On the role of the Asian monsoon in the angular momentum and kinetic energy balances of the tropics     

Abraham H. Oort  Paul H. Chan 《Pure and Applied Geophysics》1977,115(5-6):1167-1186

The balance conditions of relative angular momentum and time-mean kinetic energy and their annual variations are studied for the Northern Hemisphere tropical belt. The belt is divided into two roughly equal size parts, the monsoon and the extramonsoon regions. The data used consist of all available daily rawinsonde reports from the world areological network for the two 5-year periods 1958–63 and 1968–73.In winter, the trade winds in the monsoon and extramonsoon regions are both sources of westerly relative angular momentum for the middle latitude circulation. However, it is found that the angular momentum gained in the extramonsoon region of the Tropics is mostly destroyed by a net southward flow of mass in that region, and becomes regenerated in the monsoon region by a net northward flow of mass there. This excess of angular momentum together with the angular momentum picked up locally in the monsoon region is almost all exported across its northern boundary. It is further found that in winter the Tropics are also an important source of mean kinetic energy for middle latitudes. Again almost all export of kinetic energy was found to take place across the northern boundary of the monsoon sector. Most of this energy must be generated through the pressure gradient term inside the monsoon region itself, the transformation from transient eddy kinetic energy being very small. The proper evaluation of the pressure gradient appears to be the main stumbling block in the present study, preventing us from estimating the generation and thereby, as a residual, the frictional dissipation in the two regions.In summer, the extramonsoon region remains a source of angular momentum, but the monsoon region with its surface westerlies acts as a sink, leading to a sharp reduction (and even a midsummer reversal) of the export into middle latitudes. Also the export of mean kinetic energy almost vanishes in summer, except for a small southward transfer across the equator. The calculations for two 5-year periods give very similar estimates and thereby show the reliability of the results.Parts of this paper were presented at the International Symposium on Monsoons, March 7–12, 1977 in New Delhi, India.  相似文献   

基于无人机和MODIS数据的巴丹吉林沙漠植被分布特征与动态变化研究   总被引：2，自引：2，他引：2  

刘婵  赵文智  刘冰  孟阳阳 《中国沙漠》2019,39(4):92-102

荒漠植被在减缓沙漠扩张、阻挡风沙入侵绿洲、维护绿洲稳定性方面具有重要意义。通过无人机搭载多光谱相机获取地面样地高分辨率NDVI数据以计算植被覆盖度，结合MODIS-NDVI数据，分析了MODIS-NDVI 与无人机获取NDVI数据之间的一致性，进而建立MODIS-NDVI数据与样方植被覆盖度的空间反演模型，以研究巴丹吉林沙漠植被分布特征、动态变化及其对气温、降水的响应规律。结果表明：（1）基于无人机多光谱数据大样方植被覆盖度和MODIS-NDVI数据构建的植被覆盖度反演模型具有较高的模拟精度。（2）研究区总体植被覆盖度较低，空间上呈由东南向西北递减的趋势。（3）植被覆盖度总体呈现上升趋势，沙漠边缘的增速较沙漠内部更为明显，其中沙漠边缘增长率达1.53%/10a，而沙漠内部为0.84%/10a。沙漠边缘57.12%植被出现改善，而沙漠内部植被改善的面积约为21.26%。（4）植被生长主要受制于生长季降水量并呈现出明显的季节变化，此外植被对降水和气温的响应存在一定的滞后效应。  相似文献   

Observational constraints of sterile neutrinos in galaxies     

M. H. Chan  M.-C. Chu 《Astrophysics and Space Science》2009,319(2-4):143-147

We study a model in which degenerate sterile neutrinos account for galactic dark matter. We fit the rotation curves of 5 dwarf galaxies with the degenerate sterile neutrinos in hydrostatic equilibrium. Also we estimate the range of sterile neutrino mass by calculating the upper and lower bounds of the mass densities of sterile neutrino halos in the outermost regions of 21 normal galaxies. The observed rotation curves of 5 dwarf galaxies and 21 normal galaxies are consistent with having sterile neutrinos with mass (26–30) eV, and the similarity of the rotation curves of different galaxies emerges naturally in our model.  相似文献   

The effects of rock fragments on hydrologic and hydraulic responses along a slope     

Kou‐Chan Hung  Ken'ichirou Kosugi  Tim Hau Lee  Takahisa Misuyama 《水文研究》2007,21(10):1354-1362

A model describing the three‐dimensional matrix flow along a slope with rock fragments or impermeable blocks was developed. The model was combined with modified Picard's iteration to ensure mass conservation in the unsaturated flow. We found that rock fragments obstruct water flow along the slope. The groundwater table must be raised to provide a sufficient pore water pressure gradient to facilitate water flow, but higher pore water pressure may induce slope failure. We also conducted a bench‐scale laboratory flume experiment to examine the effects of impermeable blocks on downstream seepage flow. In addition, a numerical experiment was conducted to examine how different arrangements of impermeable blocks affect downstream seepage flow and pore water pressure. This research demonstrated that the hydraulic phenomena were affected when impermeable blocks were present, and pore water pressure increased as the position of impermeable blocks was lowered. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

某农药化工企业密集区地下水污染评价     

魏禅  马振民  朱恒华  侯玉松 《地下水》2014,(6):102-104

通过分析研究某农药化工企业密集区地下水水质及污染状况,按照国家有关标准,采用单指标评价法和内梅罗指数法对地下水的质量进行评价。评价结果显示：该区域地下水水质普遍较差,大部分为Ⅳ类及Ⅴ类水质,已不适于饮用。然后对该区域地下水污染进行评价,结果显示地下水有机和无机污染严重,并形成了以某农药化工企业为污染源并沿岩溶地下水流向的一个卤代烃类有机物污染羽,这表明造成该研究区岩溶地下水污染的主要污染源为某农药化工企业。最后根据评价结果,提出相应的控制地下水污染的建议和措施,为该地区制定地下水开发利用规划提供科学依据。  相似文献   

巴丹吉林沙漠南缘的植物固沙问题     

赵文智  白雪莲  刘婵 《中国沙漠》2022,42(1):5-11

在野外考察的基础上,选择13个典型样地,研究了巴丹吉林沙漠南缘植被组成、群落特征和植被格局。结果表明：调查的样地中有荒漠植物31种,其中灌木12种,多年生草本11种,一二年生草本8种。种群密度3 000—14 000株·hm^-2,优势种在群落中地位显著,群落稳定性较差;植被斑块面积小、最大斑块指数低。沙漠南缘年降水量虽然仅100 mm左右,但有霸王（Zygophyllum xanthoxylon）、红砂（Reaumuria soongarica）和沙蒿（Artemisia desertorum）等灌木和半灌木天然植物群落分布,有白刺（Nitraria tangutorum）和膜果麻黄（Ephedra przewalskii）灌丛沙堆,沙丘上生长着沙拐枣（Calligonum mongolicum）、沙鞭（Psammochloa villosa）等种群生存,且在沙漠延伸带流动沙丘上已成功建立了梭梭（Haloxylon ammodendron）人工固沙植被,表明在巴丹吉林沙漠南缘建立以乡土植物为主的固沙植被是可行的。在未来研究中,应在山水林田湖草沙生命共同体理念下加强从区域尺度上解析水、植被和沙的关系,系统了解生态系统的原真性、完整性和连通性及生态系统稳定性维持等问题,优化生态景观格局,确定生态保护目标和建设规模,同时要加强对乡土植物适应干旱风沙机制和扩繁保育技术研究,以便为区域生态建设和生态保护提供科技支撑。  相似文献   

A GNSS interference identification using an adaptive cascading IIR notch filter     

Chang Ho Kang  Sun Young Kim  Chan Gook Park 《GPS Solutions》2014,18(4):605-613

A low-complexity time-domain approach for global navigation satellite systems is proposed to detect and identify single-tone, multi-tone, swept continuous wave interference (CWI) and band-limited white Gaussian noise (BLWN). An adaptive notch filter and adaptive cascading filter structure are employed to identify the type of interference signals. The number of the cascading stages is selected by comparing the total power in receiver bandpass, and after, passing the adaptive notch filter. For this reason, the proposed filter structure is more efficient in identifying and mitigating interference signals than the conventional filter structure. In addition, the automatic gain control gain is used to detect the existence of continuous wave interference. The performance of the interference detection and identification method is evaluated for the cases of GPS signal in the presence of single-tone, multi-tone, swept CWI and BLWN.  相似文献   

Recombination induced softening and reheating of the cosmic plasma     

P. K. Leung  C. W. Chan  M.-C. Chu 《Monthly notices of the Royal Astronomical Society》2004,349(2):632-640

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Mass of Galaxies in Pairs     

S. Junqueira  R. Chan 《Astrophysics and Space Science》2002,279(3):271-280

We have compared the frequency distribution of the dynamical observedquantity log (V _z ² r _p), for a sample of 46 pairs of elliptical galaxies, to the distribution of this quantity obtained from numerical simulations of pairs of galaxies. From such an analysis, where we have considered the structure of the galaxies and its influence in the orbital evolution of the system, we have obtained the characteristic mass and the mass-luminosity ratio for the sample. Our results show that the hypothesis of point-mass in elliptical orbits is, for this sample, an approximation as good as the model that takes into account the structure of the galaxies. The statistical method used here gives an estimate of a more reliable mass, it minimizes the contamination of spurious pairs and it considers adequately the contribution of the physical pairs. We have obtained a characteristic mass to the 46 elliptical pairs of 1.68 × 10¹² ± 7.01 ×10¹¹ M_⊙ with M/L = 17.6 ± 7.3 (H ₀ = 60 km s^-1Mpc^-1). This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Getting Lost in Cities: Spatial Patterns of Phonetically Confusing Street Names          下载免费PDF全文

Keith Chan  Maria Vasardani  Stephan Winter 《Transactions in GIS》2015,19(4):535-562

A large street network is likely to contain duplicated or similar sounding street names. These conflicts can cause confusion in communication between people or in machine‐human interaction. Municipal authorities have begun to see the importance of uncovering these existing street name conflicts and mitigating future ones, for improved record keeping, emergency response, etc. However the commonly used Soundex phonetic algorithm is generally considered to produce poor similarity results in terms of uncovering street name conflicts. This study reports on a new fusion algorithm that combines phonetic methods and approximate string matching for street names, weighted by street type suffix (Avenue, Boulevard, Court, etc.), to quantitatively measure the collision/confusion potential presented by a pair of streets. This algorithm is then applied to the entire street network in Greater Melbourne, Australia, and the pattern of collisions at various spatial scales, and within municipalities, is mapped. The goal of this work is to produce better tools that can aid policy makers, administrators and industries dealing with location‐based services to make better decisions when assigning and disambiguating street names.  相似文献