Etude de la couche limite de surface sahelienne — Experience Yantala     

Jean-Pierre Pagès  Jean-Pierre Frangi  Pierre Durand  Claude Estournel  Aimé Druilhet 《Boundary-Layer Meteorology》1988,43(1-2):183-203

This paper presents data concerning the energy budget in the surface layer in the Sahel region (a semi-desert area). The results are drawn from a measurement campaign made in the Niamey region in the Niger, in April–May 1984 (the Yantala Campaign). The sensible heat flux is computed with the profile method, the ground heat flux is deduced from measurement of the temperature field, and the radiative net flux is measured directly with a balancemeter. The latent heat flux, which is deduced from the energy budget balance is very weak and within the accuracy limit of the method. The diurnal variation of the net flux is symmetrical, with a maximum at noon. On the other hand, the sensible heat flux variation is asymmetrical, with an afternoon decrease much slower than the morning increase. After 3.30 pm, it becomes higher than the net flux. This is compensated for by the sign change of the ground heat flux, whose maximum is found in the morning at 11 am. The second part of this paper shows the importance of one term in the surface-layer energy budget: the long-wave radiative divergence between the ground and the top of the surface layer in high superadiabatic conditions. We show, with a radiative model on the one hand and direct measurement of the radiative divergence on the other hand, that this term reaches several tens of W m^-2 in the superadiabatic conditions found in the Sahel region.   相似文献   

Raman spectroscopic studies of carbonates part I: High-pressure and high-temperature behaviour of calcite,magnesite, dolomite and aragonite     

Philippe Gillet  Claudine Biellmann  Bruno Reynard  Paul McMillan 《Physics and Chemistry of Minerals》1993,20(1):1-18

The room-temperature Raman spectra of aragonite, magnesite and dolomite have been recorded up to 30 GPa and 25 GPa, respectively and no phase changes were observed during compression, unlike calcite. The effect of temperature on the room-pressure Raman spectra of calcite, aragonite, magnesite and dolomite is reported up to 800–1100 K. The measured relative pressure and temperature-shifts of the Raman lines are greater for the lattice modes than for the internal modes of the CO₃ groups. These shifts are used to calculate the mode anharmonic parameters of the observed Raman modes; they are negative and their absolute values are smaller (close to 0) for the internal CO₃ modes than for the lattice modes (4–17 10^?5 K^?1). The temperature shifts of the lattice modes in calcite are considerably larger than those for dolomite and magnesite, and a marked non-linear increase in linewidth is observed above 400° C for calcite. This is consistent with an increasing relaxational component to the libration of the CO₃ groups about their threefold axes, premonitory to the rotational order-disorder transition at higher temperature. This behaviour is not observed for the other calcite structured minerals in this study. We examine systematic variations in the lattice mode frequencies and linewidths with composition, to begin to understand these differences in their anharmonic behaviour. Finally, we have used a simple Debye-Waller model to calculate atomic displacements in calcite, magnesite and dolomite with increasing temperature from the vibrational frequency data, to provide a direct comparison with atomic positional data from high-temperature structure refinements.  相似文献   

Fractal properties of simulated bed profiles in coarse-grained channels   总被引：2，自引：0，他引：2  

André Robert 《Mathematical Geology》1991,23(3):367-382

Bed roughness characteristics in coarse-grained channels are fairly complex. A hierarchy of roughness elements can be observed, ranging from variable particle sizes and shapes and small-scale sedimentary structures, to large-scale bedforms such as riffle-pool sequences. The effects of these scales of roughness on the flow geometry still remain to be thoroughly investigated. The semivariogram has been suggested in the past as a means of quantifying bed roughness effects on streamflow, as well as for distinguishing between scales of roughness. However, field measurements are rather time-consuming. The low number of bed profiles measured in the field precludes the identification of generally applicable relationships between the statistical properties derived from the semivariograms (such as the Hausdorff dimensions and the scale of autocorrelation corresponding to each fractal band) and the bed configuration itself (geometrical and sedimentological properties). Simulation results of gravel-bed profiles are, therefore, presented in order to complement the original investigation of Robert (1988a). The simulation experiments, based on grain characteristics of sizes and shapes and on morphological properties of small-scale bedforms, yield very significant information on boundary roughness at the microscale and give insight into the interpretation of empirical semivariograms (derived from field measurements). Bed-material sorting, variable grain shapes, and height and spacing of cluster bedforms control the fractal dimensions obtained from the semivariograms, as well as the location of the break of slope and the range of the process.  相似文献   

Mixture of populations     

Hua Zhu  André G. Journel 《Mathematical Geosciences》1991,23(4):647-671

A simple example simulating a mixture of two normal populations results in some important observations, nonnormality and nonsymmetry of the mixture conditional pdf, nonlinearity of the conditional mean as a function of the conditioning data, heteroscedasticity of the conditional variance and its nonmonotonicity as a function of distance of the unknown to the conditioning data. A comparison of the mixture statistics with those predicted by traditional models ignoring the mixture reveals the inadequacy and inappropriateness of these traditional approaches. A mixture of two multivariate normal populations is illustrated through the analytical expressions of its conditional distribution and moments.  相似文献   

Size of earthquakes in southern Mexico from indirect methods     

José A. Canas 《Pure and Applied Geophysics》1991,136(4):479-498

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Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama   总被引：3，自引：0，他引：3  

Mare J. Defant  Lee F. Clark  Robert H. Stewart  Mark S. Drummond  Jelle Z. de Boer  René C. Maury  Hervé Bellon  Thomas E. Jackson  Juan F. Restrepo 《Contributions to Mineralogy and Petrology》1991,106(3):309-324

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.  相似文献   

A theoretical approach to the kinetics of calcite ⇌ aragonite transition: application to laboratory experiments     

M. Madon  Ph. Gillet 《Earth and Planetary Science Letters》1984,67(3):400-414

A theoretical approach to the calcite ? aragonite transformation is presented for the case of a dislocation glide mechanism obeying first-order kinetics. The kinetic parameters are determined from recent experimental data from the literature; the sensitivity of the model to these and other parameters like the grain size and the deviatoric stresses are discussed.The model results in the definition of what could be the actual transformation domains of the CaCO₃ polymorphs in the (P,T) plane. These domains are different from the classical ones, i.e. different from the stability fields defined with respect to Clapeyron's curve; they can evolve in the (P,T) plane when a deviatoric stress is applied to the system. In this case, we can define a domain in which the two senses of the calcite ? aragonite transition are activated simultaneously.The application of the model to laboratory experiments needs the integration of the kinetic differential equation (K varying withP andT). Using this method, we demonstrate the role of (P,T,t) path on reaction rate and the role of experimental uncertainties on the reaction parametersK andn.Finally, we discuss the problem of the synthesis of polymorphs out of their stability field and of their formation by grinding; these problems, difficult to tackle in terms of diffusion, can be approached with the proposed dislocation transformation kinetic model.  相似文献   

TEM investigation of the crystal microstructures in a quartz-coesite assemblage of the western alps     

J. Ingrin  Ph. Gillet 《Physics and Chemistry of Minerals》1986,13(5):325-330

Atransmission electron microscope (TEM) study of quartz-coesite inclusions in garnet in crustal rocks from the Western Alps is presented. Coesite shows a low dislocation density (<10⁷ cm^?2), and quartz a higher density of defects, Brasil twins (10⁴ cm^?1) and dislocations (10⁸ cm^?2). It is concluded that coesite has been not or only slightly plastically deformed and that the yield strength of coesite is higher than that of quartz. The large scale deformation implications are briefly discussed. TEM observations show no systematic topotactic relationship between the two polymorphs and their boundaries have a scalloped morphology which suggests that growth of quartz from coesite was controlled by a diffusion process.  相似文献   

A modified parameterization of flux-profile relationships in the surface layer using different roughness length values for heat and momentum   总被引：1，自引：0，他引：1  

Patrick Mascart  Joel Noilhan  Hervé Giordani 《Boundary-Layer Meteorology》1995,72(4):331-344

The surface-layer flux-profile formulae of Louis (1979), used in many atmospheric models, are modified in a simple way to allow for different values of the roughness lengths for heat and momentum. The modified set of formulae simplifies the calculation of surface-layer fluxes over most natural land surfaces, where the roughness length for momentum can be almost an order of magnitude greater than that for heat.  相似文献   

Observed dependence of the water vapor and clear-sky greenhouse effect on sea surface temperature: comparison with climate warming experiments     

Sandrine Bony  Jean-Philippe Duvel  Hervé Le Trent 《Climate Dynamics》1995,11(5):307-320

This study presents a comparison of the water vapor and clear-sky greenhouse effect dependence on sea surface temperature for climate variations of different types. Firstly, coincident satellite observations and meteorological analyses are used to examine seasonal and interannual variations and to evaluate the performance of a general circulation model. Then, this model is used to compare the results inferred from the analysis of observed climate variability with those derived from global climate warming experiments. One part of the coupling between the surface temperature, the water vapor and the clear-sky greenhouse effect is explained by the dependence of the saturation water vapor pressure on the atmospheric temperature. However, the analysis of observed and simulated fields shows that the coupling is very different according to the type of region under consideration and the type of climate forcing that is applied to the Earth-atmosphere system. This difference, due to the variability of the vertical structure of the atmosphere, is analyzed in detail by considering the temperature lapse rate and the vertical profile of relative humidity. Our results suggest that extrapolating the feedbacks inferred from seasonal and short-term interannual climate variability to longer-term climate changes requires great caution. It is argued that our confidence in climate models' predictions would be increased significantly if the basic physical processes that govern the variability of the vertical structure of the atmosphere, and its relation to the large-scale circulation, were better understood and simulated. For this purpose, combined observational and numerical studies focusing on physical processes are needed.  相似文献