Contained density currents with high volume of release

Contained density currents with high volume of release reflect against the boundaries of the reception environment commonly leading to oscillatory flow. These flows exist in sediment clarifiers, compromising their operations, and deposited signatures of contained turbidity currents are found as part of the infill of sedimentary basins; for operation of the former and interpretation of the latter it is essential to understand the dynamic processes of these flows. Six lock‐exchange experiments with different initial densities were made in a horizontal flume, where the volume of the saline mixture in the lock was equivalent to the volume of the ambient fluid. A further two tests, with a repeated initial density, were made: one with high volume of release and very long duration; and another with low volume of release. Firstly, the movement of the current is discussed, including the oscillations within the experimental tank involving the density current and an upper layer counter‐current. It is shown that the cyclic behaviour is self‐similar with the reduced gravity of the initial density in the lock. Secondly, entrainment and water mixing processes are characterized. The time evolution of mixing is characterized qualitatively by analysing the background potential energy of the density distributions to show that mixing occurs even in the earlier stages of the current, and mainly within the first cycle of the oscillation. Quantified analysis reveals that, in currents with high volume of release, entrainment discharge is one order of magnitude higher, mainly due to the larger interface between the ambient fluid and the current. A model for the evolution of the mixing process is proposed for density currents with high volume of release. Finally, the dynamics of the head of the current is analysed. The entrainment in the head, when compared to the entrainment in the remainder body of the flows, is less important for the configuration with a larger lock.     

Role of horizontal density advection in seasonal deepening of the mixed layer in the subtropical Southeast Pacific

The mechanisms behind the seasonal deepening of the mixed layer(ML) in the subtropical Southeast Pacific were investigated using the monthly Argo data from 2004 to 2012. The region with a deep ML(more than 175 m) was found in the region of(22?–30?S, 105?–90?W), reaching its maximum depth(～200 m) near(27?–28?S, 100?W) in September. The relative importance of horizontal density advection in determining the maximum ML location is discussed qualitatively. Downward Ekman pumping is key to determining the eastern boundary of the deep ML region. In addition, zonal density advection by the subtropical countercurrent(STCC) in the subtropical Southwest Pacific determines its western boundary, by carrying lighter water to strengthen the stratification and form a "shallow tongue" of ML depth to block the westward extension of the deep ML in the STCC region. The temperature advection by the STCC is the main source for large heat loss from the subtropical Southwest Pacific. Finally, the combined effect of net surface heat flux and meridional density advection by the subtropical gyre determines the northern and southern boundaries of the deep ML region: the ocean heat loss at the surface gradually increases from 22?S to 35?S, while the meridional density advection by the subtropical gyre strengthens the stratification south of the maximum ML depth and weakens the stratification to the north. The freshwater flux contribution to deepening the ML during austral winter is limited. The results are useful for understanding the role of ocean dynamics in the ML formation in the subtropical Southeast Pacific.     

Retrieval of effective leaf area index (LAIe) and leaf area density (LAD) profile at individual tree level using high density multi-return airborne LiDAR

As an important canopy structure indicator, leaf area index (LAI) proved to be of considerable implications for forest ecosystem and ecological studies, and efficient techniques for accurate LAI acquisitions have long been highlighted. Airborne light detection and ranging (LiDAR), often termed as airborne laser scanning (ALS), once was extensively investigated for this task but showed limited performance due to its low sampling density. Now, ALS systems exhibit more competing capacities such as high density and multi-return sampling, and hence, people began to ask the questions like—“can ALS now work better on the task of LAI prediction?” As a re-examination, this study investigated the feasibility of LAI retrievals at the individual tree level based on high density and multi-return ALS, by directly considering the vertical distributions of laser points lying within each tree crown instead of by proposing feature variables such as quantiles involving laser point distribution modes at the plot level. The examination was operated in the case of four tree species (i.e. Picea abies, Pinus sylvestris, Populus tremula and Quercus robur) in a mixed forest, with their LAI-related reference data collected by using static terrestrial laser scanning (TLS). In light of the differences between ALS- and TLS-based LAI characterizations, the methods of voxelization of 3D scattered laser points, effective LAI (LAIe) that does not distinguish branches from canopies and unified cumulative LAI (ucLAI) that is often used to characterize the vertical profiles of crown leaf area densities (LADs) was used; then, the relationships between the ALS- and TLS-derived LAIes were determined, and so did ucLAIs. Tests indicated that the tree-level LAIes for the four tree species can be estimated based on the used airborne LiDAR (R² = 0.07, 0.26, 0.43 and 0.21, respectively) and their ucLAIs can also be derived. Overall, this study has validated the usage of the contemporary high density multi-return airborne LiDARs for LAIe and LAD profile retrievals at the individual tree level, and the contribution are of high potential for advancing forest ecosystem modeling and ecological understanding.     

空间层次聚类显著性判别的重排检验方法

同时顾及空间邻近与专题属性相似的空间层次聚类是挖掘空间分布模式的一种有效手段。空间层次聚类方法虽然可以获得多层次的聚集结构,但聚类结果显著性的统计判别依然是一个尚未解决的难题。为此,本文提出了一种空间层次聚类结果显著性的统计判别方法,用于确定空间层次聚类的停止准则,减少聚类过程对参数设置的依赖。通过试验分析与比较发现,该方法能够有效判别空间层次聚类结果的显著性和确定层次聚类合并过程的停止条件,同时具有很好的抗噪性,避免随机结构的干扰。     

基于规则格网的DEM插值实验

通过地形建模,将6个地形隶属函数按照101×101,112×112,126×126,144×144,168×168,201×201,257×257 7种格网密度生成6种局部地形单元的规则格网DEM;使用反距离加权(IDW)等7种插值算法,将前6种格网密度下的DEM插值成257×257规格;从原始257×257DEM中随机抽取检查点计算残差,并对残差中误差进行分析。通过分组插值实验,运用控制变量法、方差分析等方法研究地貌类型、采样密度和插值算法对DEM插值精度的影响。     

海湾水库蓄水初期的单次往复密度流

为了探讨海湾水库蓄水初期单次往复水平密度流的产生与效应,进行砂质斜坡沉积物与水体之间盐分交换的水槽实验。通过沉积物孔隙水与其上覆水的多剖面电导率动态监测,分析盐分的时空分布特征,并计算深水区、浅水区和中心区的边界层单位面积含盐总量。实验结果显示,进水在浅水区沉积物表面产生明显的渗入-溢出现象,浅水区沉积物孔隙水盐分浓度显著低于中心区与深水区同一高度的盐分浓度,深水区初始底边界层含盐量与初始边界层下边界盐分浓度均为最高;这表明进水过程在倾斜沉积物表面产生了前进密度流,水流携带的盐分在坡底累积。水槽进满水后深水区边界层含盐总量与边界层下边界盐分浓度快速降低,并且浅水区表层沉积物孔隙水与深水区同一深度水体之间的盐分快速达到平衡;这表明蓄水初期在两者之间形成了返回密度流,从浅水区表层沉积物冲刷出的盐分在密度流作用下再次进入沉积物。为避免再次进入沉积物的盐分在后期继续影响水库泛咸,建议在蓄水结束后尽快实施坡底咸水排除方案。     

基于重力全张量数据的Parker-Oldenburg算法研究

相比重力数据,重力张量数据通常包含更多的异常信息。本文根据重力数据与重力张量数据的关系,利用位场转化技术,将重力张量数据应用于传统的Parker-Oldenburg密度界面反演算法中。通过模型试验,证明了在网格间距较大或者数据存在一定噪音时,使用本文算法进行反演能得到更好的效果。实验结果说明利用重力张量数据可以有效地提高密度界面反演的分辨率。     

三维高密度电阻率法在堤坝工程中的模拟试验

为检测堤坝中裂隙、洞穴及渗漏层等隐患的存在,利用三维高密度电阻率法对数字模型和物理模型建立的裂隙隐患进行探测,结果表明室内建立的物理模型在立体图和分层切片图中裂隙的形态有些变化,但其分布范围、位置与相对各自模型中实际的比例尺寸相差不大,说明数值模拟得出的结论可以为实际工程进行理论指导,具有一定的参考价值。     

现代银杏叶脉参数初步研究

对南京现生银杏叶片的长枝阳叶、短枝阳叶和短枝阴叶的形态参数和叶脉参数进行观察和度量。结果显示:长枝阳叶、短枝阳叶和短枝阴叶的叶脉密度分别为14.78±2.44 cm、16.05±2.57cm、15.31±2.34 cm,叶脉指数分别为0.090±0.026、0.098±0.037、0.091±0.027,差别不显著。表明,银杏叶生长位置和光照强度对其叶脉参数影响不大。     

Distribution and relative density of cetaceans in the Hauraki Gulf,New Zealand

Understanding species distributions, and how they change in space and time, is vital when prioritising conservation or management initiatives. We assessed the distribution and density patterns of common dolphins (Delphinus sp.), bottlenose dolphins (Tursiops truncatus) and Bryde’s whales (Balaenoptera edeni) in the Hauraki Gulf, New Zealand. Dedicated boat-based surveys were conducted in the inner Hauraki Gulf (IHG) and off Great Barrier Island (GBI) during 2010–2012. Generalised linear models were used to investigate temporal changes in relative densities and kernel density estimation was implemented to examine spatial trends. Common dolphins were widely distributed during all seasons, with higher densities observed during winter and spring in the IHG but during autumn off GBI. There was inter-annual variation in Bryde’s whale distribution, with high densities recorded off GBI in 2011. Bottlenose dolphins were infrequently sighted in the IHG but regularly encountered off GBI, with the highest densities during spring and summer.