排序方式： 共有26条查询结果，搜索用时 31 毫秒
11.
利用2014—2015年夏季雷达资料获取雷暴发生有无数据,计算探空资料的对流指数与雷暴发生有无数据的灰色关联度,发现雷暴产生跟抬升凝结高度气压、850～700 hPa的温度和露点温度、风切变的关系最紧密。接着建立广州白云机场终端区内3类区域(离塔台中心8 km、50 km、100 km)的12小时随机森林分类模型,对不同区域的雷暴潜势进行预报和评估,发现终端区区域面积越大,雷暴发生样本比例越高,临界成功指数CSI、预报准确率AF、探测概率POD越来越高,虚假报警率FAR越来越低,表明预报出来的准确率越来越高。离塔台中心50 km和100 km区域的预报准确率AF和探测概率POD超过70%,对航空重要天气MDRS通报业务有指示作用。同时袋外错误率均低于1/3,随机森林算法的泛化性能好。 相似文献
12.
对油气勘探和大地测量中的复杂起伏地表采用POD方法进行数值模拟.从大量的观测数据中提取出标准正交的基函数,用这些基函数对起伏地表进行拟合.模型试验表明:这种方法是高效可行的,可广泛应用于数据压缩、图像处理、信号分析和流体的优化控制中. 相似文献
13.
We present a methodology conducive to the application of a Galerkin model order reduction technique, Proper Orthogonal Decomposition (POD), to solve a groundwater flow problem driven by spatially distributed stochastic forcing terms. Typical applications of POD to reducing time-dependent deterministic partial differential equations (PDEs) involve solving the governing PDE at some observation times (termed snapshots), which are then used in the order reduction of the problem. Here, the application of POD to solve the stochastic flow problem relies on selecting the snapshots in the probability space of the random quantity of interest. This allows casting a standard Monte Carlo (MC) solution of the groundwater flow field into a Reduced Order Monte Carlo (ROMC) framework. We explore the robustness of the ROMC methodology by way of a set of numerical examples involving two-dimensional steady-state groundwater flow taking place within an aquifer of uniform hydraulic properties and subject to a randomly distributed recharge. We analyze the impact of (i) the number of snapshots selected from the hydraulic heads probability space, (ii) the associated number of principal components, and (iii) the key geostatistical parameters describing the heterogeneity of the distributed recharge on the performance of the method. We find that our ROMC scheme can improve significantly the computational efficiency of a standard MC framework while keeping the same degree of accuracy in providing the leading statistical moments (i.e. mean and covariance) as well as the sample probability density of the state variable of interest. 相似文献
14.
������Ԫ���ֵĵ������˶�ѧ���ܶ��� 总被引：4，自引：3，他引：1
????????λPPP?????????????????ù???????????????????????????? ???????????????????????????????????????????????????????????????????????????????????????о????н??????? ????????????CHAMP??RACE????????????????????ο?????????????CHAMP???????????ο??????XYZ3?????????????????????0.22 m??0.22 m??0.25 m???????????????0.41 m??GRACE??A??????????????ο??????XYZ 3?????????????????????0.27 m??0.23 m??0.29 m???????????????0.45 m?? 相似文献
15.
采用聚丙烯酰胺凝胶电泳方法对广西英罗港内滩和中滩红树林中的白骨壤Avicennia marina、桐花树Ae-giceras corniculatum和秋茄Kandelia candel叶片的过氧化物酶(POD)同工酶、超氧化物歧化酶(SOD)同工酶进行了电泳分离及酶活力测定,同时测定了红树植物采样点土壤的Mn、Cr、Cd、Pb、Cu和Zn 6种重金属元素的含量。结果表明:(1)内滩的3种红树植物叶片的POD同工酶的带数或酶活力均大于中滩;桐花树和秋茄SOD同工酶的变化趋势与POD相似,但酶活力变化幅度小于POD。(2)土壤中各种重金属含量的大小顺序是Mn>Cr>Zn>Pb>Cu>Cd,中滩高于内滩。(3)桐花树POD同工酶活力变化可对土壤重金属污染起指示作用。 相似文献
16.
附加Helmert变换参数的低轨卫星约化动力学精密定轨 总被引：1，自引：0，他引：1
在运动学精密定轨以及动力学轨道积分的基础上,提出基于Helmert变换的约化动力学精密定轨模型.该模型对动力积分轨道以及运动学轨道建立Helmert变换,进而修正轨道积分中的卫星初始轨道以及各种动力学参数.应用该模型,文章采用的约化动力学精密定轨包含两个部分:运动学精密定轨以及基于Helmert变换的动力学轨道平滑.对CHAMP、GRACE两个星期的观测数据进行计算,结果显示:在引入Helmert变换平移参数的参数设置下,相对于运动学轨道,约化动力学轨道的精度平均提高了约30%;对于CHAMP卫星,约化动力学轨道与参考轨道差值在XYZ 3个方向RMS的平均值分别为(0.14,0.14,0.16) m,差值3D RMS的平均值为0.26 m;对于GRACE-A卫星,约化动力学轨道与参考轨道差值在XYZ 3个方向RMS的平均值分别为(0.17,0.15,0.13) m,差值3D RMS的平均值为0.26 m.文中还详细讨论和分析了模型中不同参数设置下轨道精度的情况. 相似文献
17.
提出一种改进的Kalman滤波算法,设计并实现了基于Kalman滤波的运动学定轨方法。通过对GRACE卫星实测数据的计算表明,该方法不仅对计算机要求低,计算效率高,而且能达到与最小二乘法相当的定轨精度,可以作为今后高采样率星载GPS数据的定轨方法。 相似文献
18.
The Earth’s non-spherical mass distribution and atmospheric drag cause the strongest perturbations on very low-Earth orbiting satellites (LEOs). Models of gravitational and non-gravitational accelerations are utilized in dynamic precise orbit determination (POD) with GPS data, but it is also possible to derive LEO positions based on GPS precise point positioning without dynamical information. We use the reduced-dynamic technique for LEO POD, which combines the geometric strength of the GPS observations with the force models, and investigate the performance of different pseudo-stochastic orbit parametrizations, such as instantaneous velocity changes (pulses), piecewise constant accelerations, and continuous piecewise linear accelerations. The estimation of such empirical orbit parameters in a standard least-squares adjustment process of GPS observations, together with other relevant parameters, strives for the highest precision in the computation of LEO trajectories. We used the procedures for the CHAMP satellite and found that the orbits may be validated by means of independent SLR measurements at the level of 3.2 cm RMS. Validations with independent accelerometer data revealed correlations at the level of 95% in the along-track direction. As expected, the empirical parameters compensate to a certain extent for deficiencies in the dynamic models. We analyzed the capability of pseudo-stochastic parameters for deriving information about the mismodeled part of the force field and found evidence that the resulting orbits may be used to recover force field parameters, if the number of pseudo-stochastic parameters is large enough. Results based on simulations showed a significantly better performance of acceleration-based orbits for gravity field recovery than for pulse-based orbits, with a quality comparable to a direct estimation if unconstrained accelerations are set up every 30 s. 相似文献
19.
The proper orthogonal decomposition (POD) method is used to construct a set of basis functions for spanning the ensemble of
data in a certain least squares optimal sense. Compared with the singular value decomposition (SVD), the POD basis functions
can capture more energy in the forecast ensemble space and can represent its spatial structure and temporal evolution more
effectively. After the analysis variables are expressed by a truncated expansion of the POD basis vectors in the ensemble
space, the control variables appear explicitly in the cost function, so that the adjoint model, which is used to derive the
gradient of the cost function with respect to the control variables, is no longer needed. The application of this new technique
significantly simplifies the data assimilation process. Several assimilation experiments show that this POD-based explicit
four-dimensional variational data assimilation method performs much better than the usual ensemble Kalman filter method on
both enhancing the assimilation precision and reducing the computation cost. It is also better than the SVD-based explicit
four-dimensional assimilation method, especially when the forecast model is not perfect and the forecast error comes from
both the noise of the initial filed and the uncertainty of the forecast model.
Supported by the National Natural Science Foundation of China (Grant No. 40705035), National High Technology Research and
Development Program of China (Grant No. 2007AA12Z144), Knowledge Innovation Project of Chinese Academy of Sciences (Grant
Nos. KZCX2-YW-217 and KZCX2-YW-126-2), and National Basic Research Program of China (Grant No. 2005CB321704) 相似文献
20.
低轨导航增强GNSS发展综述 总被引：1，自引：0，他引：1
低轨星座具有地面接收信号强度高、几何图形变化快的优势,能够与中高轨GNSS星座形成互补,对增强GNSS的精度、完好性、连续性和可用性具有显著优势,已成为当前卫星导航领域的关注热点。本文首先简要介绍了现有的GNSS增强系统;总结了国内外低轨导航增强星座发展现状;针对低轨导航增强,对比分析了高中低轨导航星座的优缺点;重点讨论了低轨导航增强在联合定轨、快速精密定位、空间天气监测和室内定位等方面带来的机遇;分析指出了低轨导航增强的空间段、地面段和用户段所面临的挑战。 相似文献