Bedload yields for sand‐bed streams in the Ngarradj Creek catchment,Northern Territory,Australia

Bedload yields were calculated by 39 methods at the East Tributary gauge, nine methods at Upper Swift Creek gauge and 11 methods at Swift Creek gauge in the Ngarradj Creek catchment in northern Australia. These methods involved combining various significant bedload rating curves determined for a measured bedload data set for a 4‐year period with either the hourly or daily hydrographs or flow duration curves for the same period, 1 September 1998 to 31 August 2002. Bedload ratings were both statistically significant (ρ ≤ 0.05) and explained at least 60% of the variance in bedload flux. Bias corrections were used with all methods based on log₁₀‐transformed ratings. Estimated mean annual bedload yields varied by three orders of magnitude at the East Tributary gauge and by two orders of magnitude at Upper Swift Creek and Swift Creek gauges. Hourly discharges usually produced higher estimated yields than daily discharges. The bedload rating‐flow duration curve technique overestimates yields and bias correction methods always produce even higher yields. Ratings using both immersed bedload weight and adjusted immersed bedload weight always under‐predicted yields because they contain an implicit threshold of motion condition that is at least four times greater than that predicted by Bagnold's threshold equation. Such a result questions the applicability of Bagnold's threshold equation to the Ngarradj Creek catchment. The best estimates of mean annual bedload yield at East Tributary, Upper Swift Creek and Swift Creek gauges are 600 ± 170 (SE), 1065 ± 150 and 1795 ± 270 t/year, respectively. © 2015 Commonwealth of Australia. Hydrological Processes © 2015 John Wiley & Sons Ltd.     

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Evaluation of dynamically downscaled reanalysis precipitation data for hydrological application

Skilful and reliable precipitation data are essential for seasonal hydrologic forecasting and generation of hydrological data. Although output from dynamic downscaling methods is used for hydrological application, the existence of systematic errors in dynamically downscaled data adversely affects the skill of hydrologic forecasting. This study evaluates the precipitation data derived by dynamically downscaling the global atmospheric reanalysis data by propagating them through three hydrological models. Hydrological models are calibrated for 28 watersheds located across the southeastern United States that is minimally affected by human intervention. Calibrated hydrological models are forced with five different types of datasets: global atmospheric reanalysis (National Centers for Environmental Prediction/Department of Energy Global Reanalysis and European Centre for Medium‐Range Weather Forecasts 40‐year Reanalysis) at their native resolution; dynamically downscaled global atmospheric reanalysis at 10‐km grid resolution; stochastically generated data from weather generator; bias‐corrected dynamically downscaled; and bias‐corrected global reanalysis. The reanalysis products are considered as surrogates for large‐scale observations. Our study indicates that over the 28 watersheds in the southeastern United States, the simulated hydrological response to the bias‐corrected dynamically downscaled data is superior to the other four meteorological datasets. In comparison with synthetically generated meteorological forcing (from weather generator), the dynamically downscaled data from global atmospheric reanalysis result in more realistic hydrological simulations. Therefore, we conclude that dynamical downscaling of global reanalysis, which offers data for sufficient number of years (in this case 22 years), although resource intensive, is relatively more useful than other sources of meteorological data with comparable period in simulating realistic hydrological response at watershed scales. Copyright © 2013 John Wiley & Sons, Ltd.     

Three‐dimensional simulations of tensile cracks in geomaterials by coupling meshless and finite element method

Failure in geotechnical engineering is often related to tension‐induced cracking in geomaterials. In this paper, a coupled meshless method and FEM is developed to analyze the problem of three‐dimensional cracking. The radial point interpolation method (RPIM) is used to model cracks in the smeared crack framework with an isotropic damage model. The identification of the meshless region is based on the stress state computed by FEM, and the adaptive coupling of RPIM and FEM is achieved by a direct algorithm. Mesh‐bias dependency, which poses difficulties in FEM‐based cracking simulations, is circumvented by a crack tracking algorithm. The performance of our scheme is demonstrated by two numerical examples, that is, the four‐point bending test on concrete beam and the surface cracks caused by tunnel excavation. Copyright © 2014 John Wiley & Sons, Ltd.     

长波区间太阳辐射对气候模拟的影响

长波区间的太阳辐射在气候模式中往往被忽略。利用国家气候中心BCC_AGCM2.0.1大气环流模式,采用矩阵算子辐射传输算法,研究了长波区间太阳辐射对气候模式辐射通量和温度模拟结果的影响。结果表明,以ISCCP和CERES辐射资料为标准,考虑长波区间太阳辐射后,长波区间晴空大气地表向下辐射通量平均误差减小2.05 W/m2,均方根误差减少1.29 W/m2;长波区间晴空大气模式顶向上辐射通量平均误差减小0.70 W/m2,均方根误差减小0.21 W/m2;长波区间有云大气地表向下辐射通量平均误差减小1.38 W/m2,均方根误差减小1.03 W/m2;长波区间有云大气模式顶向上辐射通量平均误差减小0.99 W/m2,均方根误差减小0.30 W/m2。以ECMWF再分析资料为标准,考虑长波区间太阳辐射后,赤道地区上对流层—下平流层区域温度的冷偏差得到改善,对流层顶温度平均误差减小0.27 K,均方根误差减小0.25 K。     

二流—四流球谐函数谱展开累加辐射传输方案在全球气候模式中的应用

本文首先构建了二流—四流球谐函数谱展开累加辐射传输的新方案,然后将其应用于国家气候中心第二代大气环流模式BCC_AGCM2.0.1的新版本中,并与模式中原有的Eddington累加方案进行了比较。由于新方案本质上是单层Eddington近似方案在四流上的推广。因此新方案在计算精度上要优于原方案。通过在全球气候模式中的应用与比较,本文发现新方案对气候模拟会产生比较大的影响。在晴空条件下,新方案计算的在南纬30°到60°区间、北大西洋东北部以及非洲北部的撒哈拉沙漠区域的地表向下年平均短波辐射通量要小于原方案结果,最大差别可以达到3.5 W/m2;同时,新方案计算的在南纬30°到60°区间和北大西洋东北部的大气顶向上年平均短波辐射通量要大于原方案结果,最大差别达到3 W/m2。在有云大气情况下,新方案计算的地表向下年平均短波辐射通量要小于原方案结果,并随着纬度的增加,新旧两种方案的差别逐渐变大,在南北极时达到最大5.5 W/m2;同时,新方案计算的在赤道区域的大气顶的年平均短波向上辐射通量要小于原方案结果,最大差别为2.5 W/m2,而在南北纬30°到60°区间,新方案计算的在大气顶的年平均短波向上辐射通量则要大于原方案结果,最大差别为1.5 W/m2。新方案计算的年平均短波加热率普遍高于原方案结果,特别是在800 hPa到地表之间的低层大气以及50 hPa到100 hPa的高层大气,最大差别可达0.03 K/d。因此,新方案有助于改善全球气候模式中普遍存在的赤道平流层中下层的温度冷偏差现象。     

单站多参数GLONASS码频间偏差估计及其对组合精密单点定位的影响

在分析传统GPS/GLONASS组合PPP数学模型中忽略GLONASS码IFB不足的基础上,提出一种基于"多参数"的组合PPP与码IFB估计算法。将"频间偏差"与"系统时差"参数进行合并,通过引入多个独立的"时频偏差"参数对组合PPP中的GLONASS码IFB进行函数模型补偿,同时可实现基于单个测站观测数据的码IFB精确估计。对配备6种GNSS品牌接收机的30个IGS站实测数据进行GLONASS码IFB估计与分析。结果表明:各品牌接收机不同频率通道的GLONASS码IFB可达数米,且表现出与频率的明显相关性,但难以通过简单函数建模为其提供精确的先验改正值;相同品牌接收机的GLONASS码IFB整体上具有相似的特性,而在个别测站会表现出异常特征;即使接收机类型、固件版本及天线类型完全相同的测站,GLONASS码IFB值也可能存在显著差异。新算法能实现对GLONASS码IFB的有效补偿,明显加快组合PPP的收敛速度。虽然引入多个附加参数会导致函数模型自由度减小,但对定位精度的影响有限,与传统"单参数"法进行组合PPP的定位精度相当。     

高速铁路浅埋偏压隧道安全进洞方案研究

针对傍山浅埋偏压、局部出露地表隧道工程合理施工方案的选取问题,结合现场工程地质及施工环境条件,提出"先在出露地表侧预设钢格栅混凝土护拱,随后反压回填土石,采用暗挖法进洞"的护拱暗挖技术方案,并与放坡明挖、支挡明挖方案进行对比分析.研究结果表明,明挖施工方案虽技术成熟,但用于此类隧道面临着靠山侧高陡边坡开挖及防护工程量大...     

C波段转发测轨系统测站偏差的标校

C波段转发测轨体制在GEO卫星测定轨中有突出优势,但其测量系统偏差的标校精度对定轨结果影响显著。基于L波段载波相位多星定轨结果,提出一种基于标准轨道拟合残差的系统偏差标校方法。该方法能够同时标校测量偏差及时标偏差,有利于弥补激光测距标校方法观测量较少的缺陷。利用国内监测站对中国导航卫星实测数据分析结果表明,经过标校后的C波段转发测轨精度可优于5 m,测量偏差及时标偏差具有较好的稳定性。     

Application of Laser Ablation ICP-MS to U-Th-Pb Dating of Monazite

Recent advances in laser ablation ICP-MS techniques allow accurate U-Th-Pb age dating of monazites that are as young as several tens of million years to a precision better than 2%. Accuracy of the age determinations has been improved by true real-time mass bias correction via nebulisation of a solution containing enriched ²³³U and natural Tl isotopes. The Tl-U tracer solution eliminates possible effects of variable sample matrices on the precision and accuracy of measured isotopic ratios. Mass bias corrections based on measured ²⁰⁵Tl/²³³U ratios in the tracer solution allow direct measurement of ²³⁵U in monazite. Combined with high-sensitivity laser ablation ICP-MS measurements, direct measurement of ²³⁵U particularly improves the precision of U-Pb dating of young monazites. Correction for laser-induced Pb/U and Pb/Th elemental fractionation is based on a mathematical treatment of time resolved count-rate data that is independent of laser ablation characteristics, does not require external standardisation and allows variable laser pit size or raster patterns for each measurement. The new procedures make the LA ICP-MS technique more flexible for in situ U-Th-Pb analysis.