Thin-sheet modelling of lithospheric deformation and surface mass transport

We study the effects of incorporating surface mass transport and the gravitational potential energy of both crust and lithospheric mantle to the viscous thin sheet approach. Recent 2D (cross-section) numerical models show that surface erosion and sediment transport can play a major role in shaping the large-scale deformation of the crust. In order to study these effects in 3D (planform view), we develop a numerical model in which both the dynamics of lithospheric deformation and surface processes are fully coupled. Deformation is calculated as a thin viscous layer with a vertically-averaged rheology and subjected to plane stresses. The coupled system of equations for momentum and energy conservation is solved numerically. This model accounts for the isostatic and potential-energy effects due to crustal and lithospheric thickness variations. The results show that the variations of gravitational potential energy due to the lateral changes of the lithosphere–asthenosphere boundary can modify the mode of deformation of the lithosphere. Surface processes, incorporated to the model via a diffusive transport equation, rather than just passively reacting to changes in topography, play an active role in controlling the lateral variations of the effective viscosity and hence of the deformation of the lithosphere.     

Two-dimensional horizontal modeling of fine-sediment transport at the South Channel–North Passage of the partially mixed Changjiang River estuary,China

Tidal flow and fine-sediment transport at the South Channel–North Passage of the partially-mixed Changjiang River estuary were studied using a two-dimensional horizontal (2DH) numerical model. This 2DH model was achieved by depth-integrating the momentum and convection–diffusion equations. The Alternating Direction Implicit scheme was used to solve the governing equations. The iterative method was adopted for the calculation of convection and diffusion terms of momentum equation. Comparisons between calculated and measured results (tidal elevations and depth-averaged velocities) have shown reasonable agreement. Horizontal distributions of tidal current velocity and suspended sediment concentration were qualitatively consistent with observations. Those modeled results were analyzed to elucidate the mechanisms for the formation of the turbidity maximum and intratidal variations in fine-sediment transport processes.     

Numerical simulation of the marine boundary layer characteristics over the Bay of Bengal as revealed by BOBMEX-98 Pilot experiment

An attempt has been made to study the marine boundary layer characteristics over Bay of Bengal using BOBMEX (Bay of Bengal and Monsoon Experiment) pilot experiment data sets, which was conducted between 23rd October and 12th November 1998 on board ORV Sagar Kanya. A one-dimensional multilevel atmospheric boundary layer with TKE-ε closure scheme is employed to study the marine boundary layer characteristics. In this study two synoptic situations are chosen: one represents an active convection case and the other a suppressed convection. In the present article the marine boundary layer characteristics such as temporal evolution of turbulent kinetic energy, height of the boundary layer and the airsea exchange processes such as sensible and latent heat fluxes, drag coefficient for momentum are simulated during both active and suppressed convection. Marine boundary layer height is estimated from the vertical profiles of potential temperature using the stability criterion. The model simulations are compared with the available observations.     

席状岩浆房中的岩浆冷却及动力稳定性

关于岩浆房中岩浆的结晶分异方式一直存在着争议，其基本矛盾在于对岩浆冷却历史及热不稳定性可能引起的对流形式的理解各不相同。本文在充分考虑席状岩浆房的基本物理特征的基础上，建立了稳定岩浆体系的动态冷却模型，并以攀枝花层状岩体为例，计算了相应的温度场、密度场及粘度场。同时，提出了一种基于能量守恒的、对热不稳定性进行分析的新方法。计算了热不稳定性可能引起的对流强度、对流区域及对流时间，并深入讨论了在一定的热不稳定状态下，不同的岩浆屈服强度对对流特征参量的影响。结果表明，在该岩体的整个固化过程中，如果曾发生过自发对流，其对流也是极微弱而短暂的，不会影响中下部岩浆房中岩浆的固化和结晶。这一结论与对该岩体的实际观察相符。     

基于广义垂线坐标系的三维非结构数学模型及其在珠江口的应用

基于广义垂线坐标变换,构建了非结构网格的三维水动力数学模型。模型采用半隐式有限体积法对控制方程进行数值离散,其中半隐式法用于水位梯度和垂向紊动扩散以及垂向对流项的离散,显式控制体积分法用于水平对流项等的离散。广义垂线坐标系使得模型能够灵活地对垂向网格进行布置,平面非结构网格使得模型能够适应河口海岸复杂的岸线,并可对局部进行网格加密。模型通过具有解析解的风生流和异重流对模型进行检验,应用模型模拟了珠江口的三维潮流过程,计算结果与实测水文数据的比较表明,模型能够较好地模拟珠江河口的盐水楔和三维分层水动力过程。     

The potential for free and mixed convection in sedimentary basins

 Free thermal convection and mixed convection are considered as potential mechanisms for mass and heat transport in sedimentary basins. Mixed convection occurs when horizontal flows (forced convection) are superimposed on thermally driven flows. In cross section, mixed convection is characterized by convection cells that migrate laterally in the direction of forced convective flow. Two-dimensional finite-element simulations of variable-density groundwater flow and heat transport in a horizontal porous layer were performed to determine critical mean Rayleigh numbers for the onset of free convection, using both isothermal and semi-conductive boundaries. Additional simulations imposed a varying lateral fluid flux on the free-convection pattern. Results from these experiments indicate that forced convection becomes dominant, completely eliminating buoyancy-driven circulation, when the total forced-convection fluid flux exceeds the total flux possible due to free convection. Calculations of the thermal rock alteration index (RAI=q·∇T) delineate the patterns of potential diagenesis produced by fluid movement through temperature gradients. Free convection produces a distinct pattern of alternating positive and negative RAIs, whereas mixed convection produces a simpler layering of positive and negative values and in general less diagenetic alteration. Received, January 1999/Revised, June 1999/Accepted, July 1999     

A study on the structure of the convective atmosphere over the Bay of Bengal during BOBMEX-99

Convective activity is one of the major processes in the atmosphere influencing the local and large-scale weather in the tropics. The latent heat released by the cumulus cloud is known to drive monsoon circulation, which on the other hand supplies the moisture that maintains the cumulus clouds. An investigation is carried out on the convective structure of the atmosphere during active and suppressed periods of convection using data sets obtained from the Bay of Bengal and Monsoon Experiment (BOBMEX). The cumulus convection though being a small-scale phenomenon, still influences its embedding environment by interaction through various scales. This study shows the variation in the kinematic and convective parameters during the transition from suppressed to active periods of convection. Convergence in the lower levels and strong upward vertical velocity, significant during active convection are associated with the formation of monsoon depressions. The apparent heat source due to latent heat release and the vertical transport of the eddy heat by cumulus convection, and the apparent moisture sink due to net condensation and vertical divergence of the eddy transport of moisture, are estimated through residuals of the thermodynamic equation and examined in relation to monsoon activity during BOBMEX.     

Precise temperature monitoring in boreholes: evidence for oscillatory convection? Part II: theory and interpretation

In the previous part of this work (Cermak, Safanda and Bodri, this volume p.MMM) we have described experimental data and quantified the heterogeneity features of the microtemperature time series. The spectral analysis and the local growth of the second moment technique revealed scaling structure of all observed time series generally similar and suggested the presence of two temperature forming processes. The longer-scale part can be attributed to the heat conduction in compositional and structural heterogeneous solid rocks, further affected by various local conditions. Short-scale temperature oscillations are produced by the intra-hole fluid convection due to inherent instability of water column filling the hole. Here we present how the observational evidence is supported by the results of the computer simulations. The exact modes of intra-hole convection may be different, ranging from quasi-periodic (“quiescent”) state to close of turbulence. As demonstrated by numerical modeling and referred on laboratory experiments, at higher Rayleigh numbers the periodic character of oscillation characteristic for “quiescent” regime is superseded by stochastic features. This so called “oscillatory” convection occurs due to instability within the horizontal boundary layers between the individual convectional cells. In spite of the fact that the basic convective cell motion is maintained and convection is characterized by slow motion, the oscillatory intra-hole flow and corresponding temperature patterns exhibit typical features of turbulence. The idea of boundary layer instability as a source of stochastic temperature fluctuations could explain many distinct features of borehole temperatures that previously cannot be interpreted.     

An analysis of MONTBLEX data on heat and momentum flux at Jodhpur

Parameterization of sensible heat and momentum fluxes as inferred from an analysis of tower observations archived during MONTBLEX-90 at Jodhpur is proposed, both in terms of standard exchange coefficientsC _H andC _D respectively and also according to free convection scaling. Both coefficients increase rapidly at low winds (the latter more strongly) and with increasing instability. All the sensible heat flux data at Jodhpur (wind speed at 10 m Ū₁₀ < 8 ms⁻¹) also obey free convection scaling, with the flux proportional to the ‘4/3’ power of an appropriate temperature difference such as that between 1 and 30 m. Furthermore, for Ū₁₀ < 4 ms⁻¹ the momentum flux displays a linear dependence on wind speed.     

高温下混凝土水力、热及变形耦合数学模型

针对受高温载荷的混凝土提出了一个水力-热-变形耦合的数学模型,模型中采用了3种不同的连续介质：水蒸汽与干燥空气的混合气体相,液态水以及固态混凝土骨架。在连续介质的Navier-Stokes方程基础上,推导了各相介质的质量守恒方程、动量守恒方程以及混合物整体的能量守恒方程。为了简化问题,引入了若干假设,给出了基于文献资料数据拟合的某些变量的用基本未知量表达的经验公式,建立了基于物理定律的各类运动学变量与其热力学共轭力的本构关系,并给出了系统偏微分方程。     

板内造山作用与成矿

中国大陆广泛分布强烈的板内变形和造山作用,传统的板块构造理论常常将其解释为板块边缘汇聚力的远程效应。然而,中国大陆的板内造山作用与汇聚板块边界之间缺乏可预期的动力学联系,不能简单地解释为大陆碰撞或板块俯冲的远程效应。本文提出另一种可供选择的解释,认为板内变形主要取决于岩石圈不均一性。相邻的板块拼合在一起形成统一板块之后,区域地质演化进入板内阶段。板块碰撞导致的岩石圈不均一性和重力不稳定性可以触发强烈的板内变形甚至造山作用,其延迟时间的长短取决于岩石圈不稳定性的程度和地球深部的热扰动。与板缘造山带相比,板内造山作用缺少板块俯冲-碰撞过程,板内造山带的演化历史相对简单,通常是以岩石圈拆沉作用开始,以地壳的垂向增生为特征,最后以岩石圈拆沉作用结束或形成重力不稳定岩石圈。因此,板内造山作用一般沿着古造山带发育。古造山带岩石圈结构低成熟度的特点不仅是岩石圈不稳定性的主要原因之一,而且由于挥发分和含矿元素的富集在活化过程中具有很强的成矿潜力。板内造山带的成矿作用依赖于深埋在岩石圈-软流圈系统不同深度水平上含矿流体的突然释放,主要发生在造山作用初始阶段和造山后伸展阶段。     

Precise temperature monitoring in boreholes: evidence for oscillatory convection? Part 1: Experiments and field data

Temperature was monitored in two boreholes in Kamchatka (Russia) in years 2001–2003. Ten-min reading (sampling) interval was selected for the first half-year run followed by shorter (12 days) experiment with 5-s reading interval. A similar experiment was repeated later in the test borehole Sporilov (Prague, Czech Republic), where four temperature–time series were performed with reading intervals varying from 1 to 20 s. All temperature–time series (except the record from the bottom of the hole) displayed intermittent, non-periodic oscillations of temperature of up to several hundredths of degree with sharp gradients and large fluctuations over all observed time intervals. No such oscillation was detected at the bottom of the hole. The spectral analysis revealed a high level of stochasticity in the measured signal. Calculated spectra showed “band-pass” behavior without any definite peaks, which might characterize certain periodicity. Local growth of the second moment technique revealed the presence of at least two distinct temperature-forming processes. One of them can be related to heat transfer in the structurally and compositionally complex subsurface. The second process, which presents the bulk of the measured signal, probably reflects certain intra-hole convection. We hypothesized that the oscillatory regime of such convection is responsible for the stochastic nature of measured temperatures. Results of numerical modeling describing the fluctuation of water-cells in a vertical slot support the idea of thermally unstable water column in a hole, the instability of which produces a complex oscillation system. Model solutions and their discussion is presented in Part II of this work.     

On the theory of cyclogenesis in middle latitudesZur theorie der zyklonenbildung in den mittleren breitenSur la théorie de la cyclogénèse dans les latitudes moyennes

The author gives a short introduction into theories of cyclogenesis which are based on instability. The polarfront and baroclinic instability theories are discussed along with the difference between linear and non-linear instability. The author stresses that one may consider cyclogenesis from the point of view of the production of cyclonic vorticity. The terms responsible for the production of vorticity become increasingly important as soon as the horizontal and vertical wind shear as expressed by the Rossby and Richardson numbers reach a critical combination. This critical combination can be reached more easily if heat of condensation is set free than with purely dry-adiabatic processes. Therefore, cyclogenesis is more easily possible in saturated air than in non-saturated air.     

Incipient vortex size-dependent evolution of tropical disturbances Part II — Some governing processes

Recently it was shown (Goswami and Rao 1993) that the process of intensification of tropical disturbances depends on the size of the incipient vortex in a rather nonlinear fashion. Among vortices of size ranging from 100 to 450 km (radius), embedded in the same large scale condition, it is the vortex with size about 250 km that intensifies to the most severe system. These results also showed a strong correspondence between the maximum intensity reached and the initial (3–6 hour) low level convergence field near the centre. The purpose of the present work is to identify the process(es) responsible for this scale selective intensification of tropical disturbances. It is proposed that diffusion is likely to play a crucial role in bringing about this selective intensification. In the present work a series of experiments with an axisymmetric numerical model of tropical cyclone (Wada’s model) is carried out to determine the relative roles of horizontal diffusion of momentum, moisture, heat and vertical diffusion. The results show that diffusion significantly affects the process of intensification and scale selection. While moderate diffusion does not alter the magnitude of intensification significantly, the scale selection is quite sensitive to the strength of diffusion. Interestingly, these diffusion processes, of momentum, moisture, heat and vertical do not affect the scale selection in the same fashion. The scale selection process turns out to be a result of a combined effect of these diffusion processes. However, no single diffusion process alone can give rise to a sharp selection of scale at the size of 250 km.     

Geothermal heat extraction by water circulation through a large crack in dry hot rock mass

One proposed geothermal heat extraction scheme relies on water circulation in a large vertical crack created by hydraulic fracturing in a hot dry impermeable rock mass. Water flow, heat convection and crack opening widths are analysed by finite elements. Governing field equations of the problem are first set up rigorously and then various small terms are identified and neglected, retaining the effects of pressure gradient, buoyancy, velocity head (kinetic energy) and head loss due to viscous friction in the water flow equation, and the effects of heat convection in water and heat conduction in rock in the heat transfer equation. The finite element scheme for water flow is based on a variational principle that is typical for diffusion problems, and for heat transfer it is based on the method of least-square residuals. The system of differential equations is highly non-linear. The non-linear terms and coefficients are treated in the fiaite element analysis as constant; the finite element analysisof, the steady-state pressures, fluxes and temperatures is then iterated, evaluating all non-linear terms and coefficients on the basis of the solution obtained in the previous iteration. Numerically calculated fields at various times after the start ofcooling are presented. They indicate some features favourable for the geothermal scheme, such as formation of eddy currents, and downward flux of water toward hotter rock. However, other important questions would have to be solved to gain full understanding, of this proposed geothermal scheme.     

中国大陆地幔对流格局和岩石圈层构造运动

六十年代以来,由于卫星重力测量和计算技术的迅速发展,有些研究者,如Runcorn(1967)和Liu(1976-1980),根据卫星重力数据计算获得地幔对流及其应力场图象来研究全球板块构造和区域构造。愈来愈多的事实表明,构造运动不仅是地壳和岩石圈层物质运动的表现,而更重要的是地幔物质运动的反映。因此,本文应用作者计算获得的岩石圈层下面地幔流运动图象与大地构造及近代构造运动的资料相对比,探索我国岩石圈层下的地幔对流格局形成及其对构造运动的影响。     

Some salient features of the atmosphere observed over the north Bay of Bengal during BOBMEX

This paper describes the near surface characteristics and vertical variations based on the observations made at 17.5‡N and 89‡E from ORV Sagar Kanya in the north Bay of Bengal during the Bay of Bengal Monsoon Experiment (BOBMEX) carried out in July–August 1999. BOBMEX captured both the active and weak phases of convection. SST remained above the convection threshold throughout the BOBMEX. While the response of the SST to atmospheric forcing was clearly observed, the response of the atmosphere to SST changes was not clear. SST decreased during periods of large scale precipitation, and increased during a weak phase of convection. It is shown that the latent heat flux at comparable wind speeds was about 25–50% lower over the Bay during BOBMEX compared to that over the Indian Ocean during other seasons and tropical west Pacific. On the other hand, the largest variations in the surface daily net heat flux are observed over the Bay during BOBMEX. SST predicted using observed surface fluxes showed that 1-D heat balance model works sometime but not always, and horizontal advection is important. The high resolution Vaisala radiosondes launched during BOBMEX could clearly bring out the changes in the vertical structure of the atmosphere between active and weak phases of convection. Convective Available Potential Energy of the surface air decreased by 2–3 kJ kg^-1 following convection, and recovered in a time period of one or two days. The mid tropospheric relative humidity and water vapor content, and wind direction show the major changes between the active and weak phases of convection.     

Effect of the initial density and angular-velocity profiles of pre-stellar cores on the properties of young stellar objects

The physical properties of young stellar objects are studied as functions of the initial spatial distributions of the gas surface density Σ and angular velocity Ω in pre-stellar cores using numerical hydrodynamic simulations. Two limiting cases are considered: spatially homogeneous cores with Σ = const and Ω = const and centrally concentrated cores with radius-dependent densities Σ ∝ r ⁻¹ and Ω ∝ r ⁻¹. The degree of gravitational instability and protostellar disk fragmentation is mostly determined by the initial core mass and the ratio of the rotational to the gravitational energy, and depends only weakly on the initial spatial configuration of pre-stellar cores, except for the earliest stages of evolution, when models with spatially homogeneous cores can be more gravitationally unstable. The accretion of disk matter onto a protostar also depends weakly on the initial distributions of Σ and Ω, with matter from the collapsing core falling onto the disk at a rate that is slightly higher in models with spatially homogeneous cores. An appreciable dependence of the disk mass, disk radius, and the disk-to-protostar mass ratio on the initial density and angular velocity profiles of the parent core is found only for class 0 young objects; this relationship is not systematic in the later I and II stages of stellar evolution. The mass of the central protostar depends weakly on the initial core configuration in all three evolutionary stages.