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981.
982.
在分析目前地理信息服务所面临的安全威胁问题的基础上,设计了基于SSL的令牌(Token)式地理信息服务安全访问方案,以该方案为基础开发了用户安全身份认证和强日志管理两个核心模块。研究结果表明,该方案能有效地解决地理信息服务安全访问、地理信息资源非法下载等方面的问题。 相似文献
983.
We present some algebraic and numerical simulations of the stable boundary layer. We also discuss the problem of the existence
of a critical Richardson number (Ri), beyond which the turbulence is suppressed. We compare the results of a second-order algebraic model with those of a third-order
numerical model and, to this purpose, numerical simulations of a wind-tunnel flow, which is characterized by various Richardson
numbers, were performed. As far as the second-order model is concerned, solutions, for the Richardson number greater than
any critical value, can be obtained by modifying the time scales of the second-order equation pressure correlation terms in
order to account for a buoyancy damping factor. We show that using a third-order model allows the same results (no critical
Richardson number) to be obtained without modifications to the time scales. It is suggested that the non-locality, accounted
for by the third-order moments, could allow the turbulence to persist also for Ri > 1. 相似文献
984.
985.
利用陕西苹果生产基地县及其以北地区气象观测和物候观测资料,采用时间序列分析和专家打分方法,分析了近50年来影响陕西苹果产量和质量的主要气象因子、花期霜冻和高温日数的变化特征以及气候变化对陕西苹果生长的气候适宜性评分的影响。结果表明:年平均气温和夏季平均最低气温具有明显的上升趋势,夏季相对湿度变化不大,近30年 (1979—2008年) 年降水量明显少于1961—1990年平均值;1961—1990年、1971—2000年、1979—2008年3个30年苹果基地县大部分果区气候适宜性总评分没有明显变化;温度升高、降水减少是研究区域一些县 (区) 苹果生长气候适宜性评分变化的主要原因。1961年以来, 4月中旬霜冻频率的增加使渭北西部果区和延安果区遭遇花期冻害的可能性增加;20世纪90年代以来, 高温日数的增加使果树受高温热害的影响增大。 相似文献
986.
987.
988.
一次区域性大暴雨过程中尺度诊断分析 总被引:5,自引:0,他引:5
利用实况降水、FY-2C资料、实时探空、NECP/NCAR再分析资料对2009年8月2—5日川渝地区大暴雨过程主要环流系统、水汽输送特征,以及湿位涡和垂直螺旋度演变特征进行了诊断分析。结果表明:此次强降水环流背景是高空槽东移,耦合了"天鹅"台风动力阻塞维持在川渝地区的西南低涡,南侵的冷空气和西南急流输送的暖湿气流交汇,形成明显的气旋性辐合,导致不断有中小尺度对流系统的生成、发展,且降水过程中一直维持较好的水汽输送条件;湿位涡对本次降水落区有较好的指示意义,由于强降水湿位涡负值中心上空的大尺度下沉气流、强上升气流的倾斜程度和最大锋生强迫区的位置和强度,强降水落区(重庆西部)位于负值中心(四川盆地)暖气流一侧;垂直螺旋度发展演变与暴雨有着密切的关系,当螺旋度等值线密集(稀疏)时,强降水发生(减弱或结束),且暴雨发生时,总是伴随着高(低)空正(负)垂直螺旋度的耦合发生。 相似文献
989.
An open problem that arises when using modern iterative linear solvers, such as the preconditioned conjugate gradient method or Generalized Minimum RESidual (GMRES) method, is how to choose the residual tolerance in the linear solver to be consistent with the tolerance on the solution error. This problem is especially acute for integrated groundwater models, which are implicitly coupled to another model, such as surface water models, and resolve both multiple scales of flow and temporal interaction terms, giving rise to linear systems with variable scaling. This article uses the theory of "forward error bound estimation" to explain the correspondence between the residual error in the preconditioned linear system and the solution error. Using examples of linear systems from models developed by the US Geological Survey and the California State Department of Water Resources, we observe that this error bound guides the choice of a practical measure for controlling the error in linear systems. We implemented a preconditioned GMRES algorithm and benchmarked it against the Successive Over-Relaxation (SOR) method, the most widely known iterative solver for nonsymmetric coefficient matrices. With forward error control, GMRES can easily replace the SOR method in legacy groundwater modeling packages, resulting in the overall simulation speedups as large as 7.74×. This research is expected to broadly impact groundwater modelers through the demonstration of a practical and general approach for setting the residual tolerance in line with the solution error tolerance and presentation of GMRES performance benchmarking results. 相似文献
990.
Houyuan Lu Naiqin Wu Kam-biu Liu Liping Zhu Xiangdong Yang Tandong Yao Luo Wang Quan Li Xingqi Liu Caiming Shen Xiaoqiang Li Guobang Tong Hui Jiang 《Quaternary Science Reviews》2011,30(7-8):947-966
This study investigates the distribution of modern pollen assemblages in the Qinghai-Tibetan Plateau, China, based on surface soil samples collected at 1202 sites along an altitudinal gradient of 10–5500 m asl, where mean annual precipitation (MAP) ranges from 12 to 1840 mm and mean annual temperature (MAT) from ?7 to 21.5 °C. A total of 153 pollen taxa were found with relative abundances greater than 1% in at least two samples. Canonical correspondence analysis (CCA) was used to determine the main environmental variables influencing pollen distributions. The results reveal that MAP is the most significant dominant variable. However, MAT, altitude (ALT), July temperature (MT7), and relative humidity (HHH) are also significant variables that clearly follow the gradients in the CCA ordination, suggesting that pollen assemblages probably not only reflect a single climatic parameter, but also a variety of other climatic inter-related parameters.Transfer functions, based on locally weighted weighted averaging (LWWA), were developed for MAP (R2-boot = 0.89, RMSEP = 109 mm), MAT (R2-boot = 0.78, RMSEP = 2.3 °C), ALT (R2-boot = 0.73, RMSEP = 597 m), HHH (R2-boot = 0.82, RMSEP = 4.5%), and July mean precipitation (MP7) (R2-boot = 0.87, RMSEP = 23 mm). Overall, our results confirm that pollen can provide reliable estimates of the primary climatic parameters. The application of the LWWA model to the fossil records of Chen Co Lake allowed quantitative inferences to be made about Holocene climatic changes in the southern Tibetan Plateau, suggesting that LWWA is a robust calibration method for quantitative palaeo-environmental reconstruction based on pollen data in the regions. 相似文献