Oxygen isotope exchange and closure temperatures in cooling rocks   总被引：3，自引：0，他引：3  

G. R. T. JENKIN  C. M. FARROW  A. E. FALLICK  D. HIGGINS 《Journal of Metamorphic Geology》1994,12(3):221-235

Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of δ¹⁸O within grains, and the bulk δ¹⁸O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the δ¹⁸O values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values ('slow cooling'), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures ( T _c) for the two phases and the mode of the rock. Adding a third phase into a system which undergoes 'slow' cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two-phase system. In some systems oxygen isotope reversals can be developed. If cooling is not 'slow', then the mineral δ¹⁸O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral δ¹⁸O values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems.  相似文献   

声波逾量衰减的估算          下载免费PDF全文

潘乃先  刘建强 《大气科学》1991,15(4):116-122

本文计算表明速度场湍流对逾量衰减的作用比温度场湍流的大5倍.在有限宽度声束条件下,引入有效湍流外尺度Le后,计算表明在大多数情况下逾量衰减系数难以忽略.当波束有张角时,同样的大气湍流状态下逾量衰减系数值随离声源距离增大而增大.过量衰减系数与声波波长关系近似为Ke∝λ~(-2).提供了考虑通量衰减作用的计算C_T~2的简单方法并给了几个计算实例.  相似文献   

用部分反射雷达研究低电离层的不规则性          下载免费PDF全文

杨克俊 《地球物理学报》1992,35(1):121-128

本文建议用电压干涉技术通过部分反射雷达来测量低电离层中的电子浓度不规则性及其扰动。电压干涉技术首先由Woodman提出用在Jicamarca的非相干散射雷达上,精确地确定一个高的局部散射区的位置。在单个散射体存在的情况下,这是一个简单而有效的技术。但当多个散射体同时存在时便产生了问题。Farlty等发展了一个谱干涉方法,可以有效地区分不同的散射体。后来此方法采用一对或3个天线成功地用在对流层、中层和电离层扰动的研究中。Rttger等也使用事后波束控制（Postsetqeam Steering）方法,广义上说这也是干涉方法,来确定大气扰动结构和一些参数。Adams等则更进一步提出了一个工作在中频2.66MHz的成像Doppler干涉方法,通过10个天线采用谱干涉和Doppler技术研究中层大气扰动的物理机制。  相似文献   

Estimation of flow resistance in gravel-bedded rivers: A physical explanation of the multiplier of roughness length     

N. J. Clifford  A. Robert  K. S. Richards 《地球表面变化过程与地形》1992,17(2):111-126

The need to estimate velocity and discharge indirectly in gravel-bedded rivers is a commonly-encountered problem. Semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information. Although such equations have a semi-theoretical basis, in natural gravel-bed channels, an empirical constant (6.8 or 3.5) has to be introduced to scale-up the characteristic grain size (D₅₀ or D₈₄) to represent the effective roughness length. In this paper, two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small-scale form resistance, reflecting the occurrence of microtopographic bedforms in gravel-bedded environments. First, spatial elevation dependence in short, detailed bed profiles from a single gravel-bedded river is investigated using semivariogram and zero-crossing analyses. This leads to objective identification of two discrete scales of bed roughness, associated with grain and microtopographic roughness elements. Second, the autocorrelation structure of the three-dimensional near-bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms. Apart from improving the capacity to determine friction factors for velocity and discharge estimation, the findings have implications in general for the initial motion of gravelly bed material.  相似文献   

应用涡旋相关方法对戈壁地区湍流输送特征的初步研究   总被引：36，自引：26，他引：36  

王介民  刘晓虎 《高原气象》1990,9(2):120-129

利用微机技术建立了一个较为廉价的近地层大气湍流数据实时采集处理系统,在1988年黑河预实验中工作良好,取得了一套较为完整的数据。对戈壁沙滩点的风、温、湿湍流脉动数据进行统计分析和涡旋相关方法处理,结果表明近地层大气的湍强特征符合一般规律。通量分析指出,戈壁地区的水汽通量在白天的绝大多数小时都呈负值(向下),而夜间为正。文中对湍流谱的特征也进行了初步分析。  相似文献   

二阶闭合模式在污染气象学中的某些应用     

桑建国  刘丽杰 《高原气象》1990,9(3):245-255

本文采用二阶闭合的湍流边界层模式,进行一系列数值试验以模拟边界层中连续线源的扩散状况。试验表明:无论在稳定的或不稳定的边界层中,高源的扩散能力都低于低源;在稳定层中,粗糙地表上的大气扩散能力高于光滑表面;在相同风速和地表净辐射情况下,粗糙表面上的大气扩散能力反而低于光滑表面;对流边界层中存在反梯度输送,因而K理论的应用受到限制。试验还表明,修正的Kazanski-Monin参数可能比Monin-Obukhov长度更能反映大气的扩散能力。  相似文献   

A STUDY OF 1.5-ORDER CLOSURE TURBULENCE MODEL          下载免费PDF全文

Deng Xuejiao  Li Xingsheng  Xu Xiaojin 《Acta Meteorologica Sinica》1994,8(3):305-315

Based on turbulence theory,a 1.5-order closure turbulence model is established.The model incorporating with theground surface energy budget equation is constructed by means of a vertical one-dimensional(1-D)40-levelgrid-mesh.The numerical results reveal the 24-h evolution of the clear planetary boundary layer comparing with theWangara boundary layer data of days 33—34.The model also takes into account some physical processes of radiativetransfer and baroclinicity,revealing some important characteristics observed in the boundary layer,especially for theevolution of the mixed layer and low-level jet.The calculated results are in good agreement with the observational data.On the other hand,we also run the high-resolution model of the planetary boundary layer in the Mesoscale Model Ver-sion 4(MM4)with the same physical processes and initial conditions.The results show that the high-resolution modelcan not reveal those important characteristics as the 1.5-order closure model did.In general,it is shown that the 1.5-or-der closure turbulence model based on turbulence theory is better in rationality and reality.  相似文献   

Visualization of the turbulent flow structure in a gravel-bed river     

Alistair D. Kirkbride  Stuart J. McLelland 《地球表面变化过程与地形》1994,19(9):819-825

A novel technique for visualizing turbulent flow data from a gravel-bed river is presented. The time development of flow velocity and shear stress at three heights is displayed using a computer program. This can be used to observe how the fluctuations of velocity and shear stress interact both spatially and temporally. We highlight examples of flow events which are important for the understanding of flow dynamics. The visualization suggests that the turbulent flow-field is characterized not only by coherence over time at a point, but also by spatial interdependence between points. We suggest that this new visualization approach will assist further interpretation of statistical analyses of turbulent signals, as well as focusing future measurement strategies by providing a clearer spatio-temporal picture of the flow structure.  相似文献   

Overwash occurrence consequent on morphodynamic changes following lagoon outlet closure on a coarse clastic barrier     

J. D. Orford  R. W. G. Carter  D. L. Forbes  R. B. Taylor 《地球表面变化过程与地形》1988,13(1):27-35

The reduction in sediment volume of an ebb-tide delta, as a consequence of lagoon outlet closure on a coarse clastic barrier in southeast Ireland, initiated a sequence of beachface and barrier changes downdrift. Elimination of sediment supply to the ebb-delta caused a cessation of downdrift longshore swash bar welding, and led to beach volume reductions which in turn allowed a temporal sequence of beach and nearshore morphodynamic domains to develop. These domains then controlled the sedimentation regime of the barrier adjacent to the old outlet. A temporal sequence of (a) dissipative barrier; (b) reflective barrier; and (c) inner reflective (barrier face)/outer dissipative (subtidal) wave regimes match respectively periods of (a) barrier crest build up by crestal dune development; (b) barrier crest instability (barrier width increasing, barrier height decreasing) due to rhythmic overwash; and (c) a return to barrier crest stability with limited aeolian accretion. Two barriers at different stages in this sequence are discussed.  相似文献   

Horizontal eddy diffusivities in the northern hemispheric stratosphere and lower mesosphere     

S. K. Kao  R. J. Okrasinski  N. J. Lordi  F. J. Schmidlin 《Pure and Applied Geophysics》1978,117(3):537-547

Meterological rocket soundings, launched between 1969–74 at six locations representative of low, middle, and high altitudes, are employed with the use of the statistical theory of diffusion, to determine the zonal and meridional component of eddy diffusivity between 30 and 55 km as a function of season, latitude, and altitude. A comparison is also made between annually-averaged eddy diffusivities above and below 30 km.It is shown that the zonal component of eddy diffusivity is approximately three to five times as large as the meridional component, in most cases. Both components of eddy diffusivity vary greatly with season, latitude, and altitude. Highest eddy diffusivities, found in the vicinity of the winter westerly jet, are approximately one order of magnitude higher than those present during the summer. Tropical eddy diffusivities, however, remain relatively small throughout the year. Annually, a minimum is indicated near 25 km between maximums located at the stratopause and tropopause.  相似文献