A Second-Order Closure for Neutrally Stratified Vegetative Canopy Flows

An existing second-order closure model is modified to include the effects on mean and turbulent motions of form and viscous drag in vegetative canopies. The additional physical mechanisms represented by the closure are viscous and pressure drag on canopy elements, their role in momentum absorption, in the creation of fine scale turbulent eddies and in enhancing the total viscous dissipation in the canopy airspace. Viscous dissipation is split into a standard 'isotropic contribution associated with the spectral eddy cascade and a foliage contribution associated with work against pressure and viscous drag on the foliage. Changes in the turbulent time scale that result from these mechanisms are included in the standard parameterisations of third moments and of the eddy cascade contribution to dissipation. The model is tested against a wind- tunnel 'wheat canopy, a corn canopy and a eucalypt canopy, a height range from 50 mm to 12.6 m. Model results show that the parameterisations of foliage interaction used in the closure are sufficiently robust to reproduce second-moment profiles within and above vegetative canopies to a high degree of accuracy without resorting to 'tuning of the model constants. The model also shows the natural emergence of two length scales, one associated with the familiar eddy cascade isotropic contribution to total dissipation and the other associated with the length scales of the canopy elements.     

Evaluating Models of The Neutral, Barotropic Planetary Boundary Layer using Integral Measures: Part I. Overview

Data for the cross-isobaric angle ₀, the geostrophic drag coefficient C_g, and the functions A and B of Rossby number similarity theory, obtained from meteorological field experiments, are used to evaluate a range of models of the neutral, barotropic planetary boundary layer. The data give well-defined relationships for ₀, C_g, and the integrated dissipation rate over the boundary layer, as a function of the surface Rossby number. Lettau's first-order closure mixing-length model gives an excellent fit to the data; other simple models give reasonable agreement. However more sophisticated models, e.g., higher-order closure, large-eddy simulation, direct numerical simulation and laboratory models, give poor fits to the data. The simplemodels have (at least) one free parameter in their turbulence closure that is matched toatmospheric observations; the more sophisticated models either base their closure onmore general flows or have no free closure parameters. It is suggested that all of theatmospheric experiments that we could locate violate the strict simplifying assumptionsof steady, homogeneous, neutral, barotropic flow required by the sophisticated models.The angle ₀ is more sensitive to violations of the assumptions than is C_g.The behaviour of the data varies in three latitude regimes. In middle and high latitudes the observed values of A and B exhibit little latitudinal dependence; the best estimates are A = 1.3 and B = 4.4. In lower latitudes the neutral, barotropic Rossby number theory breaks down. The value of B increases towards the Equator; the determination of A is ambiguous – the trend can increase or decrease towards the Equator. Between approximately 5° and 30° latitude, the scatter in the data is thought to be primarily due to the inherent presence of baroclinicity. The presence of the trade-wind inversion, thermal instability and the horizontal component of the Earth's rotation _H also contribute.Marked changes in the values of A and B occur in the region between the Equator andapproximately 5° latitude, as the Coriolis parameter |f| approaches zero. Although the variation of A and B with latitude suggests some similarity to the results obtained from the direct numerical simulations, the presence of additional complexities in the real atmosphere that are not included in the numerical model, precludes a meaningful direct comparison.     

断层封闭性影响因素及类型划分

影响断层封闭性的因素很多，但归结起来主要有3个，即断距、断裂充填性质和断面的紧闭程度。断距影响着断层两盘砂泥岩层的对置状态，断裂充填性质决定了断层的垂向和侧向封闭性，断面的紧闭程度决定了断层的垂向封闭能力。考虑断层在油气运移聚集过程中的作用，将断层封闭性分为侧向封闭性和垂向封闭性两大类，再根据断层在侧向和垂向的封闭程度，将断层封闭性分为完全封闭、部分开启、部分封闭和完全开启4个亚类。     

煤田地震勘探中应用偏移剖面作构造图问题初探

本文针对煤田地震勘探的特点,就如何应用偏移时间剖面直接作构造图问题进行了论述,提出了一套完整的交点闭合方法和空间校正方法。并结合实际使用PC-1500微机进行空间校正作构造图,实践证明效果良好。     

A simple closure mechanism for a compact,large-diameter,gravity corer

A simple and reliable corer closure mechanism is described, which has three principal advantages over published designs for similar automatic gravity corers. First, it allows the use of large diameter tubes (at least 80 mm internal diameter), reducing core shortening effects. Second, it allows free passage of water during descent minimising bow wave effects. Third, it is compact enough for convenient use through seasonal ice cover.     

植被微气象的数值模拟

综合考虑梯度输送和阵性穿透作用，并对植被上方的过渡层作适当处理，提出了植被微气象的一阶闭合模型。它对温、湿、风等物理量垂直分布的模拟精度良好。引用系统优化理论和方法，对模型中参数作多维非线性搜索，结果客观可靠。由于植物本身的调节适应能力和反馈作用，大气与植被间的动量与水热交换机制和过程不尽相同。在动量输送过程中，梯度产生项和阵性穿透皆不可忽视，尤其在群体下部，更为重要；而在热量和水分输送过程中，阵性穿透作用在低矮密集植被中的贡献甚微，梯度产生项起主导作用。     

Probabilistic analysis of borehole closure for through-salt well design

Probability levels, in the form of marginal cumulative distribution functions, are determined for the time it takes salt to contact a casing, which penetrates a salt canopy, and the time it takes the casing to yield once salt has contacted the casing, in support of through-salt well design activities required for deep-water, sub-salt environments. The finite-element method coupled with an advanced reliability method is used to perform the analyses. Randomness caused by uncertainties in salt properties, lithostatic stress, drilling-mud pressure, production temperature, and borehole ellipticity is handled by including statistics and probability distributions for each of these random variables. The advanced reliability method takes full account of the random variables and processes without Monte Carlo simulation, by using a method for assessing reliability statistics that computes response probabilities in a direct and efficient manner. This advanced reliability method falls under the general category of fast probability integration schemes.     

Influence of the turbulence closure scheme on the finite-element simulation of the upwelling in the wake of a shallow-water island

A three-dimensional finite-element model is used to investigate the tidal flow around Rattray Island, Great Barrier Reef, Australia. Field measurements and visual observations show both stable eddies developing at rising and falling tide in the wake of the island. The water turbidity suggests intense upwelling able to carry bed sediments upwards. Based on previous numerical studies, it remains unclear at this point whether the most intense upwelling occurs near the centre of the eddies or off the island's tips, closer to the island. All these studies resorted to a very simple turbulence closure, with a zero-equation model whereby the coefficient of vertical viscosity is computed via an algebraic expression. In this work, we aim at studying the influence of the turbulence closure on model results, with emphasis on the prediction of vertical motions. The Mellor and Yamada level 2.5 closure scheme is used and an increase in the intensity of vertical transport is observed. This increase is partly explained by the fact that the Mellor and Yamada model takes into account the hysteresis effect in the time variation of turbulence variables. The influence of the advection of turbulence variables is estimated to be negligible. By a better representation of transient coastal phenomena, the Mellor and Yamada level 2.5 turbulence closure improves the model to a significant degree.     

一种改进的水准网条件平差算法

在实际工程应用中,水准网网形复杂多样,当进行平差时,间接平差未知参数个数选取要求高,条件平差方程建立不唯一,针对此类问题,本文提出了一种利用传递数矩阵建立条件方程的算法,判断水准网中的水准路线和水准点是否是已知值,依据传递数矩阵运算规则,确定条件平差中的系数矩阵和常数项矩阵,并利用2个算例进行分析比较,证明了所提算法的有效性。     

Diffusion of Sm-Nd in Scheelite and its Significance to Isotopic Dating and Tracing

As the principal ore mineral in various tungsten (-gold) deposits, scheelite (CaWO4) plays an important role in directly dating the timing of ore formation, and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics. Since the retention of Sm-Nd systematics within scheelite is presently unconstrained, equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data. In order to better elucidate the closure of Sm-Nd in scheelite, the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein. The following Arrhenius relations were obtained: DNd = 4.00exp(?438 kJ·mol–1/RT) cm2/s DSm = 1.85exp(?427 kJ·mol–1/RT) cm2/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature. However, compared to other rare earth elements (REEs), which have markedly different atomic radii to either Nd or Sm, these are shown to exhibit a great variation in diffusivities. The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals, indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices. With this in mind, the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes. Based upon the diffusion data determined herein, Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics. Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits, and any later thermal environments. It is likely, therefore, that relevant isotopic information could be easily retained under most geological conditions, since initial crystallization of the scheelite. In addition, comparison of this mineral-element pair over a range of temperatures with some other common minerals used as geochronometers (e.g., zircon and apatite) indicates that Sm-Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz, and only a little faster than for Pb in zircon. It should be noted, within most hydrothermal deposits where zircon has crystallized, its size is typically no more than 100 μm, whereas scheelite commonly occurs as macroscopic grains. For this reason, the larger dimensions of scheelite would provide a robust Sm-Nd system more able to resist perturbations, relating to any later thermal process. As such Sm-Nd investigations of scheelite are akin to U-Pb within zircon samples used in isotopic dating. These observations indicate that Sm-Nd age and isotopic information can provide reliable data in all but the most extreme case, especially when data are extracted from macroscopic grains of scheelite that are chosen to be “pristine” (i.e., free of surface alteration and/or fractures).