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Oceanic submesoscale processes (submesoscales) with O(1–10) km horizontal scale can generate strong vertical buoyancy flux (VBF) that significantly modulate upper‐ocean stratification. Because submesoscales cannot be resolved by the prevailing ocean models, their VBFs have to be properly parameterized in order to improve model performance. Here, based on theoretical scaling analysis, we propose a new parameterization of submesoscale VBF by simultaneously considering mixed‐layer baroclinic instability (MLI) and strain‐induced frontogenesis, which are two leading generation mechanisms of submesoscales that typically co‐occur in open ocean. Compared with the parameterization of Fox‐Kemper et al. (2008, https://doi.org/10.1175/2007jpo3792.1; F08) that only considers the MLI, the new parameterization includes mesoscale strain rate and improves vertical structure function. Diagnostic validations based on submesoscale permitting simulation outputs suggest that the newly parameterized VBFs are more realistic than F08 in regard to three‐dimension distributions. How to incorporate this new parameterization into coarser‐grid ocean models, however, needs further studies. Plain Language Summary Oceanic submesoscale processes with spatial scale of O(1–10 km) can generate strong vertical buoyancy flux (VBF) in upper ocean and therefore, they significantly modulate the vertical density distribution (i.e., stratification). Because the prevailing ocean circulation models typically have horizontal resolutions of O(10–100 km), they are unable to resolve the submesoscale VBF. In order to accurately simulate the upper‐ocean stratification, the unresolved VBF needs to be expressed using the resolved larger‐scale quantities, which is called parameterization. Here, based on theoretical scaling analysis, we propose a new parameterization of submesoscale VBF by simultaneously considering contributions from mixed‐layer instability (MLI; baroclinic instability occurring in the mixed layer) and front sharpening induced by mesoscale strain, which are two important generation mechanisms of submesoscales. Compared with the previous parameterization by Fox‐Kemper et al. (2008, https://doi.org/10.1175/2007jpo3792.1; F08) that only includes MLI mechanism, the new parameterization has incorporated mesoscale strain rate and improved vertical structure function more realistically. Diagnostic analysis based on high‐resolution simulation outputs demonstrates that the newly parameterized VBFs are more realistic in aspect of both horizontal and vertical distributions than the F08 parameterization. The test and application of this parameterization in ocean models will be a focus of future studies. Key Points A new parameterization of submesoscale vertical buoyancy flux is proposed The parameterization considers simultaneously mixed‐layer baroclinic instability and strain‐induced frontogenesis Performance of the parameterization is diagnostically validated using high‐resolution simulation outputs

We investigate the coupling effect of buoyancy and shear based on an annular centrifugal Rayleigh–Bénard convection (ACRBC) system in which two cylinders rotate with an angular velocity difference. Direct numerical simulations are performed in a Rayleigh number range $10^6\\leq Ra\\leq 10^8$, at fixed Prandtl number $Pr=4.3$, inverse Rossby number $Ro^{-1}=20$, and radius ratio $\\eta =0.5$. The shear, represented by the non-dimensional rotational speed difference $\\varOmega$, varies from $0$ to $10$, corresponding to an ACRBC without shear and a radially heated Taylor–Couette flow with only the inner cylinder rotating, respectively. A stable regime is found in the middle part of the interval for $\\varOmega$, and divides the whole parameter space into three regimes: buoyancy-dominated, stable and shear-dominated. Clear boundaries between the regimes are given by linear stability analysis, meaning the marginal state of the flow. In the buoyancy-dominated regime, the flow is a quasi-two-dimensional flow on the $r\\varphi$ plane; as shear increases, both the growth rate of instability and the heat transfer are depressed. In the shear-dominated regime, the flow is mainly on the $rz$ plane. The shear is so strong that the temperature acts as a passive scalar, and the heat transfer is greatly enhanced. The study shows that shear can stabilize buoyancy-driven convection, makes a detailed analysis of the flow characteristics in different regimes, and reveals the complex coupling mechanism of shear and buoyancy, which may have implications for fundamental studies and industrial designs.

Turbulent vertical velocity measurements are scarce in regions prone to convection such as the Labrador Sea, which hinders our understanding of deep convection dynamics. Vertical velocity, w$w$ , is retrieved from wintertime glider deployments in the convective region. From w$w$ , downward convective plumes of dense waters are identified. These plumes only cover a small fraction of the convective area. Throughout the convective area, the standard deviation of w$w$agrees with scaling relations for the atmospheric surface and boundary layers. It initially depends on surface buoyancy loss in winter, and later, on wind stress after mid‐March. Both periods are characterized by positive turbulent vertical buoyancy flux. During convective periods in winter, the positive buoyancy flux is mostly forced by surface heat loss. After mid‐March, when buoyancy loss to the atmosphere is reduced, the positive buoyancy flux results from a restratifying upward freshwater flux, potentially of lateral origins and without much atmospheric influence. Plain Language Summary Deep convection is an essential component of our climate system as it uptakes and redistributes atmospheric properties, such as anthropogenic carbon and oxygen, into the abyssal ocean. Intense ocean heat loss to the atmosphere in winter triggers convection, resulting in kilometer‐sized plumes with high downward vertical velocities and deep mixed layer depth. These plumes remain challenging to observe and parameterize in climate models. Here we show that autonomous vehicles (gliders) can sample dense downwelling plumes in the Labrador Sea. Gliders sampled a positive vertical buoyancy flux that depicts downwelling of dense water parcels and upwelling of light water parcels during convection, and that compensates a buoyancy loss from the ocean to the atmosphere. At the end of convection, an observed freshwater import produces a similar buoyancy flux unmatched by the surface flux. This flux adds buoyancy and shoals the mixed layer. Additional measurements from sufficiently long glider deployments like these ones could potentially allow us to establish a crucial link between deep water formation and an expected increase in freshwater fluxes from Arctic and Greenland sources. Key Points Vertical velocity during deep ocean convection follows scalings from the atmospheric boundary layer under wind and buoyancy forcing Vertical velocity help identify convective plumes with a horizontal scale of 620 m and a downward velocity magnitude up to 4.6 cm s−1${\\mathrm{s}}^{-1}$Positive vertical buoyancy flux occurs during convection, caused by atmospheric cooling and then by freshwater flux during restratification

Global demand for space cooling is increasing due to climate change. In this study, the coupling of radiative cooling, thermal mass, and buoyancy ventilation is investigated as an alternative to mechanical air-conditioning. A scalable analytical model is calibrated from a reduced-scale experiment to investigate the effect of different thermal mass distributions between an uninsulated roof radiator and an enclosed space. It is found that a high-mass roof radiator is a viable option for summertime temperature and natural ventilation stability, but a light (or thin) roof radiator and more thermal mass indoors is a more efficient option. Maximum nighttime cooling can be achieved at the expense of temperature stability.

In a transient warming scenario, the North Atlantic is influenced by a complex pattern of surface buoyancy flux changes that ultimately weaken the Atlantic meridional overturning circulation (AMOC). Here we study the AMOC response in the CMIP5 experiment, using the near-geostrophic balance of the AMOC on interannual time scales to identify the role of temperature and salinity changes in altering the circulation. The thermal wind relationship is used to quantify changes in the zonal density gradients that control the strength of the flow. At 40°N, where the overturning cell is at its strongest, weakening of the AMOC is largely driven by warming between 1000- and 2000-m depth along the western margin. Despite significant subpolar surface freshening, salinity changes are small in the deep branch of the circulation. This is likely due to the influence of anomalously salty water in the subpolar intermediate layers, which is carried northward from the subtropics in the upper limb of the AMOC. In the upper 1000m at 40°N, salty anomalies due to increased evaporation largely cancel the buoyancy increase due to warming. Therefore, in CMIP5, temperature dynamics are responsible for AMOC weakening, while freshwater forcing instead acts to strengthen the circulation in the net. These results indicate that past modeling studies of AMOC weakening, which rely on freshwater hosing in the subpolar gyre, may not be directly applicable to a more complex warming scenario.

This paper presents a new, generalized two‐phase debris flow model that includes many essential physical phenomena. The model employs the Mohr‐Coulomb plasticity for the solid stress, and the fluid stress is modeled as a solid‐volume‐fraction‐gradient‐enhanced non‐Newtonian viscous stress. The generalized interfacial momentum transfer includes viscous drag, buoyancy, and virtual mass. A new, generalized drag force is proposed that covers both solid‐like and fluid‐like contributions, and can be applied to drag ranging from linear to quadratic. Strong coupling between the solid‐ and the fluid‐momentum transfer leads to simultaneous deformation, mixing, and separation of the phases. Inclusion of the non‐Newtonian viscous stresses is important in several aspects. The evolution, advection, and diffusion of the solid‐volume fraction plays an important role. The model, which includes three innovative, fundamentally new, and dominant physical aspects (enhanced viscous stress, virtual mass, generalized drag) constitutes the most generalized two‐phase flow model to date, and can reproduce results from most previous simple models that consider single‐ and two‐phase avalanches and debris flows as special cases. Numerical results indicate that the model can adequately describe the complex dynamics of subaerial two‐phase debris flows, particle‐laden and dispersive flows, sediment transport, and submarine debris flows and associated phenomena. Key Points This paper presents a new, generalized and unified two‐phase debris flow model Includes non‐Newtonian viscous stress, virtual mass, generalized drag, buoyancy New model adequately describes complex two‐phase debris flow, sediment transport

Supercritical pressure fluids are widely used in heat transfer and energy systems. The benefit of high heat transfer performance and the successful avoidance of phase change from the use of supercritical pressure fluids are well-known, but the complex behaviours of such fluids owing to dramatic thermal property variations pose strong challenges to the design of heat transfer applications. In this paper, the turbulent flow and heat transfer of supercritical pressure$\\textrm {CO}_2$in a small vertical tube influenced by coupled effects of buoyancy and thermal acceleration are numerically investigated using direct numerical simulation. Both upward and downward flows with an inlet Reynolds number of 3540 and pressure of 7.75 MPa have been simulated and the results are compared with corresponding experimental data. The flow and heat transfer results reveal that under buoyancy and thermal acceleration, the turbulent flow and heat transfer exhibit four developing periods in which buoyancy and thermal acceleration alternately dominate. The results suggest a way to distinguish the dominant factor of heat transfer in different periods and a criterion for heat transfer degradation under the complex coupling of buoyancy and thermal acceleration. An analysis of the orthogonal decomposition and the generative mechanism of turbulent structures indicates that the flow acceleration induces a stretch-to-disrupt mechanism of coherent turbulent structures. The significant flow acceleration can destroy the three-dimensional flow structure and stretch the vortices resulting in dissipation.

In this paper, we describe the recent developments in the field of buoyancy-driven turbulence with a focus on energy spectrum and flux. Scaling and numerical arguments show that the stably-stratified turbulence with moderate stratification has kinetic energy spectrum E u ( k ) ∼ k − 11 5 and the kinetic energy flux u ( k ) ∼ k − 4 5 , which is called Bolgiano-Obukhov scaling. However, for Prandtl number near unity, the energy flux for the three-dimensional Rayleigh-Bénard convection (RBC) is approximately constant in the inertial range that results in Kolmorogorv's spectrum ( E u ( k ) ∼ k − 5 3 ) for the kinetic energy. The phenomenology of RBC should apply to other flows where the buoyancy feeds the kinetic energy, e.g. bubbly turbulence and fully-developed Rayleigh Taylor instability. This paper also covers several models that predict the Reynolds and Nusselt numbers of RBC. Recent works show that the viscous dissipation rate of RBC scales as ∼ Ra 1.3 , where Ra is the Rayleigh number.

Body-inclusive large-eddy simulations of disk wakes are performed for a homogeneous fluid and for different levels of stratification. The Reynolds number is 5 × 10 4 and the Froude number ($Fr$) takes the values of$\\infty$, 50, 10 and 2. In the axisymmetric wake of a disk with diameter$L_{b}$in a homogeneous fluid, it is found that the mean streamwise velocity deficit ($U_{0}$) decays in two stages:$U_{0}\\propto x^{-0.9}$during$10

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