影响大兴安岭焚风的几个因子*

本文借助二维大气中尺度数值模式,讨论了我国东北地区1月份大兴安岭东坡的焚风与山区地形形状的关系,并定性地绘出了该焚风现象生消的背景条件。     

Geomorphic characteristics of the Minjiang drainage basin (eastern Tibetan Plateau) and its tectonic implications: New insights from a digital elevation model study

Abstract   The Minshan Mountain and adjacent region are the major continental escarpments along the eastern Tibetan Plateau. The Minjiang drainage basin is located within the plateau margin adjacent to the Sichuan Basin. Based on the analysis of the digital elevation model (DEM) acquired by the Shuttle Radar Topography Mission (SRTM), we know that the Minjiang drainage basin has distinct geomorphic characteristics. The regular increasing of local topographic relief from north to south is a result of the Quaternary sediment deposition within the plateau and the holistic uplift of the eastern margin of the Tibetan Plateau versus the Sichuan Basin. Results from DEM-determined Minjiang drainage sub-basins and channel profiles show that the tributaries on the opposite sides are asymmetric. Lower perimeter and area of drainage sub-basins, total channel length and bifurcation ratio within eastern flank along the Minjiang mainstream are the result of the Quaternary differential uplift of the Minshan Mountain region. Shorter stream lengths and lower bifurcation ratio might be the indications of the undergrowth and newborn features of these eastern streams, which are also representative for the eastern uplift of the Minshan Mountain.     

Flow Over Hills with Variable Roughness

The effect of variable roughness length upon the flow characteristics over hills is investigated. The changes considered herein cover a range of flow configurations such as the change from a forested (rough) valley with a moderately smooth hilltop to a grassy valley (smooth) with a “spiky” (rough) mountain top. The effect of moving the roughness with respect to the hill is also considered. Although many of the flow features change when the position of the roughness change is varied with respect to the hill these changes have very little impact upon the global properties used within orographic drag parametrization schemes.     

On the influence of stratification and tidal forcing upon mixing in sill regions

A cross-sectional non-hydrostatic model with idealized topography was used to examine the processes influencing tidal mixing in the region of sills. Initial calculations with appropriate parameters for the sill at the entrance to Loch Etive showed that the model could reproduce the main features of the observed mixing in the region. In particular, the hydraulic jump in the sill region was reproduced, as was an intense mid-water jet that was observed to separate from the lee side of the sill. Shear instabilities associated with the jet appeared to be a source of mixing within the thermocline. In addition, internal lee waves were generated on the lee side of the sill, with the observed amplification because of trapping during the flood stage. Their magnitude and hence the mixing increased with increasing Froude number (F _r). In the case of vertically varying buoyancy frequency, its value near the sill top determined the F _r number, with its value below influencing internal waves magnitude at depth. At high F _r values particularly with strong currents, short waves and overturning occurred.     

Simulation of the Askervein flow. Part 2: Large-eddy simulations

Large-eddy simulations of the neutrally stratified flow over the Askervein Hill were performed, to improve the knowledge of the flow obtained from field measurements and numerical simulations with Reynolds averaged Navier-Stokes (RANS) methods. A Lagrangian dynamic subgrid model was used but, to avoid the underdissipative character near the ground, it was merged with a damped Smagorinsky model. Simulations of a flat boundary-layer flow with this subgrid model showed that the turbulent vertical motions and shear stress were better resolved using grids with a stream to spanwise aspect ratio Δx / Δy = 2 than with an aspect ratio Δx / Δy = 1. Regarding the flow over the Askervein Hill, it was found that large-eddy simulations provide an acceptable solution for the mean-velocity field and better predictions of the turbulent kinetic energy in the upstream side of the hill than the model. However, as with the model, grid convergence was not achieved in the lee side and the size of the zone with reversed flow increased with the grid refinement. Nevertheless, the existence of the intermittent separation predicted with unsteady RANS in part one of this work seems unquestionable, due to the deceleration of the flow. In our opinion, a better modelling of the decelerating boundary layer in the lee side is required to improve the results obtained using equilibrium assumptions and achieve grid convergence.     

印度尼西亚卢瓦斯砂金矿床地质特征

卢瓦斯砂金矿位于印度尼西亚共和国苏门答腊岛东南部卢瓦斯河道及一侧阶地。矿区所处大地构造位置属喜马拉雅—马来西亚—苏门答腊火山活动带之东段。依据该砂金矿床的成矿地貌特征、地质特征、重砂矿物组合,初步认定该矿属近源型冲积型砂金矿,其砂金的物质来源于萨温河上游含金地质体,特别是火山碎屑岩中的含金石英脉和硅化脉。该砂金矿聚集于河道河漫滩及一级、二级河谷阶地中的泥、砂、砾等现代沉积物中。矿体产状受河流及两侧地形、地貌控制。     

地形地貌对地震波放大效应数值模拟研究

采用FLAC3D 软件进行大量数值模拟,研究了地形地貌对地震波放大效应规律。结果表明:对于单面坡模型,斜坡较低时,坡体对天然地震波和简谐波的峰值加速度在竖直向和顺坡面向产生线性放大效应。斜坡较高时,若坡角低,两种波形的加速度等值线近似平行于坡面,天然地震波坡顶正下方极值性不明显,但随坡角增大而明显,坡面正下方可见若干条与坡面相平行具节律性的极小值条带,随坡角增大向一条极值带转化,简谐波在高低坡脚时坡顶面正下方区域内都出现等间距相间分布具节律性的极值条带,坡面正下方的极值带状性不如天然地震波明显,随坡角增大而消失; 若坡角高,两波形加速度放大系数顺坡面呈相间的极值圈分布。双面坡坡顶宽度越小,山脊越单薄,加速度放大效应越强烈,随坡顶宽度增大,坡肩加速度放大系数趋于稳定,双面坡对加速度放大效应强于单面坡。对加速度放大效应,多峰形双峰形单峰形,坡顶越不规则,加速度放大效应越强烈。V形河谷坡面上半部分加速度放大效应要强于U形河谷,U形河谷的谷底中部对地震波动力响应要强于V形谷。坡面局部不规则性只影响到坡面局部范围内的峰值加速度分布,坡高、坡角、坡顶几何特征为整个峰值加速度分布的控制因素。     

On the topography influence on subsidence due to horizontal underground mining using the influence function method

The classical influence function method is widely used in mining subsidence prediction, but its use is typically limited to predicting the subsidence associated with horizontal stratiform underground mining when the ground surface is flat. By investigating the topography influence on subsidence under simplified mining conditions, this study attempts to improve the original influence function method to take topographic variations into account. New asymmetrical influence functions are used to simulate element subsidence. Integrating this asymmetrical approach into the influence function method increases the realism of subsidence computation when compared to the numerical simulation results. The maximum subsidence value and influence angle got from field data are introduced into the improved method, then two field subsidence cases are studied and acceptable comparison results are achieved. This improved method should serve as the foundation for future work.     

Imaging Borrelly

The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ∼60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation.