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941.
The Effect of Chessboard Variability of the Surface Fluxes on the Aggregated Turbulence Fields in a Convective Atmospheric Surface Layer 总被引:1,自引:1,他引:0
To what degree the variability of surface features can be identified in the turbulent signals observed in the atmospheric boundary layer is still an unresolved problem. This was investigated by conducting an analytical experiment for a one-dimensional 'chessboard'-type surface-flux distribution on the basis of local free convection scaling. The results showed that, due to their nonlinear dependency on the surface fluxes, the dimensionless gradients of the mean quantities and the dimensionless standard deviations are altered by the surface-flux variability. Furthermore, passive scalars, such as humidity, are considerably more sensitive to surface variability than the main active scalar, temperature. However, the response of the gradients of the mean quantities is fairly negligible in the range of variability studied herein as compared to that of the standard deviations, which were found to be more sensitive to the surface-flux variability. In addition, the phase difference between the active and the passive scalar flux distribution strongly affects the passive scalar turbulence. This dissimilarity between passive and active scalars, or between passive scalars when their source distributions are different, brings into question the use of variance methods for the measurement of a scalar flux, such as evaporation, over variable surfaces. The classical Bowen ratio method, which depends on the validity of the Reynolds analogy for the vertical gradients of the mean quantities, was shown to be relatively more robust. However, under conditions of strong surface variability, it can also be expected to fail. 相似文献
942.
An analytical one-dimensional second-order closure model is developed to describe the within canopy velocity variances, turbulent intensities, dissipation rates, Lagrangian time scale and Lagrangian far field diffusivities for vegetation canopies of arbitrary structure and density. The model incorporates and extends the model of momentum transfer developed by Massman (1997) and the model of within canopy velocity variances developed by Weil (unpublished) from the second-order closure model of Wilson and Shaw (1977). Model predictions of within and above canopy velocity variances, turbulent intensities, dissipation rates and the Lagrangian time scale are in reasonable agreement with previously measured or estimated values for these parameters. The present model suggests that the Lagrangian time scale and the far field diffusivity could be strongly dependent upon foliage structure and density through the foliage effects on the velocity variances. A simple formulation for the Lagrangian time scale at canopy height is derived from model results. Taken as a whole, the present model may provide a relatively simple way to incorporate turbulence parameters into models of soil/canopy/atmosphere mass transfer. 相似文献
943.
1.IntroductionItiswellknownthattheecosystemcangreatlyinfluencebothlocalclimateandgeneralcirculation.Onthenumericalstudyoftheturbulenceinandaboveforestcanopies,alotofsignificantstudieshavebeendone.Inallthesestudies,modelsaregenerallydividedintotwotypes:oneis'K--theory'type(Waggoner,1975;Gross,1987;Gross,1988,Jietal.,1989;Schilling,I991;Dickinsonetal.,1993;Wang,1996),theotherappliesthehigher--orderclosuremethod(Wilsonetal.,1977,Yamada,1982;Yinetal.,1989)ortheLagrangianmethod(Rampach,1987;R… 相似文献
945.
946.
中国南方新生代地幔柱活动的地球化学证据 总被引:7,自引:0,他引:7
中国南海海盆、海南岛、雷琼和龙海—明溪地区新生代玄武岩的总体相当于洋岛玄武岩(OIB),富集高场强元素(HFSE),尤其是富集Nb、Ta、U、Ti。南海海盆新生代玄武岩是玻利尼西亚型(Polynesian)、夏威夷型(Hawaii)和MORB型共存,可能是在扩张洋脊上面形成的OIB,下地幔成因的超地幔柱上升致使上地幔物质加入,海南岛和雷琼地区玄武岩具有与南海海盆玄武岩大致相似的成因。大陆板内同时代的龙海—明溪地区玄武岩具特别高的Nb/Zr和Nb/Y值,均表现为地幔柱成因的玻利尼西亚型玄武岩,其形成既可能有古老俯冲洋壳熔融后榴辉岩质残余下沉至下地幔成因,也可能有包括下地壳在内的岩石圈物质的贡献。 相似文献
947.
金平地区的镁铁-超镁铁质岩岩床群侵入在晚泥盆世地层中。岩石类型包括粒玄岩、辉绿岩、辉长岩、橄榄辉玄岩和苦橄岩。岩床群的走向与区域构造方向一致,为北西向延伸。单个岩床的厚度在1-200m之间,较厚的岩床有明显的结晶分异作用。粒玄岩和辉绿岩含相当数量的钛铁矿,称为铁钛玄武岩。化学成分以强烈富Ti为特征,TiO2含量平均达3.9%.Mg含量通常较低,是拉斑质或碱性玄武岩演化而来的。这种高Ti玄武岩还富Th、Ta、Nb,Th/Ta比值接近1,Zr/Nb比值较低,接近OIB的特征,推测产于板内环境,是地幔柱部分熔融形成的岩浆。金平岩床群中苦橄岩和橄榄辉玄岩(MgO含量在15.1%-23.3%之间)出露的比例相对较多,指示地幔柱部分熔融的温度较高,推测相当于地幔柱尾部的特征。 相似文献
948.
L. Hecht K. Thuro R. Plinninger M. Cuney 《International Journal of Earth Sciences》1999,88(2):236-252
The late-Hercynian granites of Königshain underwent multistage hydrothermal processes. Extensive high-temperature late-magmatic alteration is, for example, indicated by low Zr/Hf and an REE pattern displaying the tetrad effect. Intensive post-magmatic alteration of the granite occurred along brittle structures. At least two main stages of post-magmatic hydrothermal alteration are involved. The first high-temperature stage, which is characterized by albitization and/or quartz leaching (episyenitization), resulted from fluid–rock interaction with late-magmatic fluids that very probably mixed with external low-salinity fluids. Quartz dissolution was triggered by vapour condensation and/or the cooling of these fluids (below 450??°C) along brittle structures. The high porosity resulting from quartz leaching during stage 1 assisted subsequent circulation of low-temperature fluids at stage 2; the latter is characterized by the chloritization and illitization of episyenites. Almost all major and trace elements were enriched or depleted during one of the main alteration stages. However, Zr, Hf, Th, and Ti were immobile during post-magmatic alteration. The significant depletion of LREE and the enrichment of HREE in albitized samples is controlled by the dissolution of monazite and the new formation of HREE-rich polycrase-(Y) or aeschynite-(Y) during post-magmatic stage 1. Negative Ce anomalies of episyenites are associated with illitization and suggest oxidizing conditions during stage 2. 相似文献
949.
950.