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排序方式: 共有191条查询结果,搜索用时 15 毫秒
81.
利用离散偶极子近似法,数值计算分析了随机取向旋转椭球体沙尘气溶胶粒子在尺度参数为0.1~23时(波长0.55 μm对应有效半径为0.01~2 μm)的激光雷达线退偏比特性,通过比较其不同旋转椭球体轴半径比下的差别,研究了粒子非球形性程度对单分散和多分散沙尘气溶胶激光雷达线退偏比特性的影响.单分散旋转椭球体沙尘气溶胶的激光雷达线退偏比对粒子的轴半径比和尺度参数有很强的依赖性;对于长旋转椭球体,沙尘激光雷达线退偏比较小值出现在瑞利散射区,而较大的值出现在米散射区;对于扁旋转椭球体,在米散射区较大沙尘粒子也可以产生较小的激光雷达线退偏比,例如,轴半径比为1/16,尺度参数为3时的沙尘激光雷达线退偏比仅为0.1%.就随机取向旋转椭球体沙尘粒子而言,对于单分散系,仅当尺度参数小于0.5时,非球形特征越明显,其激光雷达线退偏比越大;而对多分散系,非球形特征越明显,其激光雷达线退偏比越大. 相似文献
82.
《Polar Science》2014,8(4):315-326
A method is proposed for detecting clouds from whole-sky color images obtained with an all-sky camera (ASC) system. In polar regions, cloud detection using whole-sky images usually suffers from large uncertainties in fractional cloud cover retrievals because of large solar zenith angles (SZAs) and high surface albedo, which cause “whitening” in the images. These problems are addressed by using differences between real images and virtual clear-sky images for a particular observation time with the same SZA. The method is applied to ASC images obtained at Ny-Ålesund, Svalbard in May of 2005–2007, and the results are compared with Micro-Pulse Lidar (MPL) measurements. When no clouds were detected by MPL, the false cloud detection rate from ASC classification was 2.1% in total hours. Conversely, when clouds were detected by MPL, the ASC classification underestimated the clouds by 11.6%. In most cases, this occurred when MPL detected very optically thin clouds. Furthermore, the variability of cloud fractions estimated by MPL and ASC was roughly constant regardless of the SZA. Thus, it is confirmed that the method developed in this study is valid for cloud detection from whole-sky color images. 相似文献
83.
The mobile incoherent Doppler lidar(MIDL),which was jointly developed by State Key Laboratory of Severe Weather(LaSW) of the Chinese Academy of Meteorological Sciences(CAMS) and Ocean University of China,provided meteorological services during the Olympic sailing events in Qingdao in 2008.In this study,two experiments were performed based on these measurements.First,the capabilities of MIDL detection of sea-surface winds were investigated by comparing its radial velocities with those from a sea buoy.MIDL radial velocity was almost consistent with sea-buoy data;both reflected the changes in wind with time.However,the MIDL data was 0.5 m s 1 lower on average than the sea-buoy data due to differences in detection principle,sample volume,sample interval,spatial and temporal resolution.Second,the wind fields during the Olympic sailing events were calculated using a four-dimensional variation data assimilation(4DVAR) algorithm and were evaluated by comparing them with data from a sea buoy.The results show that the calculations made with the 4DVAR wind retrieval method are able to simulate the fine retrieval of sea-surface wind data-the retrieved wind fields were consistent with those of sea-buoy data.Overall,the correlation coefficient of wind direction was 0.93,and the correlation coefficient of wind speed was 0.70.The distribution of retrieval wind fields was consistent with that of MIDL radial velocity;the root-mean-square error between them had an average of only 1.52 m s 1. 相似文献
84.
85.
地面Lidar在文物测量建模中的应用 总被引:1,自引:0,他引:1
应用地面激光扫描仪对"万里长城第一台"镇北台进行了全面的扫描测量,完整记录了镇北台的3维空间特征,并且就镇北台内文物局部详细扫描建模,获取了3维模型和多种线画图,为研究我国历史文化以及文物的修复重建探索了一种新的技术方法. 相似文献
86.
Frances C. O'Donnell Jonathon Donager Temuulen Sankey Sharon Masek Lopez Abraham E. Springer 《水文研究》2021,35(11):e14432
Thinning of semi-arid forests to reduce wildfire risk is believed to improve forest health by increasing soil moisture. Increased snowpack, reduced transpiration and reduced rainfall interception are frequently cited mechanisms by which reduced canopy density may increase soil moisture. However, the relative importance of these factors has not been rigorously evaluated in field studies. We measured snow depth, snow water equivalent (SWE) and the spatial and temporal variation in soil moisture at four experimental paired treatment-control thinning sites in high elevation ponderosa pine forest northern Arizona, USA. We compared snow and soil moisture measurements with forest structure metrics derived from aerial imagery and 3-dimensional lidar data to determine the relationship between vegetation structure, snow and soil moisture throughout the annual hydrologic cycle. Soil moisture was consistently and significantly higher in thinned forest plots, even though the treatments were performed 8–11 years before this study. However, we did not find evidence that SWE was higher in thinned forests across a range of snow conditions. Regression tree analysis of soil moisture and vegetation structure data provided some evidence that localized differences in transpiration and interception of precipitation influence the spatial pattern of soil moisture at points in the annual hydrologic cycle when the system is becoming increasingly water limited. However, vegetation structure explained a relatively low amount of the spatial variance (R2 < 0.23) in soil moisture. Continuous measurements of soil moisture in depth profiles showed stronger attenuation of soil moisture peaks in thinned sites, suggesting differences in infiltration dynamics may explain the difference in soil moisture between treatments as opposed to overlying vegetation alone. Our results show limited support for commonly cited relationships between vegetation structure, snow and soil moisture and indicate that future research is needed to understand how reduction in tree density alters soil hydraulic properties. 相似文献
87.
Innovative analysis and use of high‐resolution DTMs for quantitative interrogation of Earth‐surface processes
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This is the era of digital landscapes; the widespread availability of powerful sensing technologies has revolutionized the way it is possible to interrogate landscapes in order to understand the processes sculpting them. Vastly greater areas have now been acquired at ‘high resolution’: currently tens of metres globally to millimetric precision and accuracy locally. This permits geomorphic features to be visualized and analysed across the scales at which Earth‐surface processes operate. Especially exciting is the capturing of process dynamics in repeated surveying, which will only become more important with low‐cost accessible data generation through techniques such as Structure from Motion (SfM). But the key challenge remains; to interpret high resolution Digital Terrain Models (DTMs), particularly by extracting geomorphic features in robust and objective ways and then linking the observed features to the underlying physical processes. In response to the new data and challenges, recent years have seen improved processing of raw data into DTMs, development of data fusion techniques, novel quantitative analysis of topographic data, and innovative geomorphological mapping. The twelve papers collected in this volume sample this progress in interrogating Earth‐surface processes through the analysis of DTMs. They cover a wide range of disciplines and spatio‐temporal scales, from landslide prone landscapes, to agriculturally modified regions, to mountainous landscapes, and coastal zones. They all, however, showcase the quantitative exploitation of information contained in high‐resolution topographic data that we believe will underpin the improvement of our understanding of many elements of Earth‐surface processes. Most of the papers introduced here were first presented in a conference session at the European Geosciences Union General Assembly in 2011. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
88.
Fabien Gibert Irène Xuéref-Rémy Lilian Joly Martina Schmidt Juan Cuesta Kenneth J. Davis Michel Ramonet Pierre H. Flamant Bertrand Parvitte Virginie Zéninari 《Boundary-Layer Meteorology》2008,128(3):381-401
A network of remote and in-situ sensors was deployed in a Paris suburb in order to evaluate the mesoscale evolution of the
daily cycle of CO2 and related tracers in the atmospheric boundary layer (ABL) and its relation to ABL dynamics and nearby natural and anthropogenic
sources and sinks. A 2-μm heterodyne Doppler differential absorption lidar, which combines measurements of, (1) structure
of the atmosphere, (2) radial velocity, and (3) CO2 differential absorption was a particularly unique element of the observational array. We analyse the differences in the diurnal
cycle of CO, CO2, lidar reflectivity (a proxy for aerosol content) and H2O using the lidar, airborne measurements in the free troposphere and ground-based measurements made at two sites located few
kilometres apart. We demonstrate that vertical mixing dominates the early morning drawdown of CO and aerosol content trapped
in the former nocturnal layer but not the H2O and CO2 mixing ratio variations. Surface fluxes, vertical mixing and advection all contribute to the ABL CO2 mixing ratio decrease during the morning transition, with the relative importance depending on the rate and timing of ABL
rise. We also show evidence that when the ABL is stable, small-scale (0.1-km vertical and 1-km horizontal) gradients of CO2 and CO are large. The results illustrate the complexity of inferring surface fluxes of CO2 from atmospheric budgets in the stable boundary layer. 相似文献
89.
90.
Photogrammetric and laser altimetric reconstruction of water levels for extreme flood event analysis 总被引:1,自引:0,他引:1
Stuart N. Lane Timothy D. James Hamish Pritchard Mark Saunders 《The Photogrammetric Record》2003,18(104):293-307
This paper assesses the feasibility of estimating water levels using digital photogrammetry. A common problem during an extreme flood event is that automated water level recorders do not record the highest water levels, as a result of instrument malfunctioning. This paper explores two possible solutions to this problem based upon data acquired using synoptic remote sensing methods. The first method requires: (a) high-resolution elevation data (for example, in the form of a digital elevation model for the floodplain); and (b) information on the planimetric position of the maximum flood extent, such as from debris lines (known as wrack lines) visible on aerial imagery flown after the event. The planimetric data can then be used to segment the topographic data in order to identify water level elevations. The second method uses a digitial photogrammetric approach and is suitable where no topographic data are available, but aerial imagery is available, flown after the event. Provided this imagery is of the right scale, digital photogrammetric analysis may be used to identify the elevations of wrack lines visible on the imagery. In this paper, the second of these options is compared with the first. The research shows that desktop photogrammetric methods, using 1:4500 scale imagery, can yield water level estimates that are precise to ±0·147 m, on the basis of check data obtained from lidar data. This is a worst possible estimate of the acquired precision given uncertainties in the lidar data. When compared with the first option, based upon segmenting lidar data using flood outlines, the photogrammetric approach was found to be preferable given both the quality of the lidar and uncertainties over how to segment it. 相似文献