Camera derived vegetation greenness index as proxy for gross primary production in a low Arctic wetland area

The Arctic is experiencing disproportionate warming relative to the global average, and the Arctic ecosystems are as a result undergoing considerable changes. Continued monitoring of ecosystem productivity and phenology across temporal and spatial scales is a central part of assessing the magnitude of these changes. This study investigates the ability to use automatic digital camera images (DCIs) as proxy data for gross primary production (GPP) in a complex low Arctic wetland site. Vegetation greenness computed from DCIs was found to correlate significantly (R² = 0.62, p < 0.001) with a normalized difference vegetation index (NDVI) product derived from the WorldView-2 satellite. An object-based classification based on a bi-temporal image composite was used to classify the study area into heath, copse, fen, and bedrock. Temporal evolution of vegetation greenness was evaluated and modeled with double sigmoid functions for each plant community. GPP at light saturation modeled from eddy covariance (EC) flux measurements were found to correlate significantly with vegetation greenness for all plant communities in the studied year (i.e., 2010), and the highest correlation was found between modeled fen greenness and GPP (R² = 0.85, p < 0.001). Finally, greenness computed within modeled EC footprints were used to evaluate the influence of individual plant communities on the flux measurements. The study concludes that digital cameras may be used as a cost-effective proxy for potential GPP in remote Arctic regions.     

Linking satellite-based spring phenology to temperate deciduous broadleaf forest photosynthesis activity

Satellite-based remote sensed phenology has been widely used to assess global climate change. However, it is constrained by uncertain linkages with photosynthesis activity. Two dynamic threshold methods were employed to retrieve spring phenology metrics from four Moderate Resolution Imaging Spectroradiometer (MODIS) products, including fraction of Absorbed Photosynthetically Active Radiation (fAPAR), Leaf Area Index (LAI), Normalized Difference Vegetation Index (NDVI), and Enhanced Vegetation Index (EVI) for three temperate deciduous broadleaf forests in North America between 2001 and 2009. These MODIS-based spring phenology metrics were subsequently linked to the photosynthetic curves (daily gross primary productivity, GPP) measured by an eddy covariance flux tower. The 20% dynamic threshold spring onset metrics from MODIS products were closer to the photosynthesis onset metrics at the date of 2% GPP increase for NDVI and fAPAR, and closer to the date of 5% and 10% increase of GPP for EVI and LAI, respectively. The 50% dynamic threshold onset metrics were closer to the photosynthesis onset metrics at the date of 10% GPP increase for NDVI, and closer to the date of 20% GPP increase for fAPAR, LAI and EVI, respectively. These results can improve our knowledge on the photosynthesis activity status of remotely sensed spring phenology metrics.     

GIS空间数据质量控制的抗差拟合推估法

拟合推估法可用于拟合系统误差,削弱随机误差的影响,提高空间数据质量。与正常拟合推估法不同,提出了一种抗差拟合推估法以控制GIS空间数据的系统误差和异常误差的影响。在此基础上,提出了对剩余残差再拟合的思想,从而对拟合推估剩余系统误差进行控制。利用该方法,一方面可以有效地抵制系统误差的影响,另一方面又可以较好地遏制异常误差的污染。此外,对因协方差函数误差导致的信号估计偏差,也具有一定的补偿能力。最后通过实际计算与分析验证了该方法的有效性。     

生态系统碳通量估算中耦合涡度协方差与遥感技术研究进展

陆地生态系统CO^₂和水热通量的长期观测研究一直是国际上关注的热点问题。截止目前,利用微气象学原理的涡度协方差技术是唯一能直接测定生物圈与大气间物质与能量通量的标准方法,成为国际通量观测网络的主要技术。但是涡度协方差技术的测定仍然是一种小尺度观测方法,其观测结果难于直接外推到更大尺度。同时,缺乏区域、跨尺度生态系统及其时空动态观测数据一直是限制碳循环研究的主要障碍,而遥感技术的发展可望在不远的将来使大尺度、高分辨生态系统变化的长期定量观测成为可能。这些问题在当今集中体现在如何建立通量—遥感的跨尺度观测体系,并有效地将有限的通量站点测量数据与大尺度遥感资料以及生态模型有机地结合。总结过去耦合涡度协方差技术与遥感技术的工作,主要在以下3个层面展开：①涡度协方差技术与遥感技术对碳通量估算的相互验证;②涡度协方差技术为遥感反演提供地面参数;③遥感观测解译辅助分析通量贡献区（footprint）。集中在这3个方面进行探讨,通过总结各方面的研究特点与进展,可望为未来在这个领域开展工作理顺思路。     

利用MLSC法求解高程异常

运用常规的拟合模型求解大范围高程异常必然存在较大的模型误差,鉴于模型误差的不确定性,可以将模型误差看作信号采用最小二乘配置法来处理,最小二乘配置中协方差函数的确定是一个比较棘手的问题,介于此,本文提出了移动最小二乘配置法,对不同的待定点建立不同的配置模型,并通过定权的方式解决复杂的协方差函数计算问题,对一个大测区的部分GPS水准数据进行解算,获得了满意的结果。     

GSI-3DVAR背景误差协方差水平相关特征尺度对地面自动气象站资料同化的适应性研究

基于华东地区3 km分辨率WRF (Weather Research and Forecasting) 模式和高密度地面自动气象站(AWS)观测,研究GSI-3DVAR同化系统的R_HZSCL对AWS观测的地面温度和风观测同化的敏感性。结果表明:运用GSI-3DVAR同化地面AWS观测时,R_HZSCL的取值较为敏感;选取合适的R_HZSCL能有效改进地面分析场精度,相较于背景场地面温度和地面矢量风差(VWD) RMSE均可减小35％以上。当R_HZSCL过大会导致温度高、低值中心的影响范围过大,风分析场较为平滑,无法反映出中小尺度环流结构。但R_HZSCL过小则会使得温度分析场增加误差,并导致风分析场出现虚假大风。观测密度稀疏化的敏感性试验结果表明,地面温度场及风场所适应的最优R_HZSCL皆随着观测密度的增大而相应减小。     

顾及框架点坐标误差的三维基准转换严密模型

框架点坐标是由观测数据通过平差得到的,不可避免地受到观测误差的影响。针对原框架和目标框架坐标均存在误差、非公共点与公共点间存在相关性,以及转换系数矩阵中仅部分元素存在误差的实际情况,提出了同时考虑框架内误差以及转换点间相关性的基准转换严密模型,该模型将公共点和非公共点联合处理,同时计算坐标转换参数和所有点的坐标转换值,推导出了新的严格坐标转换公式,该公式为传统坐标转换公式基础上增加一改正量的形式;进一步,推导了原框架和目标框架坐标的方差不一致情况下的坐标转换模型的自适应解法;最后,利用"陆态网络工程"2000个区域站的实测坐标进行坐标转换验证,结果表明,这种严密模型较传统坐标转换模型具有更高的坐标转换精度。     

Case Comparing Study of Downstream Circulation Development during Typhoon Extratropical Transition

The interaction between extratropical transition process and the mid-latitude jet system stimulates the downstream development. In this paper, three typhoon cases were selected to study their downstream development mechanism through the analysis of the eddy kinetic energy budget and the idealized simulations. The results of Chen’s work to the Pacific region were examined. The results were consistent with the results of Chen’s Atlantic hurricane Case. ET downstream at the upper levels generated first eddies, and the disturbances triggered the low level eddy development. Then the upper and the lower coupled and formed a deep cyclone system throughout the whole troposphere. The ageostrophic geopotential flux promoted the formation and development of the downstream ridge from the typhoon. Vertical ageostrophic geopotential flux transferred energy from upper downward that convergence happened in lower, which stimulated the lower-level cyclone development. Simulation results showed that, in the process of ET, TC outflow transported low potential vorticity to mid-latitude jet, which enhanced the PV gradient and the baroclinic. Then, it is inspired the Rossby wave in the jet and propagated downstream. The formation of downstream ridge-trough couple and development of the further wave was the spread to the downstream through the Rossby wave.     

各向异性背景场误差协方差构建及在“凡亚比”台风的应用

利用相临过去时段预报结果中同一时刻不同时效的模式预报场差异,计算预报误差协方差,并基于集合-变分混合同化系统将其与静态背景场误差协方差结合,从而在同化系统中构建了具有各向异性和一定流依赖特征的背景场误差协方差。单点观测理想试验显示本方案改善了静态模型化背景场误差协方差的各向同性和流依赖性问题。“凡亚比”台风的一系列同化及模拟试验表明,从台风路径、强度等方面本文方案的效果都要优于三维变分法。本文方案在不需要集合预报,计算量与三维变分法相当的情况下,给同化系统引入了各向异性、一定流依赖特征的背景误差协方差,因此本方案适于在计算资源较为紧缺情况下,对时效要求较高的预报业务中应用。     

Multi-level CO2 fluxes over Beijing megacity with the eddy covariance method

本文基于北京325米气象塔在47,140,和280米三层高度的5年涡动相关观测资料,研究了城市下垫面与大气间的CO2交换过程.由于北京市2011年开始实行工作日汽车尾号限行,140米高度CO2通量的年增长率由2008-2010年的7.8％降低到2010-2012年的2.3％.140米高度通量源区内植被比例最小且人口密度最大,因此140米高度的5年平均CO2通量年总量)6.41 kg C m-2 yr-1(大于47米)5.78 kg C m-2 yr-1(和280米)3.99 kg C m-2 yr-1(.在年尺度上,北京汽车总保有量和总人口是最重要的CO2通量控制因子.CO2通量随风向的变化主要与风向对应的通量源区内下垫面土地利用方式有关.三层高度的夏季CO2通量均与道路的比例呈正相关关系.47,140,和280米的决定系数分别为0.69,0.57,和0.54(P＜0.05).植被比例的下降,会导致CO2年总量上升,两者存在近似于指数的关系.城市人口密度的上升会引起CO2年总量上升.