考虑基质吸力的非饱和土邓肯-张统计损伤修正模型

为反映非饱和地基土变形全过程,以某基础工程非饱和土为研究对象,开展固结排水三轴压缩试验。研究发现,非饱和土的偏应力-应变曲线形态近似双曲线,基质吸力对土体力学行为影响十分明显,基质吸力越大,土体偏应力越高。根据非饱和土变形特性及工程特点,选取邓肯-张双曲线模型作为基础模型,引入统计损伤理论,假设非饱和土微元强度服从Weibull概率密度分布,建立邓肯-张统计损伤模型。通过搭建初始切线模量与基质吸力的联系,建立一种新的考虑基质吸力的非饱和土邓肯-张统计损伤模型。给出参数解析方法,得到Weibull分布参数经验表达式,从而修正模型。分析不同基质吸力条件下非饱和土损伤累积规律,采用所建模型和传统邓肯-张模型对比验证非饱和土偏应力-应变试验曲线,证明所建模型的可行性和合理性。研究成果为非饱和土的力学特性研究及辨识模拟提供一定参考。     

Ensemble Forecasts of Tropical Cyclone Track with Orthogonal Conditional Nonlinear Optimal Perturbations

This paper preliminarily investigates the application of the orthogonal conditional nonlinear optimal perturbations(CNOPs)–based ensemble forecast technique in MM5(Fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model). The results show that the ensemble forecast members generated by the orthogonal CNOPs present large spreads but tend to be located on the two sides of real tropical cyclone(TC) tracks and have good agreements between ensemble spreads and ensemble-mean forecast errors for TC tracks. Subsequently, these members reflect more reasonable forecast uncertainties and enhance the orthogonal CNOPs–based ensemble-mean forecasts to obtain higher skill for TC tracks than the orthogonal SVs(singular vectors)–, BVs(bred vectors)– and RPs(random perturbations)–based ones. The results indicate that orthogonal CNOPs of smaller magnitudes should be adopted to construct the initial ensemble perturbations for short lead–time forecasts, but those of larger magnitudes should be used for longer lead–time forecasts due to the effects of nonlinearities. The performance of the orthogonal CNOPs–based ensemble-mean forecasts is case-dependent,which encourages evaluating statistically the forecast skill with more TC cases. Finally, the results show that the ensemble forecasts with only initial perturbations in this work do not increase the forecast skill of TC intensity, which may be related with both the coarse model horizontal resolution and the model error.     

东风急流对孟加拉湾热带气旋Nargis初始涡旋形成的影响

利用NCEP-CFSR(National Centers for Environmental Prediction Climate Forecast System Reanalysis)再分析资料和WRF模式,研究了2008年4月孟加拉湾热带气旋Nargis的初始涡旋的形成过程。结果表明:受到印度洋赤道西风急流爆发及其伴随的东传MJO事件的影响,Nargis的初始扰动生成于苏门答腊岛北部地区。另外,源于中纬度地区经南海进入孟加拉湾的东风急流(4月22—25日)对Nargis初始扰动发展到热带低压起到了重要的作用。东风急流及其携带的冷空气使得孟加拉湾东部海洋向大气输送的感热通量迅速增加,低层大气的有效位能通过非绝热加热获得能量,并向总动能转化,从而近海表涡旋性环流得到增长,Nargis初始扰动向西北移动并最终发展为热带低压。数值试验结果进一步证实了东风急流对Nargis初始涡旋生成的作用,如果没有东风急流的出现,Nargis初始扰动将不能北上发展成为热带低压。     

Initial Dilution of A Vertical Round Non-Buoyant Jet in Wavy Cross-Flow Environment

The phenomenon of wastewater discharged into coastal waters can be simplified as a turbulent jet under the effect of waves and currents. Previous studies have been carried out to investigate the jet behaviors under the current only or the wave only environment. To gain obtain better understanding of the jet behaviors in a realistic situation, a series of physical experiments on the initial dilution of a vertical round jet in the wavy cross-flow environment are conducted. The diluted processes of the jet are recorded by a high-resolution camcorder and the concentration fields of the jet are measured with a peristaltic suction pumping system. When the jet is discharged into the wavy cross-flow environment, a distinctive phenomenon, namely “effluent clouds”, is observed. According to the quantitative measurements, the jet width in the wavy cross-flow environment increases more significantly than that does in the cross-flow only environment, indicating that the waves impose a positive effect on the enhancement of jet initial dilution. In order to generalize the experimental findings, a comprehensive velocity scale ua and a characteristic length scale l are introduced. Through dimensional analysis, it is found that the dimensionless centerline concentration trajectories is in proportional to 1/3 power of the dimensionless downstream distance , and the dimensionless centerline dilution is proportional to the square of the dimensionless centerline trajectory . Several empirical equations are then derived by using the Froude number of cross-flow Frc as a reference coefficient. This paper provides a better understanding and new estimations of the jet initial dilution under the combined effect of waves and cross-flow current.     

Modeling Virgin Compression of Reconstituted Clay at Different Initial Water Contents

The observations on compressibility of reconstituted clays show that the compression line with a higher initial water content lies above the compression line with a lower initial water content for a given clay. Hence there exists additional void ratio due to initial water contents among virgin compression lines (VCLs) of reconstituted clays. In this paper, the difference in void ratio caused by different initial water contents is investigated based on the empirical equation proposed by Liu and Carter (2000) for describing the differential void ratio at the same stress between natural and reconstituted clays. The mechanism of compressibility of reconstituted clays, when the stress level is larger than the remolded yield stress, is also discussed.     

Diagnosing SST Error Growth during ENSO Developing Phase in the BCC_CSM1.1(m) Prediction System

In this study, the predictability of the El Nino-South Oscillation(ENSO) in an operational prediction model from the perspective of initial errors is diagnosed using the seasonal hindcasts of the Beijing Climate Center System Model,BCC;SM1.1(m). Forecast skills during the different ENSO phases are analyzed and it is shown that the ENSO forecasts appear to be more challenging during the developing phase, compared to the decay phase. During ENSO development, the SST prediction errors are significantly negative and cover a large area in the central and eastern tropical Pacific, thus limiting the model skill in predicting the intensity of El Nino. The large-scale SST errors, at their early stage, are generated gradually in terms of negative anomalies in the subsurface ocean temperature over the central-western equatorial Pacific,featuring an error evolutionary process similar to that of El Nino decay and the transition to the La Nina growth phase.Meanwhile, for short lead-time ENSO predictions, the initial wind errors begin to play an increasing role, particularly in linking with the subsurface heat content errors in the central-western Pacific. By comparing the multiple samples of initial fields in the model, it is clearly found that poor SST predictions of the Nino-3.4 region are largely due to contributions of the initial errors in certain specific locations in the tropical Pacific. This demonstrates that those sensitive areas for initial fields in ENSO prediction are fairly consistent in both previous ideal experiments and our operational predictions,indicating the need for targeted observations to further improve operational forecasts of ENSO.     

初值影响下误差分配的微观结构

测量数据处理中,初值对平差结果存在很大的影响,其机理影响设计矩阵的数值特征,进而影响误差的分配结构。本文着力于大地测边控制网的模拟分析,研究初值如何影响误差分配。结果表明,在奇异的网形结构下,初值越靠近真实值,对随机误差的放大作用越大。初值对线性化误差存在影响,在图形最弱方向上,线性化误差影响最大,且误差影响随初值靠近真值而减小。平差结果是随机误差的放大倍率与线性化误差的综合作用的结果,其准确度取决于线性化误差与随机误差的平衡性。     

启动对流的初始扰动对热带飑线模拟的影响

在中尺度数值模拟及数值预报中,正确的对流启动是模拟或预报是否取得成功的关键。作者针对一次热带海洋飑线过程,利用一个风暴尺度云分辨数值模式进行数值模拟,采用不同的初始扰动触发与启动对流发展,重点讨论这些不同启动对流的初始扰动对飑线演变、生命史及其成熟结构等模拟的影响。结果表明,初始扰动的结构、形态分布及其与环境场的不同配置对具有深对流的飑线生命史存在重要影响,而对飑线的成熟结构影响较小。     

初始扰动对一次华南暴雨预报的影响的研究

本文选取了2006年华南前汛期的一次暴雨过程, 采用AREMv2.3中尺度数值模式进行数值模拟, 分别在模式初始场的物理量场 (温度场、 风场、 湿度场) 上加扰动, 分析不同物理量场上的扰动对降水预报的影响, 以及物理量预报误差和扰动能量的增长情况。同时, 通过本个例讨论误差增长与湿对流的关系, 扰动振幅对误差增长的影响和华南区域的中尺度降水的可预报性问题。数值试验结果表明: 初始时刻不同物理量场加实际振幅的正态分布的随机扰动时, 对降水的影响是不同的。对于24小时降水预报, 温度场对降水的影响最大。误差的增长与湿对流不稳定有着密切的关系。小尺度小振幅误差增长很快, 而且是非线性增长。这意味着短期的较小尺度降水的可预报性很小。与大振幅扰动相比, 小振幅扰动造成的误差较小。但是小振幅扰动的迅速发展, 很快就会对降水预报造成较大的影响。因此, 只能有限地提高预报质量, 而且由于扰动非线性增长很快, 在预报时间的提前上, 不会有太大的改善。     

Study on ETKF-Based Initial Perturbation Scheme for GRAPES Global Ensemble Prediction

Initial perturbation scheme is one of the important problems for ensemble prediction. In this paper, ensemble initial perturbation scheme for Global/Regional Assimilation and PrEdiction System (GRAPES) global ensemble prediction is developed in terms of the ensemble transform Kalman filter (ETKF) method.A new GRAPES global ensemble prediction system (GEPS) is also constructed. The spherical simplex 14-member ensemble prediction experiments, using the simulated observation network and error characteristics of simulated observations and innovation-based in ation, are carried out for about two months. The structure characters and perturbation amplitudes of the ETKF initial perturbations and the perturbation growth characters are analyzed, and their qualities and abilities for the ensemble initial perturbations are given. The preliminary experimental results indicate that the ETKF-based GRAPES ensemble initial perturbations could identify main normal structures of analysis error variance and reflect the perturbation amplitudes.The initial perturbations and the spread are reasonable. The initial perturbation variance, which is approximately equal to the forecast error variance, is found to respond to changes in the observational spatial variations with simulated observational network density. The perturbations generated through the simplex method are also shown to exhibit a very high degree of consistency between initial analysis and short-range forecast perturbations. The appropriate growth and spread of ensemble perturbations can be maintained up to 96-h lead time. The statistical results for 52-day ensemble forecasts show that the forecast scores ofensemble average for the Northern Hemisphere are higher than that of the control forecast. Provided that using more ensemble members, a real-time observational network and a more appropriate inflation factor,better effects of the ETKF-based initial scheme should be shown.