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| 1 | A land surface soil moisture data assimilation framework in consideration of the model subgrid-scale heterogeneity and soil water thawing and freezing显示文摘The Ensemble Kalman Filter (EnKF) is well known and widely used in land data assimilation for its high precision and simple operation. The land surface models used as the forecast operator in a land data assimilation system are usually designed to consider the model subgrid-heterogeneity and soil water thawing and freezing. To neglect their effects could lead to some errors in soil moisture assimilation. The dual EnKF method is employed in soil moisture data assimilation to build a soil moisture data as- similation framework based on the NCAR Community Land Model version 2.0 (CLM 2.0) in considera- tion of the effects of the model subgrid-heterogeneity and soil water thawing and freezing: Liquid volumetric soil moisture content in a given fraction is assimilated through the state filter process, while solid volumetric soil moisture content in the same fraction and solid/liquid volumetric soil moisture in the other fractions are optimized by the parameter filter. Preliminary experiments show that this dual EnKF-based assimilation framework can assimilate soil moisture more effectively and precisely than the usual EnKF-based assimilation framework without considering the model subgrid-scale heteroge- neity and soil water thawing and freezing. With the improvement of soil moisture simulation, the soil temperature-simulated precision can be also improved to some extent. | TIAN XiangJun XIE ZhengHui | 2008 | Science China Earth Sciences2008，51，7： | 11 |
| 2 | A soil moisture assimilation scheme based on the ensemble Kalman filter using microwave brightness temperature显示文摘This study presents a soil moisture assimilation scheme, which could assimilate microwave brightness temperature directly, based on the ensemble Kalman filter and the shuffled complex evolution method (SCE-UA). It uses the soil water model of the land surface model CLM3.0 as the forecast operator, and a radiative transfer model (RTM) as the observation operator in the assimilation system. The assimilation scheme is implemented in two phases: the parameter calibration phase and the pure soil moisture assimilation phase. The vegetation optical thickness and surface roughness parameters in the RTM are calibrated by SCE-UA method and the optimal parameters are used as the final model parameters of the observation operator in the assimilation phase. The ideal experiments with synthetic data indicate that this scheme could significantly improve the simulation of soil moisture at the surface layer. Further- more, the estimation of soil moisture in the deeper layers could also be improved to a certain extent. The real assimilation experiments with AMSR-E brightness temperature at 10.65 GHz (vertical polariza- tion) show that the root mean square error (RMSE) of soil moisture in the top layer (0―10 cm) by as- similation is 0.03355 m3·m-3, which is reduced by 33.6% compared with that by simulation (0.05052 m3·m-3). The mean RMSE by assimilation for the deeper layers (10―50 cm) is also reduced by 20.9%. All these experiments demonstrate the reasonability of the assimilation scheme developed in this study. | BingHao Jia ZhengHui Xie XiangJun Tian ChunXiang Shi | 2009 | Science China Earth Sciences2009，52，11： | 10 |
| 3 | The Chinese carbon cycle data-assimilation system(Tan-Tracker)显示文摘In this study,the Chinese carbon cyle dataassimilation system Tan-Tracker is developed based on the atmospheric chemical transport model(GEOS-Chem)platform.Tan-Tracker is a dual-pass data-assimilation system in which both CO2concentrations and CO2fluxes are simultaneously assimilated from atmospheric observations.It has several advantages,including its advanced data-assimilation method,its highly efficient computing performance,and its simultaneous assimilation of CO2concentrations and CO2fluxes.Preliminary observing system simulation experiments demonstrate its robust performance with high assimilation precision,making full use of observations.The Tan-Tracker system can only assimilate in situ observations for the moment.In the future,we hope to extend Tan-Tracker with functions for using satellite measurements,which will form the quasioperational Chinese carbon cycle data-assimilation system. | Xiangjun Tian Zhenghui Xie Zhaonan Cai Yi Liu Yu Fu Huifang Zhang | 2014 | Chinese Science Bulletin2014，59，14： | 7 |
| 4 | A soil water and heat transfer model including changes in soil frost and thaw fronts显示文摘Freeze-thaw processes in soils,including changes in frost and thaw fronts(FTFs),are important physical processes.The movement of FTFs affects soil hydrothermal characteristics,as well as energy and water exchanges between the land surface and the atmosphere and hydrothermal processes in the land surface.This paper reduces the issue of soil freezing and thawing to a multiple moving-boundary problem and develops a soil water and heat transfer model which considers the effects of FTF on soil hydrothermal processes.A local adaptive variable-grid method is used to discretize the model.Sensitivity tests based on the hierarchical structure of the Community Land Model(CLM)show that multiple FTFs can be continuously tracked,which overcomes the difficulties of isotherms that cannot simultaneously simulate multiple FTFs in the same soil layer.The local adaptive variable-grid method is stable and offers computational efficiency several times greater than the high-resolution case.The simulated FTF depths,soil temperatures,and soil moisture values fit well with the observed data,which further demonstrates the potential application of this simulation to the land-surface process model. | WANG AiWen XIE ZhengHui FENG XiaoBing TIAN XiangJun QIN PeiHua | 2014 | Science China Earth Sciences2014，57，6： | 5 |
| 5 | An explicit four-dimensional variational data assimilation method based on the proper orthogonal decomposition: Theoretics and evaluation显示文摘The proper orthogonal decomposition (POD) method is used to construct a set of basis functions for spanning the ensemble of data in a certain least squares optimal sense. Compared with the singular value decomposition (SVD), the POD basis functions can capture more energy in the forecast ensemble space and can represent its spatial structure and temporal evolution more effectively. After the analysis variables are expressed by a truncated expansion of the POD basis vectors in the ensemble space, the control variables appear explicitly in the cost function, so that the adjoint model, which is used to de-rive the gradient of the cost function with respect to the control variables, is no longer needed. The application of this new technique significantly simplifies the data assimilation process. Several as-similation experiments show that this POD-based explicit four-dimensional variational data assimila-tion method performs much better than the usual ensemble Kalman filter method on both enhancing the assimilation precision and reducing the computation cost. It is also better than the SVD-based ex-plicit four-dimensional assimilation method, especially when the forecast model is not perfect and the forecast error comes from both the noise of the initial filed and the uncertainty of the forecast model. | TIAN XiangJun XIE ZhengHui | 2009 | Science China Earth Sciences2009，52，2： | 4 |
| 6 | An Adjoint-Free CNOP–4DVar Hybrid Method for Identifying Sensitive Areas in Targeted Observations: Method Formulation and Preliminary Evaluation显示文摘This paper proposes a hybrid method, called CNOP–4 DVar, for the identification of sensitive areas in targeted observations, which takes the advantages of both the conditional nonlinear optimal perturbation(CNOP) and four-dimensional variational assimilation(4 DVar) methods. The proposed CNOP–4 DVar method is capable of capturing the most sensitive initial perturbation(IP), which causes the greatest perturbation growth at the time of verification;it can also identify sensitive areas by evaluating their assimilation effects for eliminating the most sensitive IP. To alleviate the dependence of the CNOP–4 DVar method on the adjoint model, which is inherited from the adjoint-based approach, we utilized two adjointfree methods, NLS-CNOP and NLS-4 DVar, to solve the CNOP and 4 DVar sub-problems, respectively. A comprehensive performance evaluation for the proposed CNOP–4 DVar method and its comparison with the CNOP and CNOP–ensemble transform Kalman filter(ETKF) methods based on 10 000 observing system simulation experiments on the shallow-water equation model are also provided. The experimental results show that the proposed CNOP–4 DVar method performs better than the CNOP–ETKF method and substantially better than the CNOP method. | Xiangjun TIAN Xiaobing FENG | 2019 | Advances in Atmospheric Sciences2019，36，7： | 4 |
| 7 | Review of Coded Modulation Free Space Optical Communication System显示文摘This paper presents a constructive discussion and inclusive review on advancements in the coded modulation free-space optical(FSO) communication system and corresponding techniques. These techniques mainly include channel model,forward error correction(FEC) and modulation schemes. Firstly,a complete view of FSO system is presented which contains the description,the current research situation along with the advantages over the traditional radio frequency(RF) wireless communication and fiber optical communications. Then,the channel model,FEC,modulation schemes and complete system of coded modulation FSO are analyzed successively. At last,the review work of coded modulation FSO system is summarized and further improvements are prospected. | WANG Kaimin LIU Bo ZHANG Lijia ZHANG Qi TIAN Qinghua XIN Xiangjun | 2015 | China Communications2015，12，11： | 3 |
| 8 | GRACE terrestrial water storage data assimilation based on the ensemble four-dimensional variational method PODEn4DVar:Method and validation显示文摘Seasonal and interannual changes in the Earth's gravity field are mainly due to mass exchange among the atmosphere,ocean,and continental water sources.The terrestrial water storage changes,detected as gravity changes by the Gravity Recovery and Climate Experiment(GRACE) satellites,are mainly caused by precipitation,evapotranspiration,river transportation and downward infiltration processes.In this study,a land data assimilation system LDAS-G was developed to assimilate the GRACE terrestrial water storage(TWS) data into the Community Land Model(CLM3.5) using the POD-based ensemble four-dimensional variational assimilation method PODEn4 DVar,disaggregating the GRACE large-scale terrestrial water storage changes vertically and in time,and placing constraints on the simulation of vertical hydrological variables to improve land surface hydrological simulations.The ideal experiments conducted at a single point and assimilation experiments carried out over China by the LDAS-G data assimilation system showed that the system developed in this study improved the simulation of land surface hydrological variables,indicating the potential of GRACE data assimilation in large-scale land surface hydrological research and applications. | SUN Qin XIE ZhengHui TIAN XiangJun | 2015 | Science China Earth Sciences2015，58，3： | 3 |
| 9 | A correlation analysis method for analyzing tribological states using acoustic emission, frictional coefficient, and contact resistance signals显示文摘A multi-physical signal correlation analysis method is proposed to identify the different tribological properties of materials.The acoustic emission(AE),contact resistance,and frictional force behaviors during dry sliding between four metals,45#carbon steel,YG12 carbide,2A12 aluminum alloy,and H62 brass,have been studied.Both positive and negative correlations between the root mean square of the amplified AE(AE RMS)signal and the frictional coefficient have been found in the experiments.In addition,the AE RMS signal and the contact resistance changed with changing sliding speed and normal load in different ways.The different correlation behaviors have been attributed to diverse tribological states under different experimental conditions due to different material characteristics.The correlation analysis provides a new method of quantitatively identifying the tribological states and the AE sources during frictional interaction.The observed anomalous correlations between the AE signal and frictional coefficient should be properly considered according to the different material properties during industrial friction condition monitoring using AE technology. | Pengyi TIAN Yu TIAN Lei SHAN Yonggang MENG Xiangjun ZHANG | 2015 | Friction2015，3，1： | 3 |
| 10 | Ant colony optimization based load balancing routing and wavelength assignment for optical satellite networks显示文摘An ant colony optimization(ACO) based load balancing routing and wavelength assignment(RWA) algorithm(ALRWA) was put forward for the sake of achieving a fairy load balancing over the entire optical satellite networks. A multi-objective optimization model is established considering the characteristic of global traffic distribution. This not only employs the traffic intensity to modify the light path cost, but also monitors the wavelength utilization of optical inter-satellite links(ISLs). Then an ACO algorithm is utilized to solve this model, leading to finding an optimal light path for every connection request. The optimal light path has the minimum light path cost under satisfying the constraints of wavelength utilization, transmission delay and wavelength-continuity. Simulation results show that ALRWA performs well in blocking probability and realizes efficient load balancing. Meanwhile, the average transmission delay can meet the basic requirement of real-time business transmission. | Wen Guoli Zhang Qi Wang Houtian Tian Qinghua Zhang Wei Xin Xiangjun | 2017 | The Journal of China Universities of Posts and Telecommunications2017，24，5： | 2 |
| 11 | Accounting for CO2 Variability over East Asia with a Regional Joint Inversion System and Its Preliminary Evaluation显示文摘A regional surface carbon dioxide(CO_2) flux inversion system, the Tan-Tracker-Region, was developed by incorporating an assimilation scheme into the Community Multiscale Air Quality(CMAQ) regional chemical transport model to resolve fine-scale CO_2 variability over East Asia. The proper orthogonal decomposition-based ensemble four-dimensional variational data assimilation approach(POD-4 DVar) is the core algorithm for the joint assimilation framework, and simultaneous assimilations of CO_2 concentrations and surface CO_2 fluxes are applied to help reduce the uncertainty in initial CO_2 concentrations. A persistence dynamical model was developed to describe the evolution of the surface CO_2 fluxes and help avoid the 'signal-to-noise' problem; thus, CO_2 fluxes could be estimated as a whole at the model grid scale, with better use of observation information. The performance of the regional inversion system was evaluated through a group of single-observation-based observing system simulation experiments(OSSEs). The results of the experiments suggest that a reliable performance of Tan-Tracker-Region is dependent on certain assimilation parameter choices, for example, an optimized window length of approximately 3 h, an ensemble size of approximately 100, and a covariance localization radius of approximately 320 km. This is probably due to the strong diurnal variation and spatial heterogeneity in the fine-scale CMAQ simulation, which could affect the performance of the regional inversion system. In addition, because all observations can be artificially obtained in OSSEs, the performance of Tan-Tracker-Region was further evaluated through different densities of the artificial observation network in different CO_2 flux situations. The results indicate that more observation sites would be useful to systematically improve the estimation of CO_2 concentration and flux in large areas over the model domain. The work presented here forms a foundation for future research in which a thorough estimation of CO_2 flux variability over East Asia could be performed with the regional inversion system. | xingxia kou xiangjun tian meigen zhang zhen peng xiaoling zhang | 2017 | Journal of Meteorological Research2017，31，5： | 2 |
| 12 | Effects of sample density on the assimilation performance of an explicit four-dimensional variational data assimilation method显示文摘The concepts of sample sphere radius and sample density are proposed in this paper to help illustrate that different vector transformations result in diverse sample density with the same sample ensemble, which finally affects their assimilation performance. Several numerical experiments using a onedimensional (1-D) soil water equation and synthetic observations are conducted to evaluate this new theory in land data assimilation. | TIAN XiangJun XIE ZhengHui | 2009 | Science China Earth Sciences2009，52，11： | 2 |
| 13 | A new approach for Bayesian model averaging显示文摘Bayesian model averaging(BMA) is a recently proposed statistical method for calibrating forecast ensembles from numerical weather models.However,successful implementation of BMA requires accurate estimates of the weights and variances of the individual competing models in the ensemble.Two methods,namely the Expectation-Maximization(EM) and the Markov Chain Monte Carlo(MCMC) algorithms,are widely used for BMA model training.Both methods have their own respective strengths and weaknesses.In this paper,we first modify the BMA log-likelihood function with the aim of removing the addi-tional limitation that requires that the BMA weights add to one,and then use a limited memory quasi-Newtonian algorithm for solving the nonlinear optimization problem,thereby formulating a new approach for BMA(referred to as BMA-BFGS).Several groups of multi-model soil moisture simulation experiments from three land surface models show that the performance of BMA-BFGS is similar to the MCMC method in terms of simulation accuracy,and that both are superior to the EM algo-rithm.On the other hand,the computational cost of the BMA-BFGS algorithm is substantially less than for MCMC and is al-most equivalent to that for EM. | TIAN XiangJun XIE ZhengHui WANG AiHui YANG XiaoChun | 2012 | Science China Earth Sciences2012，55，8： | 2 |
| 14 | Pollen record of surface sediments from vertical forest zones of changhai mountain northeast China and their relations to the modern vegelation显示文摘 | Sun Xiangjun Luo Yunli Tian Jun el al | 2003 | Aeta Botanica Sinica2003，45，8： | 1 |
| 15 | Impacts of Multigrid NLS-4DVar-based Doppler Radar Observation Assimilation on Numerical Simulations of Landfalling Typhoon Haikui (2012)显示文摘We applied the multigrid nonlinear least-squares four-dimensional variational assimilation(MG-NLS4DVar)method in data assimilation and prediction experiments for Typhoon Haikui(2012)using the Weather Research and Forecasting(WRF)model.Observation data included radial velocity(Vr)and reflectivity(Z)data from a single Doppler radar,quality controlled prior to assimilation.Typhoon prediction results were evaluated and compared between the NLS-4DVar and MG-NLS4DVar methods.Compared with a forecast that began with NCEP analysis data,our radar data assimilation results were clearly improved in terms of structure,intensity,track,and precipitation prediction for Typhoon Haikui(2012).The results showed that the assimilation accuracy of the NLS-4DVar method was similar to that of the MG-NLS4DVar method,but that the latter was more efficient.The assimilation of Vr alone and Z alone each improved predictions of typhoon intensity,track,and precipitation;however,the impacts of Vr data were significantly greater that those of Z data.Assimilation window-length sensitivity experiments showed that a 6-h assimilation window with 30-min assimilation intervals produced slightly better results than either a 3-h assimilation window with 15-min assimilation intervals or a 1-h assimilation window with 6-min assimilation intervals. | Lu ZHANG Xiangjun TIAN Hongqin ZHANG Feng CHEN | 2020 | Advances in Atmospheric Sciences2020，37，8： | 1 |
| 16 | Prediction of water table depths under soil water-groundwater interaction and stream water conveyance显示文摘Water table over an arid region can be elevated to a critical level to sustain terrestrial ecosystem along the natural channel by the stream water conveyance. Estimation of water table depth and soil moisture on river channel profile may be reduced to a two-dimensional moving boundary problem with soil water-groundwater interaction. The two-dimensional soil water flow with stream water transferred is divided into an unsaturated vertical soil water flow and a horizontal groundwater flow. Therefore, a prediction model scheme for water table depths under the interaction between soil water and groundwater with stream water transferred is presented, which includes a vertical soil water movement model, a horizontal groundwater movement model, and an interface model. The synthetic experiments are conducted to test the sensitivities of the river elevation, horizontal conductivity, and surface flux, and the results from the experiments show the robustness of the proposed scheme under different conditions. The groundwater horizontal conductivity of the proposed scheme is also calibrated by SCE-UA method and validated by data collected at the Yingsu section in the lower reaches of the Tarim River, which shows that the model can reasonably simulate the water table depths. | DI ZhenHua XIE ZhengHui YUAN Xing TIAN XiangJun LUO ZhenDong CHEN YaNing | 2011 | Science China Earth Sciences2011，54，3： | 1 |
| 17 | Optomechanical soft metamaterials显示文摘We present a new type of optomechanical soft metamaterials, which is different from conventional mechanical metamaterials, in that they are simple isotropic and homogenous materials without resorting to any complex nano/microstructures. This metamaterial is unique in the sense that its responses to uniaxial forcing can be tailored by programmed laser inputs to manifest different nonlinear constitutive behaviors, such as monotonic, S-shape, plateau, and non-monotonic snapping performance. To demonstrate the novel metamaterial, a thin sheet of soft material impinged by two counterpropagating lasers along its thickness direction and stretched by an in-plane tensile mechanical force is considered. A theoretical model is formulated to characterize the resulting optomechanical behavior of the thin sheet by combining the nonlinear elasticity theory of soft materials and the optical radiation stress theory. The optical radiation stresses predicted by the proposed model are validated by simulations based on the method of finite elements. Programmed optomechanical behaviors are subsequently explored using the validated model under different initial sheet thicknesses and different optical inputs, and the first-and second-order tangential stiffness of the metamaterial are used to plot the phase diagram of its nonlinear constitutive behaviors.The proposed optomechanical soft metamaterial shows great potential in biological medicine, microfluidic manipulation,and other fields. | Xiangjun Peng Wei He Yifan Liu Fengxian Xin Tian Jian Lu | 2017 | Acta Mechanica Sinica2017，33，3： | 1 |
| 18 | Design of adaptive backoff algorithm for satellite network using grey system显示文摘In contention-based satellite communication system, collisions between data packets may occur due to the randomly sending of the packets. A proper delay before each transmission can reduce the data collision rate. As classical random multiple access protocol, the slotted ALOHA(S-ALOHA) reduces the data collision rate through time slot allocation and synchronous measures. In order to improve the stability and throughput of satellite network, a backoff algorithm based on S-ALOHA will be effective. A new adaptive backoff algorithm based on S-ALOHA using grey system was proposed, which calculates the backoff time adaptively according to the network condition. And the network condition is estimated by each user terminal according to the prediction of the channel access success ratio using the model GM(1,1) in grey system. The proposed algorithm is compared to other known schemes such as the binary exponential backoff(BEB) and the multiple increase multiple decrease(MIMD) backoff. The performance of the proposed algorithm is simulated and analyzed. It is shown that throughput of the system based on the proposed algorithm is better than of system based on BEB and MIMD backoff. And there are also some improvements of the delay performance compared to using BEB. The proposed algorithm is especially effective for large number of user terminals in the satellite networks. | Qi Xiaohang Zhang Qi Tian Qinghua Tao Ying Xin Xiangjun Liu Naijin | 2015 | The Journal of China Universities of Posts and Telecommunications2015，22，1： | 1 |
| 19 | A microwave land data assimilation system: Scheme and preliminary evaluation over China显示文摘 | Tian Xiangjun Xie Zhenghui Dai Aiguo Jia Binghao Shi Chunxiang | 2010 | Journal of Geophysical Research. Atmospheres2010，，21： | 1 |
| 20 | Rapid synthesis of bulk TiAIC2 by laser melting 显示文摘 | CHEN Deqiang TIAN Xiangjun WANG Huaming | 2014 | Materials Letters2014，129，： | 1 |
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