Two-dimensional density currents in a confined basin

We present new experimental results on the mechanisms through which steady two-dimensional density currents lead to the formation of a stratification in a closed basin. A motivation for this work is to test the underlying assumptions in a diffusive “filling box” model that describes the oceanic thermohaline circulation (Hughes, G.O. and Griffiths, R.W., A simple convective model of the global overturning circulation, including effects of entrainment into sinking regions, Ocean Modeling, 2005, submitted.). In particular, they hypothesized that a non-uniform upwelling velocity is due to weak along-slope entrainment in density currents associated with a large horizontal entrainment ratio of E _eq ?～?0.1. We experimentally measure the relationship between the along-slope entrainment ratio, E, of a density current to the horizontal entrainment ratio, E _eq, of an equivalent vertical plume. The along-slope entrainment ratios show the same quantitative decrease with slope as observed by Ellison and Turner (, 6, 423–448.), whereas the horizontal entrainment ratio E _eq appears to asymptote to a value of E eq?=?0.08 at low slopes. Using the measured values of E _eq we show that two-dimensional density currents drive circulations that are in good agreement with the two-dimensional filling box model of Baines and Turner (Baines, W.D. and Turner, J.S., Turbulent buoyant convection from a source in a confined region, J. Fluid. Mech., 1969, 37, 51–80.). We find that the vertical velocities of density fronts collapse onto their theoretical prediction that U =-2^?2/3 B ^1/3 E _eq ^2/3 (H/R) ζ, where U is the velocity, H the depth, B the buoyancy flux, R the basin width, E _eq the horizontal entrainment ratio and?ζ?= z/H the dimensionless depth. The density profiles are well fitted with?Δ?= 2^?1/3 B ^2/3 E _eq ^?2/3 H _-1 [ln(ζ )?+?τ ], where?τ?is the dimensionless time. Finally, we provide a simple example of a diffusive filling box model, where we show how the density stratification of the deep Caribbean waters (below 1850?m depth) can be described by a balance between a steady two-dimensional entraining density current and vertical diffusion in a triangular basin.     

Fluxes and Gradients in the Convective Surface Layer and the Possible Role of Boundary-Layer Depth and Entrainment Flux

We study the relation between fluxes and gradients in the very unstable surface layer by comparing recent proposals in the literature with the well-known Businger–Dyer functions. The recent proposals include results from large-eddy simulation (LES), which account for entrainment effects and effects of the boundary-layer depth. A comparison of the relationships is made with experimental data. The LES-based gradient functions show the impact of entrainment in the surface layer, but the scatter in the field data is too large to confirm this. Therefore this result is preliminary and future tests against new observations are recommended. It appears that the Businger–Dyer relationship behaves differently to the alternatives, and that it deviates from observations for large stability.     

A 2D finite volume model for bebris flow and its application to events occurred in the Eastern Pyrenees

FLATModel is a 2D finite volume code that contains several original approaches to improve debris-flow simulation. Firstly, FLATModel incorporates a ＂stop-and-go＂ technique in each cell to allow continuous collapses and remobilizations of the debris-flow mass. Secondly, flow velocity and consequently yield stress is directly associated with the type of rheology to improve boundary accuracy. Thirdly, a simple approach for entrainment is also included in the model to analyse the effect of basal erosion of debris flows. FLATMODEL was tested at several events that occurred in the Eastern Pyrenees and simulation results indicated that the model can represent rather well the different characteristics observed in the field.     

Atmospheric boundary-layer structure observed during a haze event due to forest-fire smoke

During a haze event in Baltimore, U.S.A. from July 6 to 8, 2002, smoke from forest fires in the Québec region (Canada), degraded air quality and impacted upon local climate, decreasing solar radiation and air temperature. The smoke particles in and above the atmospheric boundary layer (ABL) served as a tracer and provided a unique opportunity to investigate the ABL structure, especially entrainment. Elastic backscatter lidar measurements taken during the haze event distinctly reveal the downward sweeps (or wisps) of smoke-laden air from the free atmosphere into the ABL. Visualisations of mechanisms such as dry convection, the entrainment process, detrainment, coherent entrainment structures, and mixing inside the ABL, are presented. Thermals overshooting at the ABL top are shown to create disturbances in the form of gravity waves in the free atmosphere aloft, as evidenced by a corresponding ripple structure at the bottom of the smoke layer. Lidar data, aerosol ground-based measurements and supporting meteorological data are used to link free atmosphere, mixed-layer and ground-level aerosols. During the peak period of the haze event (July 7, 2002), the correlation between time series of elastic backscatter lidar data within the mixed layer and the scattering coefficient from a nephelometer at ground level was found to be high (R=0.96 for z =324 m, and R=0.89 for z=504 m). Ground-level aerosol concentration was at a maximum about 2 h after the smoke layer intersected with the growing ABL, confirming that the wisps do not initially reach the ground.     

A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction

For numerical weather prediction models and models resolving deep convection, shallow convective ascents are subgrid processes that are not parameterized by classical local turbulent schemes. The mass flux formulation of convective mixing is now largely accepted as an efficient approach for parameterizing the contribution of larger plumes in convective dry and cloudy boundary layers. We propose a new formulation of the EDMF scheme (for Eddy Diffusivity\Mass Flux) based on a single updraft that improves the representation of dry thermals and shallow convective clouds and conserves a correct representation of stratocumulus in mesoscale models. The definition of entrainment and detrainment in the dry part of the updraft is original, and is specified as proportional to the ratio of buoyancy to vertical velocity. In the cloudy part of the updraft, the classical buoyancy sorting approach is chosen. The main closure of the scheme is based on the mass flux near the surface, which is proportional to the sub-cloud layer convective velocity scale w _*. The link with the prognostic grid-scale cloud content and cloud cover and the projection on the non- conservative variables is processed by the cloud scheme. The validation of this new formulation using large-eddy simulations focused on showing the robustness of the scheme to represent three different boundary layer regimes. For dry convective cases, this parameterization enables a correct representation of the countergradient zone where the mass flux part represents the top entrainment (IHOP case). It can also handle the diurnal cycle of boundary-layer cumulus clouds (EUROCS\ARM) and conserve a realistic evolution of stratocumulus (EUROCS\FIRE).     

Countergradient Fluxes of Conserved Variables in the Clear convective and Stratocumulus-topped Boundary Layer: The role of the Entrainment Flux

Large-eddy simulations of a clear convective boundary layer (CBL)and a stratocumulus-topped boundary layer are studied. Bottom-upand a top-down scalars were included in the simulations, and theprinciple of linear superposition of variables was applied toreconstruct the fields of any arbitrary conserved variable.This approach allows a systematic analysis of countergradient fluxesas a function of the flux ratio, which is defined as the ratio betweenthe entrainment flux and the surface flux of the conserved quantity.In general, the turbulent flux of an arbitrary conserved quantityis counter to the mean vertical gradient if the heights where thevertical flux and the mean vertical gradient change sign do notcoincide. The regime where the flux is countergradient is thereforebounded by the so-called zero-flux and zero-gradient heights. Becausethe vertical flux changes sign only if the entrainment flux has anopposite sign to the surface flux, countergradient fluxes arepredominantly found for negative flux ratios. In the CBL the fluxratio for the virtual potential temperature is, to a good approximation,constant, and equal to -0.2. Only if the moisture contribution to thevirtual potential temperature is negligibly small will the flux ratio forthe potential temperature be equal to this value. Otherwise, theflux ratio for the potential temperature can have any arbitrary(negative) value, and, as a consequence, the fluxes for thepotential temperature and the virtual potential temperature willbe countergradient at different heights. As a practical application ofthe results, vertical profiles of the countergradient correction termfor different entrainment-to-surface-flux ratios are discussed.     

The control of salt supply on entrainment of an anhydrite layer within a salt diapir

The influence of four parameters (sedimentation rate, viscosity of salt, stratigraphic location of the anhydrite layer within the salt layer, and the perturbation width) on salt supply to down-built diapirs and its entrainment capacity are studied systematically in numerical models. Model results show that these four parameters affect salt supply, and the evolution history of a salt diapir. As such, these parameters strongly influence the style and the amount of entrainment of dense inclusions into a diapir. In active diapirs (i.e. unburied diapirs), salt supply increases with increasing sedimentation rate whereas it decreases with an increase in salt viscosity. Diapirs initiating from wide perturbation provide more salt supply to feed the diapir. Presence and initial stratigraphic location of any denser layer (e.g. an anhydrite layer) within a salt layer also affects salt supply. When lateral forces are negligible, salt supply into a diapir depends on these four parameters, which directly control the entrainment of any embedded anhydrite layer into the diapir.     

Entrainment of circumpolar water in the Indian Ocean region of the Antarctic

The net influx of the circumpolar water on the western (approximately along 10°E) and eastern (approximately 115°E) boundaries of the Indian Ocean, adopting the method of Montgomery and Stroup is computed on bivariate distribution of potential thermosteric anomaly and salinity to identify the characteristics of the flux. The zonal flux at both the boundaries indicates an alternate strong easterly and westerly flow between 36°S and 45°S, south of which the flow is mainly easterly but weak up to 56°S. At the western boundary the easterly flow is 146 Sv and westerly is 98.07 Sv, while at the eastern boundary (115°E) the corresponding fluxes are 123.46 Sv and 27.20 Sv respectively, indicating a net outflux of 48.33 Sv. This water should have been accounted by the melting of ice and influx of the Equatorial Pacific Ocean Water.     

沉积物选择性起动机理及其在砾石沉积临界值分析中的应用

混合粒径条件下沉积物的起动机理不同于均匀粒径条件下沉积物的起动机理。混合粒径条件下沉积物的起动，可由沉积物转动角φ随相对粒径及颗粒状而变化的依从关系进行解释。通过对砾石沉积物因不同粒径、形状而发生选择性起动的动力过程的分析，得出了砾石沉积物转动角选择性起动的理论模式，采用该模式来定量计算潮汐流和河流中砾石沉积物选择性起动的临界值，计算结果和测量结果吻合,肉而证实了该模式的可靠性。     

A Modelling Study of Flux Imbalance and the Influence of Entrainment in the Convective Boundary Layer

It is frequently observed in field experiments that the eddy covariance heat fluxes are systematically underestimated as compared to the available energy. The flux imbalance problem is investigated using the NCAR’s large-eddy simulation (LES) model imbedded with an online scheme to calculate Reynolds-averaged fluxes. A top–down and a bottom–up tracer are implemented into the LES model to quantify the influence of entrainment and bottom–up diffusion processes on flux imbalance. The results show that the flux imbalance follows a set of universal functions that capture the exponential decreasing dependence on u _*/w _*, where u _* and w _* are friction velocity and the convective velocity scale, respectively, and an elliptic relationship to z/z _i , where z _i is the mixing-layer height. The source location in the boundary layer is an important factor controlling the imbalance magnitude and its horizontal and vertical distributions. The flux imbalance of heat and the bottom–up tracer is tightly related to turbulent coherent structures, whereas for the top–down diffusion, such relations are weak to nonexistent. Our results are broadly consistent with previous studies on the flux imbalance problem, suggesting that the published results are robust and are not artefacts of numerical schemes.