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31.
A key non-linear mechanism in a strong-field geodynamo is that a finite amplitude magnetic field drives a flow through the Lorentz force in the momentum equation and this flow feeds back on the field-generation process in the magnetic induction equation, equilibrating the field. We make use of a simpler non-linear?α?2-dynamo to investigate this mechanism in a rapidly rotating fluid spherical shell. Neglecting inertia, we use a pseudo-spectral time-stepping procedure to solve the induction equation and the momentum equation with no-slip velocity boundary conditions for a finitely conducting inner core and an insulating mantle. We present calculations for Ekman numbers (E) in the range 2.5× 10?3 to 5.0× 10?5, for?α?=α 0cos?θ?sin?π?(r?ri ) (which vanishes on both inner and outer boundaries). Solutions are steady except at lower E and higher values of?α?0. Then they are periodic with a reversing field and a characteristic rapid increase then equally rapid decrease in magnetic energy. We have investigated the mechanism for this and shown the influence of Taylor's constraint. We comment on the application of our findings to numerical hydrodynamic dynamos. 相似文献
32.
THANASIS HADZILACOS NECTARIA TRYFONA† 《International journal of geographical information science》2013,27(2):179-203
Abstract. A formal, yet practical, GeoRelational Data Model (GRDM) is presented for the logical database design phase of the development of spatial information systems. Geographic applications are viewed in the context of information systems development. The generic needs of modelling spatial data are analyzed; it is concluded that they are not served satisfactorily by existing data models, so specifications of modelling tools for spatial application design are given. GRDM provides a set of representational constructs (relations and layers for the logical schema; virtual layers, object classes and spatial constraints for the user views) on top of well-established models. It constitutes part of a full, easily automated application design methodology. Extensive examples demonstrate the relevance, and ease-of-use of the platform-independent GRDM. 相似文献
33.
基于3S技术的锡林郭勒盟风沙灾害综合评价 总被引:1,自引:0,他引:1
本文从孕灾环境、致灾因子和承载体3个方面建立风沙灾害综合评价体系,利用内蒙古自然地理环境要素基础图件、锡林郭勒盟以及周边地区29个气象观测站点的多年气象资料和内蒙古统计年鉴等数据,应用协调模型对锡林郭勒盟风沙灾害进行综合评价研究。结果表明:锡林郭勒盟风沙灾害系统的协调性总体上由东北向西南呈越来越差之势,说明风沙灾害综合风险程度由东北向西南越来越大;锡林郭勒盟风沙灾害系统中属协调型和较协调型地区面积占全研究区的55.5%;属不协调型和极不协调型地区占全研究区的44.5%。可见,锡林郭勒盟风沙灾害子系统关系中,不协调型近占一半,政府和有关部门应提高防范意识,采取防治措施。 相似文献
34.
以季风边缘区的霍林河流域为研究对象, 利用研究区周缘9个气象站台1951-2010年的逐月气象数据, 通过对气温和降水量进行趋势分析、Mann-Kendall检验以及相关分析, 探讨流域气候变化过程、特征及周期. 结果表明: 在1951-2010年年均气温上升2.3 ℃, 其倾向率为0.38 ℃·(10a)-1, 总体呈上升的趋势. 其中, 春季气温升幅最为明显, 倾向率为0.50 ℃·(10a)-1. 同时, 年均气温以1986年为跃点, 发生突变, 突变后的1987-2010年平均气温比突变前1951-1986年气温高1.3 ℃, 并存在6~8 a和15 a的周期律. 年降水量近60 a来减少了83.9 mm, 其倾向率为-13.98 mm·(10a)-1, 呈下降的趋势. 其中, 夏季降水量的下降最为明显, 倾向率为-11.41 mm·(10a)-1. 年降水量以1998年为跃点发生突变, 突变后的1999-2010年降水量比突变前1951-1998年下降76 mm. 并存在4 a和8~9 a的振荡周期. 流域气温变化与北极涛动呈正相关, 而降水量与夏季风指数呈负相关. 相似文献
35.
This paper focuses on the formative processes of limestone pseudoconglomerates in the Gushan and Chaomidian Formations (Late Cambrian) of the North China Platform, Shandong Province, China. The Gushan and Chaomidian Formations consist mainly of limestone and shale (marlstone) interlayers, wackestone to packstone, grainstone and microbialite as well as numerous limestone conglomerates. Seventy‐three beds of limestone pseudoconglomerate in the Gushan and Chaomidian Formations were analysed based on clast and matrix compositions, internal fabric, sedimentary structures and bed geometry. These pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes (i.e. flat to undulatory disc, blade and sheet), marlstone and/or grainstone matrix and various internal fabrics (i.e. intact, thrusted, edgewise and disorganized), as well as transitional boundaries. Limestone pseudoconglomerates formed as a result of soft‐sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth. Differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates. Initial deformation (i.e. burial fragmentation, liquefaction and injection) and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates. Burial fragmentation resulted from mechanical rupture of cohesive carbonate mud, whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials (liquefied carbonate sand and water‐saturated argillaceous mud) under increased stress. Storm‐wave loading was the most probable deformation mechanism, as an external triggering force. Subsequent re‐orientation and rounding of clasts were probably prolonged under normal compactional stress. Eventually, disrupted clasts, along with matrix materials, were transformed into pseudoconglomerates by progressive lithification. Soft‐sediment deformation is prevalent in alternate layers of limestone and mud(marl)stone and/or grainstone, regardless of their depositional environments. 相似文献
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37.
Effects of nonlinearity on theairflow past an urban heat island and precipitationchange downwind, are investigated analytically in thecontext of the weakly nonlinear response of a stablystratified uniform flow to specified heating. Theheating structure is assumed to be bell-shaped in thehorizontal and exponentially decreasing with height.The forcing to the first-order equation exhibitscooling in the concentrated low-level heating region.The linear solution component shows upward motiondownstream as suggested by many previous studies. Theweakly nonlinear solution component shows downward orupward motion downstream depending on the heatingdepth. It is proposed that when the heating depth islarge, but still within a valid range of theperturbation expansion, the linear and weaklynonlinear effects constructively work together toproduce enhanced upward motion on the downstream side,not far from the heating centre. This explains toa greater extent the precipitation enhancement downstream ofthe heat island than is possible from the linear effect alone. Itis also proposed that when the heating depth is small,the linear and weakly nonlinear effects destructivelywork together to reduce upward motion on thedownstream side, not far from the heating centre. Thisexplains to a greater extent the lack of precipitation enhancementdownstream than is possible from the linear effect alone. 相似文献
38.
Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea 总被引:9,自引:0,他引:9
Abstract The Chuncheon amphibolite, part of the Gubongsan Group which overlies the Yongduri gneiss complex, is interlayered with calc-silicate rock, marble, quartzite, biotite schist and quartzofeldspathic gneiss in the central Gyeonggi massif, South Korea. Metamorphic pressures and temperatures estimated from the amphibolite are 5.5–10.6 kbar and 615–714°C. These P—T conditions are close to those defined by the reaction curve between kyanite and sillimanite, and suggest medium-pressure-type metamorphism of the Chuncheon amphibolite. For two metapelites intercalated with the amphibolite, temperatures are estimated to be 607–699° C, consistent with those obtained from the amphibolite. On the other hand, pressures estimated from these metapelites are significantly different, 4–6 kbar and 9–13 kbar, when rim and core compositions of garnet are, respectively, used. These P—T estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3–7 kbar during denudation. Rapid decompression is likely on the basis of the results of mineral chemistry, phase equilibria and geothermobarometer. Moreover, in conjunction with the occurrence of kyanite in the adjacent Gyeonggi gneiss complex, P—T estimates of the Chuncheon amphibolite and metapelite suggest a clockwise P—T—t path. This evolutionary path may be related to the amalgamation of continents during the late Proterozoic event which corresponds to the Jinningian orogeny in the Qinling belt of China. 相似文献
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