO2 is treated to have a heterogeneous structure consisting of vapor-like fluid and liquid-like fluid. A physical model of scCO2 heat transfer in vertical heating tube was established containing a gas-like layer near the wall and a liquid-like fluid layer. Detailed distribution of thermophysical properties and turbulence in radial direction show that scCO2 heat transfer is greatly affected by the thickness of gas-like film, thermal properties of gas-like film and turbulent kinetic energy in the near-wall region. Buoyancy parameters Bu < 10−5, Bu* < 5.6 × 10−7 and flow acceleration parameter Kv < 3 × 10−6 in this paper, which indicate that buoyancy effect and flow acceleration effect has no influence on heat transfer of scCO2 under high mass fluxes. This work successfully explains the heat transfer mechanism of supercritical fluid under high mass flux.
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Cessi, Paola. "An Energy-Constrained Parameterization of Eddy Buoyancy Flux." Journal of Physical Oceanography 38, no. 8 (August 1, 2008): 1807–19. http://dx.doi.org/10.1175/2007jpo3812.1.
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Abstract A parameterization for eddy buoyancy fluxes for use in coarse-grid models is developed and tested against eddy-resolving simulations. The development is based on the assumption that the eddies are adiabatic (except near the surface) and the observation that the flux of buoyancy is affected by barotropic, depth-independent eddies. Like the previous parameterizations of Gent and McWilliams (GM) and Visbeck et al. (VMHS), the horizontal flux of a tracer is proportional to the local large-scale horizontal gradient of the tracer through a transfer coefficient assumed to be given by the product of a typical eddy velocity scale and a typical mixing length. The proposed parameterization differs from GM and VMHS in the selection of the eddy velocity scale, which is based on the kinetic energy balance of baroclinic eddies. The three parameterizations are compared to eddy-resolving computations in a variety of forcing configurations and for several sets of parameters. The VMHS and the energy balance parameterizations perform best in the tests considered here.
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Caldwell, Peter, Christopher S. Bretherton, and Robert Wood. "Mixed-Layer Budget Analysis of the Diurnal Cycle of Entrainment in Southeast Pacific Stratocumulus." Journal of the Atmospheric Sciences 62, no. 10 (October 1, 2005): 3775–91. http://dx.doi.org/10.1175/jas3561.1.
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Abstract Mixed-layer budgets of boundary layer mass, moisture, and liquid water static energy are estimated from 6 days of data collected at 20°S, 85°W (a region of persistent stratocumulus) during the East Pacific Investigation of Climate (EPIC) stratocumulus cruise in 2001. These budgets are used to estimate a mean diurnal cycle of entrainment and, by diagnosing the fluxes of humidity and liquid water static energy necessary to maintain a mixed-layer structure, of buoyancy flux. Although the entrainment rates suggested by each of the budgets have significant uncertainty, the various methods are consistent in predicting a 6-day mean entrainment rate of 4 ± 1 mm s−1, with higher values at night and very little entrainment around local noon. The diurnal cycle of buoyancy flux suggests that drizzle, while only a small term in the boundary layer moisture budget, significantly reduces subcloud buoyancy flux and may induce weak decoupling of surface and cloud-layer turbulence during the early morning hours, a structure that is maintained throughout the day by shortwave warming. Finally, the diurnal cycle of entrainment diagnosed from three recently proposed entrainment closures is found to be consistent with the observationally derived values.
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Nowak, Jakub L., Holger Siebert, Kai-Erik Szodry, and Szymon P. Malinowski. "Coupled and decoupled stratocumulus-topped boundary layers: turbulence properties." Atmospheric Chemistry and Physics 21, no. 14 (July 20, 2021): 10965–91. http://dx.doi.org/10.5194/acp-21-10965-2021.
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Abstract. We compare turbulence properties in coupled and decoupled marine stratocumulus-topped boundary layers (STBLs) using high-resolution in situ measurements performed by the helicopter-borne Airborne Cloud Turbulence Observation System (ACTOS) platform in the region of the eastern North Atlantic. The thermodynamically well-mixed coupled STBL was characterized by a comparable latent heat flux at the surface and in the cloud-top region, and substantially smaller sensible heat flux in the entire depth. Turbulence kinetic energy (TKE) was efficiently generated by buoyancy in the cloud and at the surface, and dissipated with comparable rate across the entire depth. Structure functions and power spectra of velocity fluctuations in the inertial range were reasonably consistent with the predictions of Kolmogorov theory. The turbulence was close to isotropic. In the decoupled STBL, decoupling was most obvious in humidity profiles. Heat fluxes and buoyant TKE production at the surface were similar to the coupled case. Around the transition level, latent heat flux decreased to zero and TKE was consumed by weak stability. In the cloud-top region, heat fluxes almost vanished and buoyancy production was significantly smaller than for the coupled case. The TKE dissipation rate inside the decoupled STBL varied between its sublayers. Structure functions and power spectra in the inertial range deviated from Kolmogorov scaling. This was more pronounced in the cloud and subcloud layer in comparison to the surface mixed layer. The turbulence was more anisotropic than in the coupled STBL, with horizontal fluctuations dominating. The degree of anisotropy was largest in the cloud and subcloud layer of the decoupled STBL. Integral length scales, of the order of 100 m in both cases, indicate turbulent eddies smaller than the depth of the coupled STBL or of the sublayers of the decoupled STBL. We hypothesize that turbulence produced in the cloud or close to the surface is redistributed across the entire coupled STBL but rather only inside the sublayers where it was generated in the case of the decoupled STBL. Scattered cumulus convection, developed below the stratocumulus base, may play a role in transport between those sublayers.
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Yannopoulos, Panayotis C. "Unique Superposition Solution of Multiple Plane or Round Buoyant Jets for Tracer and Buoyancy Fluxes." Journal of Environmental Engineering 138, no. 9 (September 2012): 985–89. http://dx.doi.org/10.1061/(asce)ee.1943-7870.0000554.
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Taylor, John R. "Accumulation and Subduction of Buoyant Material at Submesoscale Fronts." Journal of Physical Oceanography 48, no. 6 (June 2018): 1233–41. http://dx.doi.org/10.1175/jpo-d-17-0269.1.
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AbstractThe influence of submesoscale currents on the distribution and subduction of passive, buoyant tracers in the mixed layer is examined using large-eddy simulations. Submesoscale eddies are generated through an ageostrophic baroclinic instability associated with a background horizontal buoyancy gradient. The simulations also include various levels of surface cooling, which provides an additional source of three-dimensional turbulence. Submesoscales compete against turbulent convection and restratify the mixed layer while generating strong turbulence along a submesoscale front. Buoyant tracers accumulate at the surface along the submesoscale front where they are subducted down into the water column. The presence of submesoscales strongly modifies the vertical tracer flux, even in the presence of strong convective forcing. The correlation between high tracer concentration and strong downwelling enhances the vertical diffusivity for buoyant tracers.