scholarly journals On the Vorticity Balance over Steep Slopes: Kuroshio Intrusions Northeast of Taiwan

2020 ◽  
Vol 50 (8) ◽  
pp. 2089-2104
Author(s):  
Xiaohui Liu ◽  
Dong-Ping Wang ◽  
Jilan Su ◽  
Dake Chen ◽  
Tao Lian ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Joint Effect ◽  
Vorticity Equation ◽  
3D Analysis ◽  
Viscous Stress ◽  
Bottom Pressure ◽  
Sensitivity Experiment ◽  
Nonlinear Advection ◽  
Steep Slopes ◽  
Vorticity Balance

AbstractThe circulation of the Kuroshio northeast of Taiwan is characterized by a large anticyclonic loop of surface intrusion and strong upwelling at the shelfbreak. To study the mechanisms of Kuroshio intrusions, the vorticity balance is examined using a high-resolution nested numerical model. In the 2D depth-averaged vorticity equation, the advection of geostrophic potential vorticity (APV) term and the joint effect of baroclinicity and relief (JEBAR) term are dominant. On the other hand, in the 2D depth-integrated vorticity equation, the main balance is between nonlinear advection and bottom pressure torque. It is shown that JEBAR and APV tend to compensate, and their difference is comparable to bottom pressure torque. Perhaps most significantly, a general framework is provided for examination of vorticity balance over steep slopes through a full 3D depth-dependent vorticity equation. The 3D analysis reveals a well-defined bottom boundary layer over the shelfbreak, about 40 m deep and capped by the vertical velocity maximum. In the upper frictionless layer from the surface to about 100 m, the primary balance is between nonlinear advection and horizontal divergence. In the lower frictional layer, viscous stress is balanced by nonlinear advection and horizontal divergence. The bottom pressure torque, which corresponds to the depth-integrated viscous effect, is a proxy for viscous stress divergence at the bottom. The importance of nonlinear advection is further demonstrated in a sensitivity experiment by removing advective terms from momentum equations. Without nonlinear advection, the bottom pressure torque becomes trivial, the boundary layer vanishes, and the on-shelf intrusion is considerably weakened.

scholarly journals Barotropic vorticity balance of the North Atlantic subpolar gyre in an eddy-resolving model

2019 ◽  
Author(s):  
Mathieu Le Corre ◽  
Jonathan Gula ◽  
Anne-Marie Tréguier
Keyword(s):  
North Atlantic ◽  
Subpolar Gyre ◽  
Bottom Pressure ◽  
First Order ◽  
The North ◽  
Sverdrup Balance ◽  
North Atlantic Subpolar Gyre ◽  
The North Atlantic ◽  
Barotropic Vorticity ◽  
Vorticity Balance

Abstract. The circulation in the North Atlantic Subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics is traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first order balance between the planetary vorticity advection, the bottom pressure torque and the wind stress curl. However, this dynamics has been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic Subpolar gyre using a high resolution simulation (≈ 2-km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow-topography interactions. Our findings highlight that, locally, there is a first order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between each other. However, balances integrated over different regions of the gyre – shelf, slope and interior – still highlight the important role played by nonlinearities and the bottom drag curls. In particular the topographic Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are the nonlinear terms due to eddies generated along eastern boundary currents and the time-mean nonlinear terms from the Northwest Corner. Our results suggest that a good representation of the mesoscale activity along with a good positioning of the Northwest corner are two important conditions for a better representation of the circulation in the North Atlantic Subpolar Gyre.

Tropical Cyclone Intensification Simulated in the Ooyama-type Three-layer Model with a Multilevel Boundary Layer

2021 ◽  
Author(s):  
Rong Fei ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Vertical Mixing ◽  
Type Model ◽  
Layer Model ◽  
Coriolis Parameter ◽  
Wind Structure ◽  
Model Configuration ◽  
Nonlinear Advection ◽  
Boundary Layer Dynamics

AbstractThe first successful simulation of tropical cyclone (TC) intensification was achieved with a three-layer model, often named the Ooyama-type three-layer model, which consists of a slab boundary layer and two shallow water layers above. Later studies showed that the use of a slab boundary layer would produce unrealistic boundary layer wind structure and too strong eyewall updraft at the top of TC boundary layer and thus simulate unrealistically rapid intensification compared to the use of a height-parameterized boundary layer. To fully consider the highly height-dependent boundary layer dynamics in the Ooyama-type three-layer model, this study replaced the slab boundary layer with a multilevel boundary layer in the Ooyama-type model and used it to conduct simulations of TC intensification and also compared the simulation with that from the model version with a slab boundary layer. Results show that compared with the simulation with a slab boundary layer, the use of a multilevel boundary layer can greatly improve simulations of the boundary-layer wind structure and the strength and radial location of eyewall updraft, and thus more realistic intensification rate due to better treatments of the surface layer processes and the nonlinear advection terms in the boundary layer. Sensitivity of the simulated TCs to the model configuration and to both horizontal and vertical mixing lengths, sea surface temperature, the Coriolis parameter, and the initial TC vortex structure are also examined. The results demonstrate that this new model can reproduce various sensitivities comparable to those found in previous studies using fully physics models.

The global dominance of the Atlantic circulation, seen through boundary pressures.

2020 ◽  
Author(s):  
Chris W. Hughes ◽  
Joanne Williams ◽  
Adam Blaker ◽  
Andrew C. Coward
Keyword(s):  
Continental Slope ◽  
Large Scale ◽  
Strong Influence ◽  
Atlantic Meridional Overturning Circulation ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Mesoscale Variability ◽  
Bottom Pressure ◽  
Meridional Overturning ◽  
Vorticity Balance

<p>The rapid propagation of boundary waves (or, equivalently, the strong influence of topography on vorticity balance) ensures that bottom pressure along the global continental slope reflects large scale ocean processes, making it possible to see through the fog of the mesoscale, which obscures many observable quantities. This fact is exploited in systems to monitor the Atlantic Meridional Overturning Circulation (AMOC). Here, we use diagnostics from an ocean model with realistic mesoscale variability to demonstrate two things. First: boundary pressures form an efficient method of monitoring AMOC variability. Second: pressures are remarkably constant along isobaths around the global continental slope, varying by less than 5 cm sea-level-equivalent over vast distances below the directly wind-driven circulation. In the latter context, the AMOC stands out as a clear exception, leading to a link between the AMOC and differences in the hydrography of entire ocean basins.</p>

Diapycnal Transport and Mixing Efficiency in Stratified Boundary Layers near Sloping Topography

2011 ◽  
Vol 41 (2) ◽  
pp. 329-345 ◽  
Author(s):  
Lars Umlauf ◽  
Hans Burchard
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Slope Angle ◽  
Low Frequency ◽  
Moment Closure ◽  
Mixing Efficiency ◽  
Small Scale ◽  
Second Moment Closure ◽  
Steep Slopes ◽  
Sloping Topography

Abstract The interaction of shear, stratification, and turbulence in boundary layers on sloping topography is investigated with the help of an idealized theoretical model, assuming uniform bottom slope and homogeneity in the upslope direction. It is shown theoretically that the irreversible vertical buoyancy flux generated in the boundary layer is directly proportional to the molecular destruction rate of small-scale buoyancy variance, which can be inferred, for example, from microstructure observations. Dimensional analysis of the equations shows that, for harmonic boundary layer forcing and no rotation, the problem is governed by three nondimensional parameters (slope angle, roughness number, and ratio of forcing and buoyancy frequencies). Solution of the equations with a second-moment closure model for the turbulent fluxes reveals the periodic generation of gravitationally unstable boundary layers during upslope flow, consistent with available observations. Investigation of the nondimensional parameter space with the help of this model illustrates a systematic increase of the bulk mixing efficiencies for (i) steep slopes and (ii) low-frequency forcing. Except for very steep slopes, mixing efficiencies are substantially smaller than the classical value of Γ = 0.2.

scholarly journals A Form of Potential Vorticity Equation for Depth-Integrated Flow with a Free Surface

2008 ◽  
Vol 38 (5) ◽  
pp. 1131-1136 ◽  
Author(s):  
Chris W. Hughes
Keyword(s):  
Free Surface ◽  
Potential Vorticity ◽  
Vorticity Equation ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Frequency Effects ◽  
Ocean Bottom Pressure ◽  
Forcing Term ◽  
Scalar Variable

Abstract A form of linear, barotropic potential vorticity equation is derived for an ocean with a free surface, in which only one scalar variable appears (ocean bottom pressure, or subsurface pressure). Unlike quasigeostrophic or rigid-lid derivations, the only approximation made (apart from linearization) is that changes in the circulation must be slow compared with the inertial frequency. Effects of stratification are included, but only parametrically in the sense that density is treated as a given quantity or forcing term rather than a variable.

Transient Method for Convective Heat Transfer Measurement With Lateral Conduction—Part II: Application to an Isolated Spherical Roughness Element

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
J. Bons ◽  
Daniel Fletcher ◽  
Brad Borchert
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Convective Heat Transfer ◽  
Finite Volume ◽  
Convective Heat ◽  
Roughness Element ◽  
Measurement Techniques ◽  
Stanton Number ◽  
3D Analysis ◽  
Transient Conduction

The effect of lateral conduction on convective heat transfer measurements using a transient infrared technique over an isolated spherical roughness element (bump) is evaluated. Comparisons are made between a full 3D finite-volume analysis and a simpler 1D transient conduction model. The surface temperature history was measured with a high resolution infrared camera during an impulsively started hot-gas flow at a flow Reynolds number of 860,000. The boundary layer was turbulent with the bump heights equivalent to 0.75, 1.5, and 3 times the boundary layer momentum thickness. When considering transient conduction effects only in the bump wake, the 1D approximate method underestimates the actual Stanton number estimated with the 3D model. This discrepancy is only 10% for a 75% change in St number occurring over a surface distance of 10 mm (the half-width of the wake). When the actual bump topology is accounted for in estimating the Stanton number on the bump itself with the 3D analysis technique, the increased surface area of the finite-volume cells on the protruding bump actually decreases the predicted value of St locally. The net result is that the two effects can cancel each other, and in some cases the 1D approximate technique can provide a reasonably accurate estimate of the surface heat transfer without the added complexity of the 3D finite-volume method. For the case of the largest bump tested, with maximum surface angularity exceeding 60 deg, the correction for 3D topology yields a 1D St estimate that is within 20–30% of the 3D estimate over much of the bump surface. These observed effects are valid for transient measurement techniques while the opposite is true for steady-state measurement techniques.

scholarly journals CFD Investigation of Fluid Flow Over Aerofoil With and Without Dimple

2020 ◽  
Author(s):  
Uzair Sajjad ◽  
Khalid Hamid ◽  
Naseem Abbas
Keyword(s):  
Boundary Layer ◽  
Fluid Flow ◽  
Cross Section ◽  
Turbulence Model ◽  
Aerodynamic Performance ◽  
3D Analysis ◽  
Cad Model ◽  
Uniform Cross Section ◽  
Solid Works ◽  
Lift And Drag

Abstract This work labels the effect of dimples on aerodynamic performance of an airfoil. NACA 0018 having a uniform cross section has been evaluated in this study. Eclipse dimpled airfoil is tested and compared with plain airfoil and with the airfoil in the literature [23,24]. Flows taken into consideration are subsonic. The CAD model is drawn in Solid works 2016, while the simulations are performed in Ansys 18.3. A 2-D CFD investigation is performed on both models using k-w turbulence model, subsequently the better one is selected based on the results. 3D analysis is performed on a segment of airfoil having one dimple. Lift and drag coefficients are calculated for various angles of attack. This investigation tells that dimples affect the aerodynamics of airfoil, particularly for various angle of attacks. For smaller angle of attacks, plain airfoil showed less drag and higher lift, but totally different trend is achieved with increasing angle of attack whereas 20° was found to be the optimum angle. The findings proved that dimples on the surface delay the separation of boundary layer by generating additional turbulence on the surface and consequently reduce the formation of wake, which in turn decreases drag significantly.

scholarly journals Barotropic vorticity balance of the North Atlantic subpolar gyre in an eddy-resolving model

Ocean Science ◽  
2020 ◽  
Vol 16 (2) ◽  
pp. 451-468 ◽  
Author(s):  
Mathieu Le Corre ◽  
Jonathan Gula ◽  
Anne-Marie Tréguier
Keyword(s):  
North Atlantic ◽  
Subpolar Gyre ◽  
Bottom Pressure ◽  
First Order ◽  
The North ◽  
Sverdrup Balance ◽  
North Atlantic Subpolar Gyre ◽  
The North Atlantic ◽  
Barotropic Vorticity ◽  
Vorticity Balance

Abstract. The circulation in the North Atlantic subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics are traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first-order balance between the planetary vorticity advection, the bottom pressure torque, and the wind stress curl. However, these dynamics have been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic subpolar gyre using a new eddy-resolving simulation (with a grid space of ≈2 km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow–topography interactions. Our findings highlight that, locally, there is a first-order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between them. However, balances integrated over different regions of the gyre – shelf, slope, and interior – still highlight the important role played by nonlinearities and bottom drag curls. In particular, the Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are nonlinear terms due to eddies generated along eastern boundary currents and time-mean nonlinear terms in the northwest corner. Our results suggest that a good representation of the mesoscale activity and a good positioning of mean currents are two important conditions for a better representation of the circulation in the North Atlantic subpolar gyre.

scholarly journals The Influence of Soil Moisture on the Planetary Boundary Layer and on Cumulus Convection over an Isolated Mountain. Part I: Observations

2013 ◽  
Vol 141 (3) ◽  
pp. 1061-1078 ◽  
Author(s):  
Xin Zhou ◽  
Bart Geerts
Keyword(s):  
Boundary Layer ◽  
Soil Moisture ◽  
Monsoon Season ◽  
Convective Available Potential Energy ◽  
Cloud Base ◽  
Cumulus Convection ◽  
Sensitivity Experiment ◽  
Dry Soil ◽  
Mountain Part ◽  
Santa Catalina Mountains

Abstract Data collected around the Santa Catalina Mountains in Arizona as part of the Cumulus Photogrammetric, In Situ and Doppler Observations (CuPIDO) experiment during the 2006 summer monsoon season are used to investigate the effect of soil moisture on the surface energy balance, boundary layer (BL) characteristics, thermally forced orographic circulations, and orographic cumulus convection. An unusual wet spell allows separation of the two-month campaign in a wet and a dry soil period. Days in the wet soil period tend to have a higher surface latent heat flux, lower soil and air temperatures, a more stable and shallower BL, and weaker solenoidal forcing resulting in weaker anabatic flow, in comparison with days in the dry soil period. The wet soil period is also characterized by higher humidity and moist static energy in the BL, implying a lower cumulus cloud base and higher convective available potential energy. Therefore, this period witnesses rather early growth of orographic cumulus convection, growing rapidly to the cumulonimbus stage, often before noon, and producing precipitation rather efficiently, with relatively little lightning. Data alone do not allow discrimination between soil moisture and advected airmass characteristics in explaining these differences. Hence, the need for a numerical sensitivity experiment, in Part II of this study.

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