Investigating a Hierarchy of Eulerian Closure Models for Scalar Transfer Inside Forested Canopies

Modelling the transfer of heat, water vapour, and CO₂ between the biosphere and the atmosphere is made difficult by the complex two-way interaction between leaves and their immediate microclimate. When simulating scalar sources and sinks inside canopies on seasonal, inter-annual, or forest development time scales, the so-called well-mixed assumption (WMA) of mean concentration (i.e. vertically constant inside the canopy but dynamically evolving in time) is often employed. The WMA eliminates the need to model how vegetation alters its immediate microclimate, which necessitates formulations that utilize turbulent transport theories. Here, two inter-related questions pertinent to the WMA for modelling scalar sources, sinks, and fluxes at seasonal to inter-annual time scales are explored: (1) if the WMA is to be replaced so as to resolve this two-way interaction, how detailed must the turbulent transport model be? And (2) what are the added predictive skills gained by resolving the two-way interaction vis-à-vis other uncertainties such as seasonal variations in physiological parameters. These two questions are addressed by simulating multi-year mean scalar concentration and eddy-covariance scalar flux measurements collected in a Loblolly pine (P. taeda L.) plantation near Durham, North Carolina, U.S.A. using turbulent transport models ranging from K-theory (or first-order closure) to third-order closure schemes. The multi-layer model calculations with these closure schemes were contrasted with model calculations employing the WMA. These comparisons suggested that (i) among the three scalars, sensible heat flux predictions are most biased with respect to eddy-covariance measurements when using the WMA, (ii) first-order closure schemes are sufficient to reproduce the seasonal to inter-annual variations in scalar fluxes provided the canonical length scale of turbulence is properly specified, (iii) second-order closure models best agree with measured mean scalar concentration (and temperature) profiles inside the canopy as well as scalar fluxes above the canopy, (iv) there are no clear gains in predictive skills when using third-order closure schemes over their second-order closure counterparts. At inter-annual time scales, biases in modelled scalar fluxes incurred by using the WMA exceed those incurred when correcting for the seasonal amplitude in the maximum carboxylation capacity (V _{cmax, 25}) provided its mean value is unbiased. The role of local thermal stratification inside the canopy and possible computational simplifications in decoupling scalar transfer from the generation of the flow statistics are also discussed.

“The tree, tilting its leaves to capture bullets of light; inhaling, exhaling; its many thousand stomata breathing, creating the air”. Ruth Stone, 2002, In the Next Galaxy

     

Difference in the Interdecadal Variability of Spring and Summer Sensible Heat Flux over Northwest China

The present study investigates the difference in interdecadal variability of the spring and summer sensible heat fluxes over Northwest China by using station observations from 1960 to 2000. It was found that the spring sensible heat flux over Northwest China was greater during the period from the late 1970s to the 1990s than during the period from the 1960s to the mid-1970s. The summer sensible heat flux was smaller in the late 1980s through the 1990s than it was in the 1970s through the early 1980s. Both the spring and summer land-air temperature differences over Northwest China displayed an obvious interdecadal increase in the late 1970s. Both the spring and summer surface wind speeds experienced an obvious interdecadal weakening in the late 1970s. The change in the surface wind speed played a more important role in the interdecadal variations in sensible heat flux during the summer, whereas the change in the land-air temperature difference was more important for the interdecadal variations in sensible heat flux in the spring. This difference was related to seasonal changes in the mean land-air temperature difference and the surface wind speed. Further analysis indicated that the increase in the spring land surface temperature in Northwest China was related to an increase in surface net radiation.     

基于LiDAR的海岛岸线自动提取研究

为了探讨栅格化方法与图像分割法对海岛岸线提取的效果,本文主要研究了基于Li DAR数据利用这两种方法对某岛进行瞬时海岸线的提取。利用栅格化方法通过对Li DAR点云数据进行粗差剔除、滤波去噪、构建Terrain数据集、创建栅格表面、生成TIN模型及自动生成等高线,从而实现了瞬时海岸线的提取;利用图像分割法是通过对Li DAR点云数据进行粗差剔除、滤波处理、构建TIN模型、生成二值栅格图像、图像处理与图像边缘提取的过程实现瞬时海岸线的提取。对两种方法提取的海岸线进行叠加显示分析,试验结果表明:两种方法提取的海岸线形态结构基本吻合,海岸线提取效率较传统方法均有提高,但栅格化方法提取的海岸线比图像分割方法提取的海岸线更平滑、更细化,边缘信息较为丰富,与实际海岸线更贴切,效果更加理想。     

Deriving 3D coseismic deformation field by combining GPS and InSAR data based on the elastic dislocation model

The density of GPS measurements is usually one of the key issues in resolving 3-D coseismic deformation field from integrating GPS and Interferometric Synthetic Aperture Radar (InSAR) measurements with pure mathematic interpolation methods An approach that combines the elastic dislocation model with the Best Quadratic Unbiased Estimator (BQUE) or a robust estimation method named IGG (Institute of Geodesy and Geophysics) is proposed to reconstruct 3-D coseismic deformation field, in which only a small amount of GPS data is needed to produce a reasonable initial 3-D coseismic deformation. Then the BQUE and IGG are used to weight the InSAR and GPS measurements to avoid computational issues caused by the negative variance problem and to decrease the impact from gross errors. The Wenchuan earthquake is used to test the proposed method. We find that the developed method makes it possible to use only a few GPSs and InSAR data to recover the 3-D coseismic deformation field, which offers extensive future usage for measuring earthquake deformation, particularly in some tectonically active regions with sparse GPS measurements.     

植被净初级生产力模型估算及其对气候变化的响应研究进展

近年来随着遥感和地理信息系统技术的广泛应用,植被净初级生产力研究经历了从小范围的传统测量阶段到大范围的模型估算阶段的重要转变,并参与全球变化研究。其研究手段和研究内容大大拓宽,在植被净初级生产力模型估算以及对气候变化（如温度、降水、ＣＯ₂浓度等）的响应等方面的研究取得了可喜的进展。     

玉米冠层反射率及净辐射的估算

在1994~1996年3年田间试验的基础上,根据玉米生长状况及其冠层特征,选取包括植株高度、平均叶面积密度、农田覆被率等植物特征量组成的植物参数——综合植被系数来描述玉米冠层状况,并以此为因子来估算玉米冠层的反射率和净辐射,取得了良好的结果。     

1D elastic full‐waveform inversion and uncertainty estimation by means of a hybrid genetic algorithm–Gibbs sampler approach

Stochastic optimization methods, such as genetic algorithms, search for the global minimum of the misfit function within a given parameter range and do not require any calculation of the gradients of the misfit surfaces. More importantly, these methods collect a series of models and associated likelihoods that can be used to estimate the posterior probability distribution. However, because genetic algorithms are not a Markov chain Monte Carlo method, the direct use of the genetic‐algorithm‐sampled models and their associated likelihoods produce a biased estimation of the posterior probability distribution. In contrast, Markov chain Monte Carlo methods, such as the Metropolis–Hastings and Gibbs sampler, provide accurate posterior probability distributions but at considerable computational cost. In this paper, we use a hybrid method that combines the speed of a genetic algorithm to find an optimal solution and the accuracy of a Gibbs sampler to obtain a reliable estimation of the posterior probability distributions. First, we test this method on an analytical function and show that the genetic algorithm method cannot recover the true probability distributions and that it tends to underestimate the true uncertainties. Conversely, combining the genetic algorithm optimization with a Gibbs sampler step enables us to recover the true posterior probability distributions. Then, we demonstrate the applicability of this hybrid method by performing one‐dimensional elastic full‐waveform inversions on synthetic and field data. We also discuss how an appropriate genetic algorithm implementation is essential to attenuate the “genetic drift” effect and to maximize the exploration of the model space. In fact, a wide and efficient exploration of the model space is important not only to avoid entrapment in local minima during the genetic algorithm optimization but also to ensure a reliable estimation of the posterior probability distributions in the subsequent Gibbs sampler step.     

基于一阶弹性波方程的能量互相关成像条件

基于地震波场能量构建的能量互相关成像条件,具有易实现、物理意义明确及背向散射压制效果明显等优势.但是,目前构建的能量互相关成像条件仅适用于二阶弹性波方程,难以直接应用于一阶弹性波方程.为此,本文针对一阶弹性波方程,基于能量守恒定理及能量密度,构建以速度-应力为参数的能量范数以表征弹性波场能量,将速度-应力能量范数拓展为能量内积以提取弹性波场反射能量.震源端与检波端的基矢量正方向保持一致的基础上,构建得到可有效压制背向散射的弹性波能量成像条件.数值模拟结果表明:该成像条件可以得到背向散射压制、振幅有效保持的能量成像结果.     

大陆架科学钻探CSDP-2井的垂直地震剖面测量

南黄海海相地层的地震波场特征和层位标定一直是困扰地震勘探的重要问题.为了近距离、高精度和高分辨率地观测井周围构造特征和岩石性质引起的波场变化,为地震资料的采集、处理与解释提供地震波衰减规律、速度与层位标定等信息,对大陆架科学钻探CSDP-2井实施了近零偏移距垂直地震剖面（VSP）观测.针对海相地层顶部强反射界面地震波穿透难的问题,采用了大容量气枪震源并设计了气枪阵列组合方式,提高了激发地震波的能量,获得了强反射界面之下清晰的PP、PS下行波和上行波信号.采用了三分量偏振合成、组合滤波和波场分离等处理方法,对VSP观测数据进行处理,获得了海相三叠系—志留系的精细的纵波、横波速度结构和地层吸收因子等物性数据,建立了钻井地层、测井、VSP上行波和多道地震剖面对应关系,实现了不同尺度的地质和地球物理属性资料的有效衔接,标定了钻井地质剖面上各深度地质体的地震反射特性,厘定了过井地震剖面上反射同相轴的地质属性.此次观测取得的纵波、横波速度信息,成为建立南黄海海相地层速度模型主要的资料来源,也是地震资料的岩性反演处理不可缺少的信息.     

油水两相渗流多孔介质弹性波传播动力学模型研究

弹性波在储层渗流场中的传播与衰减规律是研究波场强化采油动力学机理的重要基础.基于等效流体理论和饱和静态流体弹性波传播Biot理论,建立油水两非混相流体渗流条件下储层多孔介质中弹性波传播的动力学模型,通过算例求解与分析,发现含油水两相渗流储层多孔介质中同时存在着3种纵波P1、P2、P3和1种横波S;受频率和含水饱和度的影响,各波波速和品质因子呈现出不同变化规律,4种体波波速与频率、饱和度正相关,P1、P2波品质因子与饱和度正相关,P3和S波品质因子与饱和度负相关;最后,通过与传统静态弹性波模型结果对比,进一步分析了宏观渗流场对弹性波传播特征的影响规律,为揭示低频人工地震波辅助强化采油技术的动力学机理和工艺参数优化提供了重要理论依据.