Total Buoyancy Flux Research Articles

Under pressure: turbulent plumes in a uniform crossflow

Direct numerical simulation is used to investigate the integral behaviour of buoyant plumes subjected to a uniform crossflow that are infinitely lazy at the source. Neither a plume trajectory defined by the centre of mass of the plume $z_c$ nor a trajectory defined by the central streamline $z_{U}$ is aligned with the average streamlines inside the plume. Both $z_c$ and $z_{U}$ are shown to correlate with field lines of the total buoyancy flux, which implies that a model for the vertical turbulent buoyancy flux is required to faithfully predict the plume angle. A study of the volume conservation equation shows that entrainment due to incorporation of ambient fluid with non-zero velocity due to the increase in the surface area (the Leibniz term) is the dominant entrainment mechanism in strong crossflows. The data indicate that pressure differences between the top and bottom of the plume play a leading role in the evolution of the horizontal and vertical momentum balances and are crucial for appropriately modelling plume rise. By direct parameterisation of the vertical buoyancy flux, the entrainment and the pressure, an integral plume model is developed which is in good agreement with the simulations for sufficiently strong crossflow. A perturbation expansion shows that the current model is an intermediate-range model valid for downstream distances up to $100\ell _b$ – $1000 \ell _b$ , where $\ell _b$ is the buoyancy length scale based on the flow speed and plume buoyancy flux.

Seasonal thermocline in the China Seas and northwestern Pacific Ocean

Climatological seasonal variations of the thermocline in the China Seas and northwestern Pacific Ocean were studied using historical data from 1930 through 2001 (707,624 profiles). The quantitative roles of surface thermal buoyancy (Bq), haline buoyancy flux (Bp), and total buoyancy flux (B) against the wind‐induced mixing (τ) in different seasons and regions were also explored using the buoyancy ratio (R = ∣Bq/Bp∣) and the Monin‐Obukhov depth ratio (δ), respectively. The thermocline has obvious seasonal variations in the study area north of 20°N. There is no thermocline along the west coast of the Bohai Sea (BS), Yellow Sea (YS), and northern East China Sea from December to March resulting from surface cooling and wind mixing. The significantly different variation of the thermocline strength on and off the Chinese shelf is mainly caused by the fact that the thermal stratification is enhanced by bottom tidal mixing on the shelf. The δ indicates that the thermocline depth on the Chinese shelf is mainly dominated by B in summer, while it is dominated by τ in winter. It reveals an opposite feature in the Kuroshio region; the dominating factor is B in winter, associated with the large heat buoyancy loss there. South of 20°N, the dominating factor is similar to that on the shelf, with the more obvious B dominant characteristic during the monsoon transition periods. The R demonstrates that B is mainly controlled by Bq all year round, with some sporadically Bp‐dominated regions in the tropical area in winter and in the BS and eastern YS in September.

A practical indicator for surface ocean heat and freshwater buoyancy fluxes and its application to the NCEP reanalysis data

The buoyancy flux at the air/sea interface plays a key role in water mass transformation and mixing as it modifies surface water density and in turn drives overturning and enhances stratification. It is the interplay of these two independent heat and freshwater buoyancy flux components that is of central importance when analysing mechanisms of the ocean/atmosphere interaction. Here, a diagnostic quantity (ΘB) is presented that allows to capture the relative contribution of both components on the buoyancy flux in one single quantity. Using NCEP reanalysis of heat and freshwater fluxes (1948–2009) demonstrates that ΘB is a convenient tool to analyse both the temporal and spatial variability of their corresponding buoyancy fluxes. For the global ocean the areal extent of buoyancy gain and loss regions changed by 10%, with the largest extent of buoyancy gain during the 1970–1990 period. In the subpolar North Atlantic, and likewise in the South Pacific, decadal variability in freshwater flux is pronounced and, for the latter region, takes control over the total buoyancy flux since the 1980s. Some of the areal extent time series show a significant correlation with large-scale climate indices.

Turbulent mixing in a strongly forced salt wedge estuary

Turbulent mixing of salt is examined in a shallow salt wedge estuary with strong fluvial and tidal forcing. A numerical model of the Merrimack River estuary is used to quantify turbulent stress, shear production, and buoyancy flux. Little mixing occurs during flood tides despite strong velocities because bottom boundary layer turbulence is dislocated from stratification elevated in the water column. During ebbs, bottom salinity fronts form at a series of bathymetric transitions. At the fronts, near‐bottom velocity and shear stress are low, but shear, stress, and buoyancy flux are elevated at the pycnocline. Internal shear layers provide the dominant source of mixing during the early ebb. Later in the ebb, the pycnocline broadens and moves down such that boundary layer turbulence dominates mixing. Mixing occurs primarily during ebbs, with internal shear mixing accounting for about 50% of the total buoyancy flux. Both the relative contribution of internal shear mixing and the mixing efficiency increase with discharge, with bulk mixing efficiencies between 0.02 and 0.07. Buoyancy fluxes in the estuary increase with discharge up to about 400 m3 s−1 above which a majority of the mixing occurs offshore. Observed buoyancy fluxes were more consistent with the k‐ɛ turbulence closure than the Mellor‐Yamada closure, and more total mixing occurred in the estuary with k‐ɛ. Calculated buoyancy fluxes were sensitive to horizontal grid resolution, as a lower resolution grid yielded less integrated buoyancy flux in the estuary and exported lower salinity water but likely had greater numerical mixing.

The effect of mesoscales on the tracer equation in z-coordinates OGCMs

In z-coordinates ocean codes, mesoscale fluxes entering the T,S equations are represented by three terms: an eddy-induced velocity, a diapycnal flux Σ and a diffusion (Redi-like) term. Several eddy resolving codes have shown that the diapycnal flux Σ is quite large. However, all ocean codes have been run with zero diapycnal flux, Σ = 0. We model Σ and show that its contribution is of the same order of magnitude as the other two mesoscale terms usually accounted for. We also assess the validity of the two arguments most frequently cited to neglect Σ: (1) in an adiabatic regime, fluxes across isopycnal surfaces must vanish and so must the diapycnal flux Σ (we show that since Σ is not the total buoyancy flux but only part of it, there is no justification in demanding that Σ should satisfy the same conditions as the total flux) and (2) the results of a z-coordinate ocean code without Σ can be re-interpreted as those derived from the TRM (temporal residual mean) in which there is no Σ almost by definition since TRM is quite close to an isopycnal model.

Colliding turbulent plumes

The collision of axisymmetric turbulent plumes with buoyancy fluxes of opposite sign is examined experimentally. The total buoyancy flux loss of each plume as a result of the collision is measured. The measurements are made using a new experimental technique for measuring the buoyancy flux of a plume based on the ventilation theory of Linden, Lane-Serff & Smeed (J. Fluid Mech. vol. 212, 1990, p. 309). The experimental results are presented as functions of the buoyancy flux ratio 0.25. This plume–plume collision is modelled using a modified set of entrainment equations. The model allows for the exchange of buoyancy and deflection of the plumes as they pass by each other. We present predictions of total buoyancy flux loss as a function of both plume strength and separation. The model predictions are compared to the experimental measurements of buoyancy flux loss, and show good agreement.

Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth

In order to quantitatively investigate the role of leads and sea-ice in air-mass modification, aircraft observations were conducted over the partially ice-covered Sea of Okhotsk. We investigated two cold-air outbreak events with different sea-ice concentrations. In both cases, the difference between the temperatures of surface air and the sea surface (ΔT) dropped rapidly with the accumulated fetch-width of leads up to about 35-40 km, and then decreased very slowly. The surface sensible heat flux originating from open water was about 300 W m−2 within a few kilometres from the coast and decreased with increasing accumulated fetch-width. The sensible heat flux was about 100 W m−2 on average. These results indicate that the downwind air-mass modification depends mainly on the total (accumulated) extent of open water. The total buoyancy flux \(\left(\overline {w'T \, '_v}\right)\) calculated by the joint frequency distribution method correlated very well with ice concentration. Such a relationship was not clear in the case of the moisture flux \((\overline {w'q'})\). The ratio between rising thermals \(\left(\overline {w^+T^+_v}\right) \) and cold downdrafts \(\left(\overline {w^-T^-_v}\right)\) differed significantly between upwind and downwind regions; that is, the buoyancy flux was dominated by \(\overline {w^+T^+_v} \) in the developing stage of the boundary layer, while \(\overline {w^-T^-_v} \) also became important after the development of the boundary layer.

Aircraft Observations of Air-Mass Modification Upstream of the Sea of Japan during Cold-Air Outbreaks

Russian research aircraft observed the atmospheric boundary layer in 2001 during weak (29 Jan.), intense (2 Feb.), and very intense (3 Feb.) cold air outbreaks. These observations occurred during a field experiment of Winter Meso-scale convective systems Observations, over the Sea of Japan in 2001 (WMO-01). Conditional sampling techniques were used to elucidate air-mass modification processes upstream of the Sea of Japan during cold-air outbreaks. Buoyancy fluxes in the lower boundary layer near the sea surface differed significantly among the three cases. TKE budgets suggest that the differences were characterized mainly by differences in buoyancy production. However, common properties in heat transfer did exist, namely intensity (3.0) and fractional coverage (28%) of rising thermals (w + T + v ). Scale analysis showed that heating by convective-scale motions dominated in the lower boundary layer. Heating by meso-y scale motions became more important in the upper boundary layer. Near the cloud top, on the other hand, contributions by cold downdrafts (w - T - v ) to the total buoyancy flux (w'T' v ) dominated. The cloud-top w'T' v , was about half of that near the sea surface, suggesting that cooling processes near the cloud top are also important in TKE production. Comparisons of fractional spatial coverage of w + T + v with thermal motions over other warm oceans show good agreement with the AMTEX and GALE cases. However, flux intensities in the present case, and in the GALE case, are stronger than in the AMTEX case. Impacts on ocean convection by the upstream, large surface heat fluxes are discussed.

Layer-averaged modeling of two-dimensional turbidity currents with a dissipative-Galerkin finite element method Part II: Sensitivity analysis and experimental verification

Choi (1998) proposed a finite element model for the simulation of turbidity currents spreading two-dimensionally in deep ambient water. The dissipative-Galerkin formulation was used for the hyperbolic feature of the governing equations. Accurate front tracking was accomplished by employing the deforming grid generation technique. In the present paper, mass conservative property of the two-dimensional numerical method is tested in the case of a buoyancy-conserving saline current. Sensitivity analyses are also performed to see the model response to such parameters as total buoyancy flux, inlet velocity, flow resistance co-efficient, slope, and sediment particle. Finally, laboratory experiments are conducted by generating saline density currents for the verification of the numerical model. The experimental data are obtained with an emphasis on bulk characteristics of the density current such as propagating pattern, longitudinal and lateral spreading distances, and opening angle. The observed result appears to be in a good agreement with the computed solution, which suggests that the developed numerical model is capable of simulating the turbidity current propagating on a slope with no lateral restriction.

Organization and Representation of Boundary Layer Clouds

Abstract This study is intended to summarize and to simplify the complicated processes in boundary layer cloud regimes using a single parameter, Q1, the normalized saturation deficit. With the aid of large eddy simulation (LES) data from different boundary layer cloud regimes it is illustrated i) that the in-cloud buoyancy flux is maximized when the fractional cloudiness approaches zero; ii) that the ensemble average buoyancy flux possesses two maxima, one for the trade wind cumulus case and one for the stratocumulus case; and iii) that the preferred mode for boundary layer clouds is either small cumuli or high values of cloudiness, and that cloudiness transitions from one regime to the other are difficult to represent numerically as in the transition regime the cloud-water-related variables are very parameter sensitive. In addition, the importance of the contribution of the liquid water flux to the in-cloud and total buoyancy flux is outlined, and simple analytical and empirical methods are presented to ...

The use of ocean microstructure to quantify both turbulent mixing and salt-fingering

A method is developed whereby the vertical fluxes of heat and salt can be deduced from oceanic microstructure measurements of temperature and shear variance. This method is appropriate when both turbulent mixing and double-diffusive convection contribute to the vertical property fluxes. Previous methods that deduce property fluxes from microstructure measurements have assumed that either turbulent mixing or double-diffusive convection is the cause of the observed microstructure; here we present a method suitable for the more general situation where both mixing processes contribute to the property fluxes in the region of interest. The key assumptions are that the mixing efficiency of the turbulence and the flux ratio of the salt fingers are unchanged by the presence of the other process. It is found that sufficiently accurate total heat and salt fluxes can be deduced from the microstructure data, but our present imprecise knowledge of the salt-finger buoyancy flux ratio places large error bars on the total buoyancy flux.