Including the Drag Effects of Canopies: Real Case Large-Eddy Simulation Studies     

Pierre Aumond  Valery Masson  Christine Lac  Benoit Gauvreau  Sylvain Dupont  Michel Berengier 《Boundary-Layer Meteorology》2013,146(1):65-80

We use the mesoscale meteorological model Meso-NH, taking the drag force of trees into account under stable, unstable and neutral conditions in a real case study. Large-eddy simulations (LES) are carried out for real orography, using a regional forcing model and including the energy and water fluxes between the surface (mostly grass with some hedges of trees) and the atmosphere calculated using a state-of-the-art soil-vegetation-atmosphere-transfer model. The formulation of the drag approach consists of adding drag terms to the momentum equation and subgrid turbulent kinetic energy dissipation, as a function of the foliage density. Its implementation in Meso-NH is validated using Advanced Regional Prediction System simulation results and measurements from Shaw and Schumann (Boundary-Layer Meteorol, 61(1):47?C64, 1992). The simulation shows that the Meso-NH model successfully reproduces the flow within and above homogeneous covers. Then, real case studies are used in order to investigate the three different boundary layers in a LES configuration (resolution down to 2 m) over the ??Lannemezan 2005?? experimental campaign. Thus, we show that the model is able to reproduce realistic flows in these particular cases and confirm that the drag force approach is more efficient than the classical roughness approach in describing the flow in the presence of vegetation at these resolutions.  相似文献   

Wind-Flow Dynamics Over a Vineyard     

Ali Chahine  Sylvain Dupont  Carole Sinfort  Yves Brunet 《Boundary-Layer Meteorology》2014,151(3):557-577

Wind-flow dynamics has been extensively studied over horizontally uniform canopies, but agricultural plantations structured in rows such as vineyards have received less attention. Here, the wind flow over a vineyard is studied in neutral stratification from both large-eddy simulation (LES) and in situ measurements. The impact of row structure on the wind dynamics is investigated over a range of wind directions from cross-row to down-row, and a typical range of row aspect ratio (row separation/height ratio). It is shown that the mean flow over a vineyard is similar to that observed in uniform canopies, especially for wind directions from cross-row to diagonal. For down-row winds, the mean flow exhibits noticeable spatial variability across each elementary row-gap pattern, as the wind is channeled in the inter-row. This spatial variability increases with the aspect ratio. With down-row winds the turbulent structures are also more intermittent and generate larger turbulent kinetic energy and momentum flux. The displacement height and roughness length of the vineyard vary with the aspect ratio in a way similar to their variation with canopy density in uniform canopies. Both parameters take smaller values in down-row wind flow, for which the canopy appears more open. The analysis of velocity spectra and autocorrelation functions shows that vineyard canopies share similar features to uniform canopies in terms of turbulent coherent structures, with only minor changes with wind direction.  相似文献   

Initial Results from Fitting $$-Modes Using Intensity Observations from the Helioseismic and Magnetic Imager     

Sylvain G. Korzennik 《Solar physics》2017,292(9):138

The Helioseismic and Magnetic Imager project recently started processing the continuum-intensity images following global helioseismology procedures similar to those used to process the velocity images. The spatial decomposition of these images has produced time series of spherical harmonic coefficients for degrees up to \(\ell=300\), using a different apodization than the one used for velocity observations. The first 360 days of observations were processed and are made available. I present initial results from fitting these time series using my fitting method and compare the derived mode characteristics to those estimated using coeval velocity observations.  相似文献   

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars          下载免费PDF全文

Roger H. Hewins  Brigitte Zanda  Munir Humayun  Alexander Nemchin  Jean‐Pierre Lorand  Sylvain Pont  Damien Deldicque  Jeremy J. Bellucci  Pierre Beck  Hugues Leroux  Maya Marinova  Laurent Remusat  Christa Göpel  Eric Lewin  Marion Grange  Allen Kennedy  Martin J. Whitehouse 《Meteoritics & planetary science》2017,52(1):89-124

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a ¹⁴⁷Sm‐¹⁴³Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ¹⁷O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).  相似文献   

On the Use of Hydrological Models and Satellite Data to Study the Water Budget of River Basins Affected by Human Activities: Examples from the Garonne Basin of France     

Eric Martin  Simon Gascoin  Youen Grusson  Clément Murgue  Mélanie Bardeau  François Anctil  Sylvain Ferrant  Romain Lardy  Patrick Le Moigne  Delphine Leenhardt  Vincent Rivalland  José-Miguel Sánchez Pérez  Sabine Sauvage  Olivier Therond 《Surveys in Geophysics》2016,37(2):223-247

Natural and anthropogenic forcing factors and their changes significantly impact water resources in many river basins around the world. Information on such changes can be derived from fine scale in situ and satellite observations, used in combination with hydrological models. The latter need to account for hydrological changes caused by human activities to correctly estimate the actual water resource. In this study, we consider the catchment area of the Garonne river (in France) to investigate the capabilities of space-based observations and up-to-date hydrological modeling in estimating water resources of a river basin modified by human activities and a changing climate. Using the ISBA–MODCOU and SWAT hydrological models, we find that the water resources of the Garonne basin display a negative climate trend since 1960. The snow component of the two models is validated using the moderate-resolution imaging spectroradiometer snow cover extent climatology. Crop sowing dates based on remote sensing studies are also considered in the validation procedure. Use of this dataset improves the simulated evapotranspiration and river discharge amounts when compared to conventional data. Finally, we investigate the benefit of using the MAELIA multi-agent model that accounts for a realistic agricultural and management scenario. Among other results, we find that changes in crop systems have significant impacts on water uptake for agriculture. This work constitutes a basis for the construction of a future modeling framework of the sociological and hydrological system of the Garonne river region.  相似文献   

Silver and lead in high-altitude lake sediments: Proxies for climate changes and human activities     

Marion Garçon  Catherine Chauvel  Emmanuel Chapron  Xavier Faïn  Mingfang Lin  Sylvain Campillo  Sarah Bureau  Marc Desmet  Marie-Christine Bailly-Maître  Laurent Charlet 《Applied Geochemistry》2012

High-altitude lake sediments are often used as archives for environmental changes and their chemical and isotopic compositions provide significant constraints on natural and anthropogenic long-term changes that have occurred in their catchment area. Here, trace-element concentrations and Pb isotopes are presented for two sedimentary cores from Lake Blanc Huez in the French Alps, to trace the impact of climate changes and human activities over the Holocene. Lead and Ag contents are very high and clearly dominated by input from a Pb–Ag vein located a few meters from the lakeshore, a vein that also buffers the Pb isotopes. Mining of this vein in medieval times is recorded in the corresponding lake sediments with high Ag content coupled with high Pb/U ratio. These chemical characteristics can be used to constrain the major Holocene climate changes. Significant advances of glaciers next to the lake produced sediments with Ag and Pb concentration peaks and high Pb/U ratios due to accelerated erosion of the Pb–Ag vein, similar to the effects of the medieval mining. In contrast, reduced glacier activity led to the formation of organic-rich sediments with high U and As contents and low Pb/U ratios. More generally, the observed combination of chemical changes could be used elsewhere to decipher environmental changes over long periods of time.  相似文献   

Albedo control of seasonal South Polar cap recession on Mars     

Frédéric Schmidt  Sylvain Douté  Bernard Schmitt  Mathieu Vincendon  Jean-Pierre Bibring  Yves Langevin  The OMEGA Team 《Icarus》2009,200(2):374-394

Over the last few decades, General Circulation Models (GCM) have been used to simulate the current martian climate. The calibration of these GCMs with the current seasonal cycle is a crucial step in understanding the climate history of Mars. One of the main climatic signals currently used to validate GCMs is the annual atmospheric pressure cycle. It is difficult to use changes in seasonal deposits on the surface of Mars to calibrate the GCMs given the spectral ambiguities between CO₂ and H₂O ice in the visible range. With the OMEGA imaging spectrometer covering the near infra-red range, it is now possible to monitor both types of ice at a spatial resolution of about 1 km. At global scale, we determine the change with time of the Seasonal South Polar Cap (SSPC) crocus line, defining the edge of CO₂ deposits. This crocus line is not symmetric around the geographic South Pole. At local scale, we introduce the snowdrop distance, describing the local structure of the SSPC edge. Crocus line and snowdrop distance changes can now be used to calibrate GCMs. The albedo of the seasonal deposits is usually assumed to be a uniform and constant parameter of the GCMs. In this study, albedo is found to be the main parameter controlling the SSPC recession at both global and local scale. Using a defrost mass balance model (referred to as D-frost) that incorporates the effect of shadowing induced by topography, we show that the global SSPC asymmetry in the crocus line is controlled by albedo variations. At local scale, we show that the snowdrop distance is correlated with the albedo variability. Further GCM improvements should take into account these two results. We propose several possibilities for the origin of the asymmetric albedo control. The next step will be to identify and model the physical processes that create the albedo differences.  相似文献   

Influence of Random Inclusion of Synthetic Wicks Fibers of Hair on the Behavior of Clayey Soils     

Armand Sylvain Ludovic Wouatong  Wamba Danny Love Djukem  François Ngapgue  Valentine Katte  Véronique Kamgang Kabeyene Beyala 《Geotechnical and Geological Engineering》2017,35(6):2637-2646

Several soil improvement methods are used to enhance the engineering properties of soil, among which, reinforcement by fibers is considered as an effective ground improvement method because of its cost effectiveness, and easy adaptability. The present investigation chooses synthetic wick and vinifera raphia fibers as reinforcement. The synthetic wick fiber was randomly included into the soil at four different percentages i.e. 0, 2, 4, 6% by volume of raw soil. Vinifera raphia fiber was used at one percentage (4%) as control. The main objective of this research is to focus on the strength behavior of clayey soil reinforced with randomly included synthetic wick fiber. The physical and mineralogical characterization was carried out on ten soil samples. The compression, flexion, abrasion, water absorption and capillarity tests were performed on synthetic wick reinforced specimens with various fiber contents. The results of these tests have clearly shown an improvement in the compression strength values from 1.65 to 2.84 MPa for the wicks fibers, and the flexural strength values which varied between 1.08 and 1.96 MPa. Hence, the waste of synthetic wick fibers is therefore an efficient reinforcement for compressed earth blocks, which are very significant for construction of durable and economic infrastructures.  相似文献   

Carbon sources supporting a diverse fish community in a tropical coastal ecosystem (Gazi Bay,Kenya)     

J. Nyunja  M. Ntiba  J. Onyari  K. Mavuti  K. Soetaert  S. Bouillon 《Estuarine, Coastal and Shelf Science》2009

Interlinked mangrove–seagrass ecosystems are characteristic features of many tropical coastal areas, where they act as feeding and nursery grounds for a variety of fishes and invertebrates. The autotrophic carbon sources supporting fisheries in Gazi bay (Kenya) were studied in three sites, two located in the tidal creeks flowing through extensive mangrove forests, another site located in the subtidal seagrass meadows, approximately 2.5 km away from the forest. Carbon and nitrogen stable isotope composition of 42 fish species, 2 crustacean species and a range of potential primary food sources (e.g., mangroves, seagrasses and epiphytes, macroalgae) were analysed. There was considerable overlap in the δ¹³C signatures between fish (−16.1 ± 2.1‰), seagrasses (−15.1 ± 3.0‰), seagrass epiphytes (−13.6 ± 3.3‰), and macroalgae (−20.4 ± 3.1‰). Nevertheless, the signatures for most primary producers were sufficiently distinct to indicate that the dominant carbon sources for fish were mainly derived from the seagrass and their associated epiphytic community, and possibly macroalgae. Mangrove-derived organic matter contributes only marginally to the overall fish food web. Carbon supporting these fish communities was derived directly through grazing by herbivorous and some omnivorous fishes, or indirectly through the benthic food web. Fishes from the mangrove creeks had distinctly lower δ¹³C signatures (−16.8 ± 2.0‰) compared to those collected in the adjacent seagrass beds (−14.7 ± 1.7‰). This indicated that these habitats were used as distinct sheltering and feeding zones for the fishes collected, with minimal degree of exchange within the fish communities despite their regular movement pattern.  相似文献   

Long-lasting floods buffer the thermal regime of the Pampas   总被引：1，自引：0，他引：1  

Javier?HouspanossianEmail author  Sylvain?Kuppel  Marcelo?Nosetto  Carlos?Di Bella  Patricio?Oricchio  Mariana?Barrucand  Matilde?Rusticucci  Esteban?Jobbágy 《Theoretical and Applied Climatology》2018,131(1-2):111-120

The presence of large water masses influences the thermal regime of nearby land shaping the local climate of coastal areas by the ocean or large continental lakes. Large surface water bodies have an ephemeral nature in the vast sedimentary plains of the Pampas (Argentina) where non-flooded periods alternate with flooding cycles covering up to one third of the landscape for several months. Based on temperature records from 17 sites located 1 to 700 km away from the Atlantic coast and MODIS land surface temperature data, we explore the effects of floods on diurnal and seasonal thermal ranges as well as temperature extremes. In non-flooded periods, there is a linear increase of mean diurnal thermal range (DTR) from the coast towards the interior of the region (DTR increasing from 10 to 16 K, 0.79 K/100 km, r ² = 0.81). This relationship weakens during flood episodes when the DTR of flood-prone inland locations shows a decline of 2 to 4 K, depending on surface water coverage in the surrounding area. DTR even approaches typical coastal values 500 km away from the ocean in the most flooded location that we studied during the three flooding cycles recorded in the study period. Frosts-free periods, a key driver of the phenology of both natural and cultivated ecosystems, are extended by up to 55 days during floods, most likely as a result of enhanced ground heat storage across the landscape (~2.7 fold change in day-night heat transfer) combined with other effects on the surface energy balance such as greater night evaporation rates. The reduced thermal range and longer frost-free periods affect plant growth development and may offer an opportunity for longer crop growing periods, which may not only contribute to partially compensating for regional production losses caused by floods, but also open avenues for flood mitigation through higher plant evapotranspirative water losses.  相似文献