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41.
The gravity method is one of the geophysical tools used for engineering and environmental investigations where the detection of cavities, karst phenomena, subsoil irregularities, or landfills is essential. In many cases, deep or small-scale heterogeneities generating low-amplitude anomalies have to be detected and the reliability of further interpretation requires highly accurate measurements, carefully corrected for any quantifiable disturbing effects. The purpose of this study is to investigate the factors likely to limit measurement quality and how to make improvements.Calibrations of a Scintrex gravimeter were made between French relative and absolute base stations, and the relative uncertainties on the calibration factors were estimated for these links. Ranging from 10−3, for calibration on an old gravity net, to 10−4, for a high amplitude absolute base line, this accuracy will be generally sufficient for microgravity surveys.Continuous gravity recordings of Scintrex gravimeters, installed at the same stable site, enabled the estimation of the stability and accuracy of the instruments and revealed that some of the time variations of g measurements, such as instrumental drift, tidal effects and seismic noise, are not entirely removed by standard processing procedures. The accuracy of corrected gravity measurements is mainly limited by inadequate corrections of tidal effects and by a poor estimation of ocean loading effects. In comparison with residual defaults in tidal corrections, instrumental and seismic noises are taken more properly into account by statistical data processing.In field operation, residual tidal effects are generally integrated into an experimental terrain drift estimated on the basis of frequent repeated measurements. A differential gravity approach, based on a fixed gravimeter reference whose recordings are used to correct measurements made with a mobile gravimeter, has also been investigated at a test site. Compared to standard processing, this method can help improve repeatability of gravity measurements.Microgravity surveys in the urban environment require effective and accurate consideration of the effects of infrastructures, nearby buildings and basements, as well as those of topography, in the vicinity of a gravity station. Correction procedures, applied at the same experimental site, where gravity points are located close to buildings, walls and basement slope, appear to have almost totally eliminated these disturbances. 相似文献
42.
Turbulent Dissipation Rate In The Boundary Layer Via Uhf Wind Profiler Doppler Spectral Width Measurements 总被引：2，自引：0，他引：2
B. Campistron S. Bernard B. Bénech F. Ardhuin-Girard J. Dessens E. Dupont B. Carissimo 《Boundary-Layer Meteorology》2002,103(3):361-389
The study is focused on the retrieval and validation of the turbulent kinetic energy dissipation rate from spectral width measurements made by a UHF wind profiler in the convective atmospheric boundary layer. The possibility to deduce , which is one of the most important parameters for boundary-layer study and monitoring, from Doppler radar spectral width measurements has a firm theoretical basis established by numerous earlier works. However, the major drawback of this approach lies in various meteorological and instrumental sources of non-turbulent spectral width broadening which have to be recognised and accounted for. In the first part of the study, the theoretical background of the retrieval is detailed and all possible causes of spectral broadening are listed and evaluated. In the second part, the method is applied to four days of UHF diurnal boundary-layer observations, collected during the TRAC-98 experiment, for which in situ aircraft measurements were available. Comparison between radar-retrieved and in situ aircraft measurements yields a fairly good agreement with a linear correlation coefficient of about 0.9 and a residual bias less than 2 × 10-4 m2 s-3. The analysis of derived from vertical and off-zenith observation leads to the recommendation, that in the boundary layer, where the wind is usually moderate, data collected by off-zenith beams should be used. Indeed, the measurement of the vertical spectral width, less affected by the large-scale broadening factor, can still be altered by the ground clutter removal. 相似文献
43.
Simulation of Meteorological Fields Within and Above Urban and Rural Canopies with a Mesoscale Model 总被引：3，自引：4，他引：3
Accurate simulation of air quality at neighbourhood scales (on order of 1-km horizontal grid spacing) requires detailed meteorological fields inside the roughness sub-layer (RSL). Since the assumptions of the roughness approach, used by most of the mesoscale models, are unsatisfactory at this scale, a detailed urban and rural canopy parameterisation, called DA-SM2-U, is developed inside the Penn State/NCAR Mesoscale Model (MM5) to simulate the meteorological fields within and above the urban and rural canopies. DA-SM2-U uses the drag-force approach to represent the dynamic and turbulent effects of the buildings and vegetation, and a modified version of the soil model SM2-U, called SM2-U(3D), to represent the thermodynamic effects of the canopy elements. The turbulence length scale is also modified inside the canopies. SM2-U(3D) assesses the sensible and latent heat fluxes from rural and urban surfaces in each of the computational layers inside the canopies by considering the shadowing effect, the radiative trapping by the street canyons, and the storage heat flux by the artificial surfaces. DA-SM2-U is tested during one simulated day above the city of Philadelphia, U.S.A. It is shown that DA-SM2-U is capable of simulating the important features observed in the urban and rural RSL, as seen in the vertical profiles of the shear stress, turbulent kinetic energy budget components, eddy diffusivity, potential air temperature, and specific humidity. Within the canopies, DA-SM2-U simulates the decrease of the wind speed inside the dense canopies, the skirting of the flow around the canopy blocks, warmer air inside the vegetation canopy than above open areas during the night and conversely during the day, and constantly warmer air inside the urban canopy. The comparison with measurements shows that the surface air temperature above rural and urban areas is better simulated by DA-SM2-U than by the `standard version' of MM5. 相似文献
44.
Ahmed Benamar HuaQing Wang Nasre-dine Ahfir Abdellah Alem Nicolas Masséi Jean-Paul Dupont 《Comptes Rendus Geoscience》2005,337(5):497-504
We study in this paper the transport and deposition of suspended particles in saturated porous media. Some chromatographic short-pulse tests were performed in a laboratory column with a tracer and two types of suspended particles. A mathematical model was used to determine the hydrodispersive parameters and deposition rates from the breakthrough curves. The results show that the fitted parameters obtained vary with the flow rate and the characteristics of the suspended particles. We also observe, at high flow rates, that particles have a migration velocity higher than that of the conservative tracer. To cite this article: A. Benamar et al., C. R. Geoscience 337 (2005). 相似文献
45.
Jean-Charles Dupont Martial Haeffelin Alain Protat Dominique Bouniol Neda Boyouk Yohann Morille 《Boundary-Layer Meteorology》2012,143(1):207-225
A suite of active and passive remote sensing instruments and in-situ sensors deployed at the SIRTA Observatory (Instrumented
Site for Atmospheric Remote Sensing Research), near Paris, France, for a period of six months (October 2006–March 2007) document
simultaneously radiative, microphysical and dynamic processes driving the continental-fog life cycle. The study focuses on
a 6-day period between 23 and 29 December 2006 characterized by several stratus-cloud lowering and lifting events and almost
18 h of visibility below 1 km. Conceptual models and different possible scenarios are presented here to explain the formation,
the development and the dissipation phases of three major stratus–fog events and to quantify the impact of each driving process.
For example, slowly evolving large-scale conditions characterized by a slow continuous cloud-base lowering, followed by a
rapid transient period conductive to fog formation and dissipation, are observed for cases 1 and 3. During this stable period,
continuous cloud-top radiative cooling (≈ −160 Wm−2) induces a progressive and slow lowering of the cloud base: larger droplets at cloud top (cloud reflectivity approximately
equals to −20 dBZ) induce slow droplet fall to and beyond cloud base (Doppler velocity ≈ −0.1 ms−1), cooling the sub-cloud layer by evaporation and lowering the saturation level to 100 m (case 1) or to the surface (cases
2 and 3). Suddenly, a significant increase in Doppler velocity magnitude ≈ −0.6 ms−1 and of turbulent kinetic energy dissipation rate around 10−3 m2s−3 occurs at cloud base (case 1). These larger cloud droplets reach the surface leading to fog formation over 1.5 h. The Doppler
velocity continues to increase over the entire cloud depth with a maximum value of around −1 ms−1 due to the collection of fog droplets by the drizzle drops with high collection efficiency. As particles become larger, they
fall to the ground and lead to fog dissipation. Hence, falling particles play a major role in both the formation and also
in the dissipation of the fog. These roles co-exist and the balance is driven by the characteristics of the falling particles,
such as the concentration of drizzle drops, the size distribution of drizzle drops compared to fog droplets, Doppler velocity
and thermodynamic state close to the surface. 相似文献
46.
Ilham Bouimetarhan Lydie Dupont Enno Schefuß Gesine Mollenhauer Stefan Mulitza Karin Zonneveld 《Quaternary Research》2009,72(2):188-197
Pollen and organic-walled dinoflagellate cyst assemblages from core GeoB 9503-5 retrieved from the mud-belt ( 50 m water depth) off the Senegal River mouth have been analyzed to reconstruct short-term palaeoceanographic and palaeoenvironmental changes in subtropical NW Africa during the time interval from ca. 4200 to 1200 cal yr BP. Our study emphasizes significant coeval changes in continental and oceanic environments in and off Senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca. 2900 and 2500 cal yr BP. After ca. 2500 cal yr BP, the environment slowly became drier again as indicated by slight increases in Sahelian savannah and desert elements in the pollen record. Around ca. 2200 cal yr BP, this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen, dinoflagellate cysts, fresh-water algae and plant cuticles, suggesting “episodic flash flood” events of the Senegal River. The driest phase developed after about 2100 cal yr BP. 相似文献
47.
The Tian Shan range formed in the late Cenozoic in response to the northward propagation of deformation related to the India–Eurasia continental collision. Precise timing of the Tian Shan uplift is required to understand possible mechanisms of continental lithosphere deformation and interactions between climate, tectonism and erosion. Here, we provide magnetostratigraphic age control on the northern Chinese Tian Shan foreland successions. A thorough rock magnetic analysis identifies haematite‐ and magnetite‐bearing alluvial fan deposits in the upper portion of the sampled strata as more reliable palaeomagnetic recorders than magnetite‐bearing fluvial and lacustrine deposits that are often maghaemitized in the lower part of the record. As a result, a robust correlation to the geomagnetic polarity time scale is obtained from 6 to 2 Ma while a tentative correlation is proposed from 6 to 16 Ma. Sediment accumulation rates increase from 155 to 260 m Myr?1 at 3.9±0.3 Ma. This change coincides with a gradual lithologic transition from fluvial (sandstone‐dominated) to alluvial fan (conglomerate‐dominated) deposits that likely records an approaching erosional source related to tectonically increased subsidence rather than differential compaction. Clear evidence for growth strata starting at an estimated age of ～2 Ma provides a minimum age for folding. These results are compared with previous magneotstratigraphic studies from the same and other sections of the northern Tian Shan foreland basin fill, thus enabling a critical assessment of the reliability of magnetostratigraphic dating and the significance of sediment accumulation rate variations with respect to facies variations and growth strata. Our results in the Taxi He section provide a sequence of events that is consistent with enhanced tectonic forcing starting at ～4 Ma, although a climatic contribution must be considered given the close relationship of these ages with the Pliocene climate deterioration. 相似文献
48.
Hanane Zaïdi Eric Dupont Maya Milliez Luc Musson-Genon Bertrand Carissimo 《Boundary-Layer Meteorology》2013,147(2):237-259
We simulate the microscale heterogeneities of turbulent variables observed at a complex site for different wind directions. The atmospheric computational fluid dynamics (CFD) results are compared with an ensemble of 36 months of data collected at the experimental site SIRTA “Site Instrumental de Recherche par Télédétection Atmosphérique”, located near Paris (France) in a semi-urban environment. The experimental data show that the normalized turbulent kinetic energy (TKE) k/U 2 (where k is TKE and U is the wind speed) at 10-m height, for two different locations, is highly dependent on wind direction and strongly influenced by trees. These measurements show a strong increase of the normalized TKE downstream of the forest canopies with a large variability within the 36-month period in part due to the variation of the tree foliage. The numerical simulations are carried out using the CFD code Code_Saturne with the standard k?ε closure, in neutral stratification. The buildings are taken into account explicitly in the mesh and the forested areas are modelled with two approaches: the classical roughness wall law and a drag porosity. A comparison has been performed between the calculated values and the median of measured values of the normalized TKE and the normalized friction velocity, for each wind sector of 10°. A very good agreement is obtained with the drag porosity model, whereas the classical roughness law leads to a strong underestimation downstream of the forested areas. However, this large improvement of the results using the drag porosity model can only be obtained with a refinement of the grid, especially in forested areas, and an accurate land-use map. 相似文献
49.
Combined influence of atmospheric physics and soil hydrology on the simulated meteorology at the SIRTA atmospheric observatory 总被引：1，自引：0，他引：1
F. Cheruy A. Campoy J.-C. Dupont A. Ducharne F. Hourdin M. Haeffelin M. Chiriaco A. Idelkadi 《Climate Dynamics》2013,40(9-10):2251-2269
The identification of the land-atmosphere interactions as one of the key source of uncertainty in climate models calls for process-level assessment of the coupled atmosphere/land continental surface system in numerical climate models. To this end, we propose a novel approach and apply it to evaluate the standard and new parametrizations of boundary layer/convection/clouds in the Earth System Model (ESM) of Institut Pierre Simon Laplace (IPSL), which differentiate the IPSL-CM5A and IPSL-CM5B climate change simulations produced for the Coupled Model Inter-comparison Project phase 5 exercise. Two different land surface hydrology parametrizations are also considered to analyze different land-atmosphere interactions. Ten-year simulations of the coupled land surface/atmospheric ESM modules are confronted to observations collected at the SIRTA (Site Instrumental de Recherche par Télédection Atmosphérique), located near Paris (France). For sounder evaluation of the physical parametrizations, the grid of the model is stretched and refined in the vicinity of the SIRTA, and the large scale component of the modeled circulation is adjusted toward ERA-Interim reanalysis outside of the zoomed area. This allows us to detect situations where the parametrizations do not perform satisfactorily and can affect climate simulations at the regional/continental scale, including in full 3D coupled runs. In particular, we show how the biases in near surface state variables simulated by the ESM are explained by (1) the sensible/latent heat partitionning at the surface, (2) the low level cloudiness and its radiative impact at the surface, (3) the parametrization of turbulent transport in the surface layer, (4) the complex interplay between these processes. We also show how the new set of parametrizations can improve these biases. 相似文献
50.
Costa Alexandre C. Dupont Fanny Bier George van Oel Pieter Walker David W. Martins Eduardo S. P. R. 《Hydrogeology Journal》2023,31(3):751-769
Particularly in arid and semiarid areas, more and more populations rely almost entirely on imported water. However, the extent to which intentional discharge into transiting river systems and unintentional leakage may be augmenting water resources for communities along and down gradient of the water transfer scheme has not previously been subject to research. The objective of this study was to assess both the potential of a large-scale water transfer (WT) scheme to increase groundwater availability by channel transmission losses in a large dryland aquifer system (2,166 km²) in Brazil, and the capability of the receiving streams to transport water downstream under a prolonged drought. An integrated surface-water/groundwater model was developed to improve the estimation of the groundwater resources, considering the spatio-temporal variability of infiltrated rainfall for aquifer recharge. Aquifer recharge from the WT scheme was simulated under prolonged drought conditions, applying an uncertainty analysis of the most influential fluxes and parameters. The annual recharge (66 mm/year) was approximately twice the amount of water abstracted (1990–2016); however, the annual recharge dropped to 13.9 mm/year from 2012 to 2016, a drought period. Under similar drought conditions, the additional recharge (6.89 × 106 m³/year) from the WT scheme did not compensate for the decrease in groundwater head in areas that do not surround the receiving streams. Actually, the additional recharge is counteracted by a decrease of 25% of natural groundwater recharge or an increase of 50% in pumping rate; therefore, WT transmission losses alone would not solve the issue of the unsustainable management of groundwater resources.
相似文献