Gap Filling and Quality Assessment of CO2 and Water Vapour Fluxes above an Urban Area with Radial Basis Function Neural Networks

Vertical turbulent fluxes of water vapour, carbon dioxide, and sensible heat were measured from 16 August to the 28 September 2006 near the city centre of Münster in north-west Germany. In comparison to results of measurements above homogeneous ecosystem sites, the CO₂ fluxes above the urban investigation area showed more peaks and higher variances during the course of a day, probably caused by traffic and other varying, anthropogenic sources. The main goal of this study is the introduction and establishment of a new gap filling procedure using radial basis function (RBF) neural networks, which is also applicable under complex environmental conditions. We applied adapted RBF neural networks within a combined modular expert system of neural networks as an innovative approach to fill data gaps in micrometeorological flux time series. We found that RBF networks are superior to multi-layer perceptron (MLP) neural networks in the reproduction of the highly variable turbulent fluxes. In addition, we enhanced the methodology in the field of quality assessment for eddy covariance data. An RBF neural network mapping system was used to identify conditions of a turbulence regime that allows reliable quantification of turbulent fluxes through finding an acceptable minimum of the friction velocity. For the data analysed in this study, the minimum acceptable friction velocity was found to be 0.15 m s⁻¹. The obtained CO₂ fluxes, measured on a tower at 65 m a.g.l., reached average values of 12 μmol m⁻² s⁻¹ and fell to nighttime minimum values of 3 μmol m ⁻² s⁻¹. Mean daily CO₂ emissions of 21 g CO₂ m⁻²d ⁻¹ were obtained during our 6-week experiment. Hence, the city centre of Münster appeared to be a significant source of CO₂. The half-hourly average values of water vapour fluxes ranged between 0.062 and 0.989 mmol m⁻² s⁻¹and showed lower variances than the simultaneously measured fluxes of CO₂.     

A Re-examination of Density Effects in Eddy Covariance Measurements of CO2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems are used. To account for these effects, mean vertical velocity and perturbation of the density of dry air are two critical parameters in treating those physical processes responsible for density variations. Based on various underlying assumptions, different studies have obtained different formulas for the mean vertical velocity and perturbation of the density of dry air, leading to a number of approaches to correct density effects. In this study, we re-examine physical processes related to different assumptions that are made to formulate the density effects. Specifically, we re-examine the assumptions of a zero dry air flux and a zero moist air flux in the surface layer, used for treating density variations, and their implications for correcting density effects. It is found that physical processes in relation to the assumption of a zero dry air flux account for the influence of dry air expansion/compression on density variations. Meanwhile, physical processes in relation to the assumption of a zero moist air flux account for the influence of moist air expansion/compression on density variations. In this study, we also re-examine mixing ratio issues. Our results indicate that the assumption of a zero dry air flux favors the use of the mixing ratio relative to dry air, while the assumption of a zero moist air flux favors the use of the mixing ratio relative to the total moist air. Additionally, we compare different formula for the mean vertical velocity, generated by air-parcel expansion/compression, and for density effect corrections using eddy covariance data measured over three boreal ecosystems.     

石英显微变形机制及流变学特征研究进展

石英是组成地壳的主要矿物之一,其物理性质与地壳的变形行为及流变学属性密切相关,因此石英的显微变形机制及流变学特征研究意义重大。随着石英显微变形机制及流变学属性实验学研究的深入,我们对石英在地壳变形过程中重要作用的认识也随之提升。本文从石英结构入手,对升温过程中石英主滑移系的转换进行了总结,并且升温过程中显微变形机制也发生了改变,对流变学内涵也不同,所以分述了石英的主要变形机制,及其流变学参数;由于应变局部化为地壳变形的研究热点,并且石英对应变局部化具有一定的指示意义,所以本文对石英在应变局部化过程中的表现进行了深入讨论。考虑到地壳变形的研究过程中,石英不常作为单矿物发生变形,所以本文对多相矿物的变形表现也进行了讨论。最后我们将简单介绍一些现今主流的石英显微变形机制的研究手段。     

Growing season evapotranspiration in boreal fens in the Athabasca Oil Sands Region: Variability and environmental controls

Current efforts to assess changes to the wetland hydrology caused by growing anthropogenic pressures in the Athabasca Oil Sands Region (AOSR) require well-founded spatial and temporal estimates of actual evapotranspiration (ET), which is the dominant component of the water budget in this region. This study assessed growing season (May–September) and peak growing season (July) ET variability at a treed moderate-rich fen and treed poor fen (in 2013–2018), open poor fen (in 2011–2014), and saline fen (in 2015–2018) using eddy covariance technique and a set of complementary environmental data. Seasonal fluctuations in ET were positively related to net radiation, air temperature and vapour pressure deficit and followed trends typical for the Boreal Plains (BP) and AOSR with highest rates in June–July. However, no strong effect of water table position on ET was found. Strong surface control on ET is evident from lower ET values than potential evapotranspiration (PET); the lowest ET/PET was observed at saline fen, followed by open fen, moderately treed fen, and heavily treed fen, suggesting a strong influence of vegetation on water loss. In most years PET exceeded precipitation (P), and positive relations between P/PET and ET were observed with the highest July ET rates occurring under P/PET ~1. However, during months with P/PET > 1, increased P/PET was associated with decreased July ET. With respect to 30-year mean values of air temperature and P in the area, both dry and wet, cool and warm growing seasons (GS) were observed. No clear trends between ET values and GS wetness/coldness were found, but all wet GS were characterized by peak growing seasons with high daily ET variability.     

The impacts of mountain pine beetle disturbance on the energy balance of snow during the melt period

Mountain snowpacks provide most of the annual discharge of western US rivers, but the future of water resources in the western USA is tenuous, as climatic changes have resulted in earlier spring melts that have exacerbated summer droughts. Compounding changes to the physical environment are biotic disturbances including that of the mountain pine beetle (MPB), which has decimated millions of acres of western North American forests. At the watershed scale, MPB disturbance increases the peak hydrograph, and at the stand scale, the ‘grey’ phase of MPB canopy disturbance decreases canopy snow interception, increases snow albedo, increases net shortwave radiation, and decreases net longwave radiation versus the ‘red’ phase. Fewer studies have been conducted on the red phase of MPB disturbance and in the mixed coniferous stands that may follow MPB‐damaged forests. We measured the energy balance of four snowpacks representing different stages of MPB damage, management, and recovery: a lodgepole pine stand, an MPB‐infested stand in the red phase, a mixed coniferous stand (representing one successional trajectory), and a clear‐cut (representing reactive management) in the Tenderfoot Creek Experimental Forest in Montana, USA. Net longwave radiation was lower in the MPB‐infested stand despite higher basal area and plant area index of the other forests, suggesting that the desiccated needles serve as a less effective thermal buffer against longwave radiative losses. Eddy covariance observations of sensible and latent heat flux indicate that they are of similar but opposite magnitude, on the order of 20 MJ m^?2 during the melt period. Further analyses reveal that net turbulent energy fluxes were near zero because of the temperature and atmospheric vapour pressure encountered during the melt period. Future research should place snow science in the context of forest succession and management and address important uncertainties regarding the timing and magnitude of needlefall events. Copyright © 2015 John Wiley & Sons, Ltd.     

一种用于篡改定位及近似恢复的水印算法

针对遥感影像易被非法篡改的问题,提出了一种用于篡改定位及近似恢复的遥感影像半脆弱水印方案。首先将二值伪随机矩阵作为认证水印,通过量化Contourlet变换各方向子带中绝对值最大的系数将其嵌入,然后采用影像4×4分块后的灰度平均值作为恢复水印,将其嵌入到最低有效位。水印检测不需要原始遥感影像参与,实现了盲提取。实验结果表明,算法对于噪声、JPEG压缩等合理性失真具有一定的鲁棒性,而对于恶意篡改等攻击具有脆弱性,能够实现篡改区域的精确定位和近似恢复,使恢复后的遥感影像满足基本的视觉要求,可有效保护遥感影像的安全。     

WiFi指纹定位的一种新组合算法

探讨AP选取策略和贝叶斯位置估计算法对基于RSS的WiFi室内定位技术位置估计精度的影响。国内外学者分别对AP选取算法和贝叶斯位置估计算法进行了大量的研究。为了进一步深入研究不同算法的优劣性,利用组合优化的思想对不同算法进行组合,通过找出最优算法组合从而提升WiFi室内定位系统的性能。基于互信息最小化的AP选取策略和考虑AP相关性的贝叶斯位置估计算法,提出一种新的WiFi指纹定位组合算法。实验分析表明:新算法具有良好的实用性和定位性能。     

基于青藏高原春季感热异常信号的中国东部夏季降水统计预测模型

使用1980-2014年由青藏高原中东部的地面气象观测台站观测资料计算得到的地表感热通量以及中国东部高分辨率的降水格点资料,在年代际变化和年际变率两个时间尺度上,使用最大协方差分析方法研究了青藏高原春季感热与中国东部夏季6、7和8月降水的关系,基于最大协方差关联因子的时间尺度分解回归分析方法建立了一个降水统计预测模型。青藏高原春季感热的各个关联预报因子与中国东部夏季各月降水的相关分析表明,在年代际成分中,6、7和8月在中国东部绝大部分地区均存在显著相关,方差贡献分别为75.6%、99.9%和79.7%;在年际成分中,相关区域在6月是华南地区、华北沿海地区和江淮流域,7月是华南地区西南部、长江流域、东北地区东南部和黄河中下游地区,8月是东北地区和华南地区西部,方差贡献分别为42.7%、43.4%和32.0%。预测模型的解释方差分析和后报试验检验表明,7月对整个中国东部地区预测效果最好,6月主要在长江以南地区,而8月主要在东北地区和华南地区西部预测效果较好。该预测模型能很好描述青藏高原春季感热与中国东部夏季各月降水的关联性,并对局地降水实现较好的定量预测,具有在短期气候预测业务应用的价值。      

Evapotranspiration of a Populuseuphratica Oliv. forest and its controlling factors in the lowerHeihe RiverBasin, Northwest China

Evapotranspiration (ET) within an ecosystem is crucial for the water-limited environment that currently lacks adequate quantification in the arid region of Northwest China, mainly covered by phreatophytes, such as the Populus euphratica Oliv. tree and the Tamarix ramosissima Ledeb. shrub species. Accordingly, ET was measured for an entire year using eddy covariance (EC) in P. euphratica stands in the lower Heihe River Basin, Northwest China. During the growing season, the total ET was 850 mm, with a mean of 4.0 mm/d, which is obviously more than that observed at tree-level and standlevel scales, which was likely due to the different level of soil evaporation induced by irrigation via water conveyance. Factors associated with ET fall into either environmental or plant eco-physiological categories. Environmental factors account for at least 79% variation of ET, and the linear relationship between ET and the groundwater table (GWT) revealed the potential water use of P. euphratica forests under the non-water stress condition with the GWT less than 3 m deep. Plant eco-physiological parameters, specifically the leaf area index (LAI), have direct impact on the seasonal pattern of ET, which provides a valuable reference to the wide-area estimates of ET for riparian forests by using LAI. In conclusion, P.euphratica forests have high water use after water conveyance, which may be the result of long-term adapting to local climates and limited water availability.     

基于指纹识别的室内定位中的隐私保护

基于指纹识别的定位是最流行的室内定位方法.在离线阶段,服务器测量指纹,比如来自特定空间已知位置的不同接入点（AP）的接收信号强度（RSS）,测量后服务器将测量结果保存在数据库中;在线上阶段,用户同时向服务器发送他当前指纹的测量结果以及位置查询请求,服务器将在数据库中查找与测量结果最接近的指纹.虽然这种方法已经被研究了很久,但现有的工作并没有考虑2个隐私要求：供应商希望保护他们花大代价收集的指纹,用户想要对服务器保留他们的指纹测量结果,以避免泄漏位置.为了实现隐私保护,本文提出一种使用加密技术的指纹匹配方案,这个方案在加密情况下计算由用户测量的指纹与服务器存储的指纹的距离,服务器存储的指纹在这一过程中仍处于密文空间.本文证明了这个方案在进行单点定位时能够很好地保证两者的隐私要求.为了减少高昂的时间开销,本文还提出了一个基于网格划分的改进方案,以及以有限的隐私损失为代价的扩展方案.为加强安全性,最后提出了有效对抗特定攻击的对策,在这种攻击中恶意用户可以通过重复定位获得服务器存储的指纹.使用公众 RSS指纹数据集的扩展实验结果显示本文方案足以在实现实时定位的同时保留定位精度.