Search

Evidence has been provided that land-cover changes such as deforestation can have an impact on cloudiness and precipitation. However, conflicting results have been obtained at different scales and places, highlighting our poor understanding of the physical processes involved. Here we focus on mesoscale summer cloudiness in a temperate region, as influenced by a large forest massif (the Landes forest in France). Our study is based on an up-to-date atmosphere-surface mesoscale model (Meso-NH coupled with SURFEX). Based on observational data, we first optimise the model configuration for our purpose, and show that with a 500 m horizontal resolution we can successfully simulate the higher summer cloud cover observed over the forest, compared to its surroundings. Secondly, we investigate the physical processes leading to cloud formation in a representative case study. Based on a comparative analysis of diagnostics and budgets over forest and non-forest areas, we find that the larger sensible heat flux over the forest and its higher roughness are the main drivers of cloudiness, enhancing vertical velocity and boundary-layer mixing. Third, we simulate the impact of the 2009 Klaus storm that led to the loss of about one third of the trees. Considering fifteen representative convective summer days, we show that the model simulates well the resulting decrease in summer cloudiness that was reported in a previous study based on satellite observations. As a complementary tool, the mesoscale simulations allow to quantify the impacts of the Klaus storm windthrough on the diurnal cycle of the boundary layer.

We study joint operational and financing decisions of a cashconstrained manufacturing firm in the presence of bank financing and trade credit, where banks set credit limit based on borrowers repayment histories. Trade credit financing enables the manufacturing firm to build credit history, which affects bank financing availability in the future periods. This can make trade credit valuable, even though it is costly in the current period. We quantify the value of trade credit as a way of securing access to future bank financing and investigate how financing considerations distort the firm’s operational decisions.

International audience; A key issue for predicting and mitigating the risk of wind damage in forests is to understand the mechanics of damage propagation during windstorms. Until now the acting processes have been poorly understood due to the difficulty of performing measurements in such extreme conditions. Here we use an innovative wind–tree interaction model, which allows for large deflection and tree breakage, to unravel for the first time the mechanisms of damage propagation at forest scale. We find that damage propagation involves two stages. Firstly, initial damage is caused by the impact of strong downward wind gusts. Trees break preferentially at the end of such critical passing sweeps, as the tree motion decelerates. The second stage starts when the damaged areas reach about 5 canopy heights in length and 1 canopy height in width: as the flow accelerates within the damaged areas the mean wind load becomes sufficient to break newly-created edge trees. From this bifurcation point tree damage increases drastically, irrespective of the tree motion state and the type of passing gusts. In addition to demonstrating the possibility of simulating wind damage propagation, these results have considerable potential for improving wind risk models.

International audience; Forested landscapes often exhibit large spatial variability in vertical and horizontal foliage distributions. This variability may impact canopy-atmosphere ex- changes through its action on the development of turbulent structures. Here we inves- tigate in neutral stratification the turbulent structures encountered in a maritime pine forest characterised by a high, dense foliated layer associated with a deep and sparse trunk space. Both stand and edge regions are considered. In situ measurements and the outputs of large-eddy simulations are used and analysed together. In stand conditions, far from the edge, canopy-top structures appear stronlgy damped by the dense crown layer. Turbulent wind fluctuations within the trunk space, where the momentum flux vanishes, are closely related to these canopy-top structures through pressure diffusion. Consequently, auto-correlation and spectral analyses are not quite appropriate to characterise the vertical scale of coherent structures in this type of canopy, as pressure diffusion enhances the actual scale of structures. At frequencies higher than those associated with canopy-top structures, wind fluctuations related to wake structures developing behind tree stems are observed within the trunk space. They manifest themselves in wind velocity spectra as secondary peaks in the inertial subrange re- gion, confirming the hypothesis of spectral short-cuts in vegetation canopies. In the edge region specific turbulent structures develop just below the crown layer, in addi- tion to canopy-top structures. They are generated by the wind shear induced by the sub-canopy wind jet that forms at the edge. These structures provide a momentum exchange mechanism similar to that observed at the canopy top but in the opposite di- rection and with a lower magnitude. They may develop as in plane mixing-layer flows, with some perturbations induced by canopy-top structures. Wake structures are also observed within the trunk space in the edge region.

International audience; Experiments on two-dimensional fluidized beds of glass beads are presented for two fluids of typical Reynolds numbers of about 1 and a few hundred. The presence of inertial effects leads to the formation of wakes behind the particles and consequently to an anisotropy of the microstructure. The microstructures, studied by video imaging, are characterized by pair probability distribution functions and structure factors.

During the last few decades, many studies have been performed to determine water and carbon budgets of broadleaf and deciduous forests, crops and grasslands. However, since most measurements have been made in different regions and at different periods, it is difficult to compare the results directly. In order to evaluate accurately the respective contribution of various agroecosystems to global water and carbon exchanges, it is necessary to compare data obtained in similar climatic and weather conditions. To address this question, we present the results from simultaneous measurements carried out during one year over three typical agroecosystems of the Les Landes region in South-West France:~an agricultural field with maize from 29 May to 18 October, a young (5 year-old) pine forest and a mature (37 year-old) pine forest. All measurements were collected as part of the Regional Experiment component of the CarboEurope-IP project. During most of the year, the agricultural field without vegetation is a source of CO2, but from late June to early September the maize crop becomes a stronger carbon sink than the forests. Over the whole measurement period the three agroecosystems behave as CO2 sinks with carbon storage of about 335, 210 and 160 g C m−2 for the young forest, the mature forest and the agricultural field, respectively. We investigated the influence of climatic conditions on Gross Primary Production (GPP) of the three ecosystems and observed a predominant role of vapour pressure deficit (VPD) for forests and of photosynthetic photon flux density (FPP) for maize. Daily Water Use Efficiencies (WUE) of the three ecosystems were evaluated and expressed as functions of the mean daily vapour pressure deficit (VPD). Similar trends were observed for the two forests, which suggests that for a given species WUE is independent of stand age. The WUE of the maize crop at maturity was also found to depend upon VPD, but it is about twice as large as for the forests, owing to the physiological advantages of C4 species.

An analysis of 10 variables caracterising the distribution of the Québec's cities size, permitted identification of 3 distinct phases of evolution of the urban system since 1871. Primatial growth reached a maximum in the 1920s and was followed by a transition-maturation period where, around 1956, the number of centers stabilized at 60. Lognormality was then at a maximum. Since that peak, the metropolisation process induced a new spell of primatial growth. The Quebec urban structure is still markedly unbalanced, the transition from primacy to lognormality is still going on. Its longing could be explained by the fact that in Quebec the "center-periphery" spatial dualism is doubled with an ethno-linguistic dualism. Without the french-english opposition one can think that the transition period would have resolved rapidly with the development of non-metropolitan industrial complexes.

The Advanced Regional Prediction System (ARPS) has been modified so as to simulate turbulent flow at very fine scale within and above vegetation canopies using a large-eddy simulation (LES) approach. It is first shown that this new version of ARPS is able to reproduce accurately all essential features of turbulent flow over homogeneous canopies. A sensitivity study of the flow to the morphology of the canopy, i.e. the density and vertical leaf-area (LAI) distribution, is then performed numerically over three types of canopies with five levels of leaf-area index, from 1 to 5. This study confirms the universal characteristics of turbulent flow over vegetation canopies, as previously observed from wind-tunnel and in situ experiments. It shows that the typical features of canopy flow become more pronounced as canopy density increases, and quantifies the extent to which differences in canopy morphology can explain the experimental variability observed between canopies. This variability in turbulent characteristics is mostly visible in the subcanopy space, and depends significantly on the density of the upper foliated layers. The mean longitudinal separation Λ w between adjacent coherent structures has also been computed for each canopy using the wavelet transform and approximating the convection velocity of coherent structures as 1.8 times the average wind velocity at canopy top. Except for the sparsest canopies, the analogy between the atmospheric flow near the top of a vegetation canopy and a plane mixing layer is well verified: Λ w is directly related to the shear length scale L s = U ( h ) / U ′ ( h ) , where h is canopy height, U mean velocity and U ′ is the vertical gradient d U / d z , in a way close to the prediction Λ w / h = 8.1 L s / h .

International audience; As forest edges are often associated with wind damage, it may be of interest to modify the edge region in order to reduce wind-induced risks. To this purpose, this study investigates tree vulnerability to wind load downwind from leading edges designed with various treatments: sharp, tapered, sparse, dense, tall and small edges. Using a large-eddy simulation flow model, instantaneous wind and turbulence fields are simulated on either side of each edge. These fields are then used to compute mean and extreme tree bending moments as well as their ratio, the gust factor. The behaviour of these variables downwind from the edge agrees well with previous wind tunnel measurements. The gust factor increases at some distance behind the edge, due to the development of coherent eddy structures generated at the canopy-air interface. Unlike wind gusts in the vicinity of the edge, these structures penetrate deep within the canopy through sweep motions. Tree vulnerability is slightly reduced downwind from tapered, sparse and small edges and enhanced downwind from dense ones. Behind tall edges, the gust factor is reduced in the edge region but enhanced further downstream due to the interaction of the canopy with the wake of the edge treatment.

International audience; Sharp heterogeneities in forest structure, such as edges, are often responsible for wind damage. In order to better understand the behaviour of turbulent flow through canopy edges, large-eddy simulations (LES) have been performed at very fine scale (2 m) within and above heterogeneous vegetation canopies. A modified version of the Advanced Regional Prediction System (ARPS), previously validated in homogeneous conditions against field and wind-tunnel measurements, has been used for this purpose. Here it is validated in a simple forest-clearing-forest configuration. The model is shown to be able to reproduce accurately the main features observed in turbulent edge flow, especially the "enhanced gust zone" (EGZ) present around the canopy top at a few canopy heights downwind from the edge, and the turbulent region that develops further downstream. The EGZ is characterized by a peak in streamwise velocity skewness, which reflects the presence of intense intermittent wind gusts. A sensitivity study of the edge flow to the forest morphology shows that with increasing canopy density the flow adjusts faster and turbulent features such as the EGZ become more marked. When the canopy is characterized by a sparse trunk space the length of the adjustment region increases significantly due to the formation of a sub-canopy wind jet from the leading edge. It is shown that the position and magnitude of the EGZ are related to the mean upward motion formed around canopy top behind the leading edge, caused by the deceleration in the sub-canopy. Indeed, this mean upward motion advects low turbulence levels from the bottom of the canopy; this emphasises the passage of sudden strong wind gusts from the clearing, thereby increasing the skewness in streamwise velocity as compared with locations further downstream where ambient turbulence is stronger.

A multi-purpose model for small-scale atmospheric flows over heterogeneous landscapes is being developed. The aim of this research is to build a tool able to predict the dynamical (wind, turbulence) and diffusive (gases, particles) fields over landscapes characterised by heterogeneous plant cover. In its present stage of development the model is based on the numerical integration of neutral atmospheric flow equations, using an energy-dissipation closure scheme and over a domain that may include vegetation layers. Three validation cases of the model are presented: (i) response of the airflow to a change in surface roughness; (ii) airflow within and above a horizontally homogeneous plant canopy; (iii) airflow over two complex forest-to-clearing and clearing-to-forest transitions. All simulations provide results in good agreement with the experimental data, except for turbulent kinetic energy just after a clearing-to-forest transition. This result is not surprising for a statistical k−e model in a flow region characterised by strong distorsion and intermittent turbulence. However the overall good performance of the model is promising for environmental research at fine scales over heterogeneous landscapes.

A modelling study is performed in order to quantify the relative effect of allowing for the physiological properties of an undergrowth grass sward on total canopy water and carbon fluxes of the Le-Bray forest (Les-Landes, South-western France). The Le-Bray forest consists of maritime pine and an herbaceous undergrowth (purple moor-grass), which is characterised by a low stomatal control of transpiration, in contrast to maritime pine. A CO2-responsive land surface model is used that includes responses of woody and herbaceous species to water stress. An attempt is made to represent the properties of the undergrowth vegetation in the land surface model Interactions between Soil, Biosphere, and Atmosphere, CO2-responsive, ISBA-A-gs. The new adjustment allows for a fairly different environmental response between the forest canopy and the understory in a simple manner. The model's simulations are compared with long term (1997 and 1998) micro-meteorological measurements over the Le-Bray site. The fluxes of energy, water and CO2, are simulated with and without the improved representation of the undergrowth vegetation, and the two simulations are compared with the observations. Accounting for the undergrowth permits one to improve the model's scores. A simple sensitivity experiment shows the behaviour of the model in response to climate change conditions, and the understory effect on the water balance and carbon storage of the forest. Accounting for the distinct characteristics of the undergrowth has a substantial and positive effect on the model accuracy and leads to a different response to climate change scenarios.

The current emphasis on global climate studies has led the scientific community to set up a number of sites for measuring long-term biospheric fluxes, and to develop a wide range of biosphere–atmosphere exchange models. This paper presents a new model of this type, which has been developed for a pine forest canopy. In most coniferous species the canopy layer is well separated from the understorey and several cohorts of needles coexist. It was therefore found necessary to distinguish several vegetation layers and, in each layer, several leaf classes defined not only by their light regime and wetness status but also by their age. This model, named MuSICA, is a multilayer, multileaf process-based model. Each submodel is first independently parameterized using data collected at a EUROFLUX site near Bordeaux (Southwestern France). Particular care is brought to identify the seasonal variations in the various physiological parameters. The full model is then evaluated using a two-year long data set, split up into 12 day-type classes defined by the season, the weather type and the soil water status. Beyond the good overall agreement obtained between measured and modelled values at various time scales, several points of further improvement are identified. They concern the seasonal variations in the stomatal response of needles and the soil/litter respiration, as well as their interaction with soil or litter moisture. A sensitivity analysis to some of the model features (in-canopy turbulent transfer scheme, leaf age classes, water retention, distinction between shaded and sunlit leaves, number of layers) is finally performed in order to evaluate whether significant simplifications can be brought to such a model with little loss in its predictive quality. The distinction between several leaf classes is crucial if one is to compute biospheric fluxes accurately. It is also evidenced that accounting for in-canopy turbulent transfer leads to better estimates of the sensible heat flux.

International audience; Forest soils are often covered with a litter that influences the rate of mass and energy transfer between the soil and the air above, thereby modifying the temperature and moisture fields in the soil. The presence of a litter should therefore be accounted for in forest SVAT models, especially when long-term simulations are to be performed. A heat and moisture litter model has been developed by adding two dynamical equations to a force-restore type soil model. The experimental data used for the model validation was collected in a pine forest canopy in the South-West of France, that was part of the Euroflux network. The model is tested and validated over a two-year period. It is shown to provide a fairly good simulation of soil and litter moisture, soil and litter temperature and turbulent fluxes measured above the forest floor. It is also shown that simulations without the litter layer are unable to reproduce all these variables simultaneously. We then perform a sensitivity analysis to the parameters whose values are either uncertain or likely to be variable in time and space, such as the litter thickness, the rainfall fraction intercepted by the litter or the maximum value of the surface resistance. A threshold value of the litter moisture used in the surface resistance parameterisation turns out to be the most critical parameter. Further work is needed to investigate the possible relationships between the various parameters describing the litter, but the present litter model can already be used in combination with other forest SVAT models.

Resume Les mesures des fractions volumiques locales sont rares en ecoulements diphasiques fluide-particules. En general, la concentration est supposee spatialement homogene dans les experiences fondamentales concernant les ecoulements sedimentaires. Ceci est loin d'etre verifie hors du regime de Stokes, en particulier au voisinage de parois. Cette note presente deux methodes qui, moyennant quelques restrictions, donnent acces a cette fraction volumique dans un ecoulement diphasique tridimensionnel.

International programmes such as EUROFLUX focus on the analysis of long-term fluxes and energy budgets in the biosphere. Reliable estimates of hourly energy budgets require an accurate estimation of soil heat flux, that is often non-negligible even in a forest, and can be predominant during the night. Over long periods of time such as one to several months, its contribution can also be significant. The present work has been carried out to get good estimates of the soil heat flux in a maritime pine stand in the southwest of France, one of the 15 EUROFLUX sites. Using a whole year’s worth of data, soil heat flux was estimated by a two-step version of the null-alignment method using soil temperature, water content and bulk density measurements between the soil surface and a depth of 1 m. A data subset was firstly used to estimate and model the soil thermal conductivity at various depths. The full data set was then used with the modelled conductivity to estimate heat storage between the surface and a reference depth, and calculate the heat flux at the soil surface. Throughout the investigated year and at a 30 min time scale, the soil heat flux represents 5‐10% of the incident net radiation, i.e. 30‐50% of the net radiation over the understorey. Cumulative values from September 1997 to March 1998 reach a maximum of 70 MJ m 2 , which represents nearly 50% of the cumulative values of transmitted net radiation (140 MJ m 2 ) over the same period. These estimates of soil heat flux allowed the energy budgets of the whole stand and the understorey to be closed, and showed that the storage terms are significant not only at a 30 min time scale but also at longer time scales (a few weeks). An attempt was finally made to model soil heat flux from meteorological data, which has rarely been done for a forest soil and over a long-term data set. In most of the existing models, soil heat flux is taken as a fraction of net radiation or sensible heat flux. Here, the litter acts as a mulch at the soil surface so that the only significant terms of the energy balance at this level are soil heat flux, transmitted net radiation and turbulent sensible heat flux. Soil heat flux is shown to be a linear combination of (1) net radiation above the understorey with a clear dependence of the coefficient on the soil cover fraction, and (2) the difference between the air and litter temperatures, with little influence of soil water content or wind speed on the coefficient. © 2001 Elsevier Science B.V. All rights reserved.

Resume La rencontre d'un jet et d'un ecoulement perpendiculaire engendre des structures tourbillonnaires instationnaires, a la frontiere du jet. Cette note decrit, a partir de visualisations quantitatives, l'evolution de ces structures. Une des evolutions possibles est particulierement detaillee: le processus d'appariement entre deux tourbillons consecutifs.

International audience; Two-point, space-time correlations of streamwise and vertical velocity were obtained from a wind tunnel simulation of an atmospheric surface layer with an underlying model wheat canopy constructed of flexible nylon stalks. Velocity data extend from 1/6 canopy height to several canopy heights, with in excess of 2000 three-dimensional vector separations of the two x-wire probes. Isocorrelation contours over anx, z slice show the streamwise velocity autocorrelation to be roughly circular, such that vertical velocities at the same horizontal position but different heights are closely in phase. Cross-correlations between the two velocity components reflect this difference to some extent. Lateral displacements of the probes revealed side lobes with correlations of reversed sign but we cannot positively link this pattern to particular vorticular structures. Integral length scales obtained directly from the spatial correlations match similar scales deduced from single-point time series with Taylor's hypothesis at 2 to 3 times the canopy height but greatly exceed such scales at lower levels, particularly within the wheat. We conclude that the reversed sign lateral lobes are important components of the correlation field and that an integral length scale for the lateral direction must be defined such that they are included. Convective velocities obtained from the time lag to optimally restore correlation lost by physical separation of the probes change only slowly with height and greatly exceed the mean wind velocity within and immediately above the canopy. Thus, mean wind velocity is not a suitable proxy for convective velocity in the application of Taylor's hypothesis in this situation. The ratio of vertical to longitudinal convective velocity for the streawise velocity signal yields a downwind tilt angle of about 39° which is probably a better estimate of the slope of the dominant fluid motions than the tilt of the major axis of the isocorrellation contours mentioned previously.

Workshop on Thermal Remote Sensing of the Energy and Water Balance Over Vegetation in Conjunction with Other Sensors, La-Londe-des-Maures, 20-24 septembre 1993; International audience; Surface temperature is a key parameter for assessing fluxes at the surface-atmosphere interface. Proper estimation of radiative surface temperature requires corrections for perturbating factors such as atmospheric contributions and angular effects. Several models have been derived to address angular effects, but relevant data for validating such models is still scarce. This paper describes a field experiment dedicated to collecting angular measurements of brightness surface temperature over several types of surfaces (bare soils with different roughnesses, corn, grass, alfalfa), using a unique measurement protocol with simultaneous temperature readings at two angles. For each surface zenithal and azimuthal angular effects are quantified. In some cases (unstressed, fully-covering alfalfa) the difference between oblique and vertical brightness temperatures is within ±0.5 K. Over stressed corn the temperature measured at angles of ±60° is about 4 K less than the nadir looking temperature, but it is 3.5 K higher over a ploughed bare soil, when the inclined radiometer faces the sunlit side of the furrows. Over a bare smooth soil the observed angular variations are shown to be compatible with those due to possible angular variations in emissivity. All the results are discussed in terms of surface geometry and microclimatic conditions, and compared to previous studies. Implications are deduced for the interpretation of satellite measurements of surface temperature.

International audience; A new method for estimating scalar fluxes, called surface renewal analysis, was developed and successfully tested with air temperature data from a maize crop, an orchard, and a forest canopy. The method employs the scalar trace and is based on the concept of turbulent coherent structures causing most of the scalar fluxes. It was generally more accurate than other scalar based methods such as the Tillman variance method, and can be applied to data gathered under stable stability conditions. Filtering the temperature trace, based on the wind speed at the canopy height, resulted in more accurate surface renewal estimates than when raw 10 Hz data were used.

Various models have been proposed to predict changes in scalar concentrations and surface fluxes following an abrupt change in surface conditions. In most cases the boundary conditions used to describe the surface represent limiting cases such as a step change in either concentration, flux or surface resistance. Three main conclusions emerge from a review of experimental results published thus far. Firstly, a dry-to-wet transition can be treated as a step change in concentration, after which the difference between the downwind and upwind surface fluxes show a power-law dependence on distance from the leading edge with an exponent of the order of − 1 6 . Secondly, wet-to-dry transitions should be treated as resulting in a step change in flux. This has not always been appreciated. Thirdly, a dry-to-moist (e.g. irrigated) transition is shown to resemble more closely a step change in flux than a step change in surface resistance. This can only be explained if stomatal resistance varies in conjunction with downwind changes in saturation deficit. We propose a simple analytical model to describe the streamwise change in surface resistance in the limit of a step change in flux. A scarcity of published data suitable for validation of this model has led us to undertake a new experimental investigation. This experiment is the subject of a companion paper.

It has been suggested in Part I of this paper that, after a dryland-to-irrigated transition, the surface sensible and latent heat fluxes may exhibit little change with downstream distance from the leading edge. It was argued that such step changes in surface fluxes could be caused by a feedback mechanism between surface resistance and changing saturation deficit. A simple model was derived for describing the streamwise change in surface resistance leading to such a step change in flux. The aim of the present paper is to test these assumptions against experimental data acquired specifically for this purpose. Measurements were performed over a transition between dryland and irrigated barley. Eddy covariance determinations of sensible heat flux at a height of 1 m showed little variation downwind of the transition and, once corrected from the footprint effect, exhibited patterns roughly compatible with a step change in surface flux. Transects of radiative temperature, and air temperature and humidity above the crop, also showed downwind changes in good agreement with the expected variation. The most convincing results were obtained from measurements of stomatal and canopy resistance, whose downwind variation was close to that required by a step change in evapotranspiration.

Should firms in developed economies work with more or fewer suppliers than firms in developing economies? More generally, how does the number of suppliers for a firm depend on the firm’s economic environment? To answer these questions we identify several economic and business factors that might affect the number of suppliers (and that separate developed and developing economies): supply risk, fixed costs of working with suppliers, and access to financing (particularly trade-credit financing).

International audience; The classical flux-gradient relationships derived from the surface layer similarity theory have been known for a long time not to be valid just above plant canopies; within the framework of first-order closure, all the published studies have indeed shown that in the so-called 'roughness sublayer' the momentum and/or scalar diffusivities are enhanced above their inertial layer values. Alternate relationships that would be adequate in this region of the flow are, therefore, required for estimating heat and water vapour fluxes by the aerodynamic method or for calculating the aerodynamic resistances above tall canopies. A simple enhancement model may be a suitable basis for such relationships, provided it can be validated against existing or new experimental data. For this purpose, measurements of wind speed, air temperature and humidity profiles were made above a maize crop, along with other standard energy budget measurements. They confirm the existence of a roughness sublayer just above the canopy, in which the profiles depart significantly from the inertial layer similarity forms. This sublayer was found to extend up to about twice the canopy height. The measurements, performed throughout the entire sublayer and above it, give a precise picture of the evolution of the discrepancies with height, which can be interpreted in terms of enhanced diffusivities. Our results allow us to derive simple generalized flux-profile relationships that can be used to calculate the fluxes of heat and water vapour (and any other scalar) from two or more measurement levels, in the same way as with the classical aerodynamic method. These new relationships are shown to be compatible with most of the results already published, obtained over a wide range of canopies. The errors made when using the inertial layer relationships are quantified for typical experimental situations.

We applied a site evaluation approach combining Lagrangian Stochastic footprint modelling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.

Les supraconducteurs sont essentiellement utilises aujourd’hui au plan industriel pour creer des inductions moderees a fortes dans des volumes qui peuvent etre considerables. Ces dispositifs exploitent les grandes densites de courant des supraconducteurs, associees a des pertes nulles en induction constante. Leur utilisation apporte aussi une stabilite temporelle exceptionnelle de l’induction magnetique, indispensable pour certaines applications. Les applications industrielles actuelles des supraconducteurs avec les materiaux « conventionnels », le NbTi et le Nb3Sn sont principalement presentees.

La decouverte de la supraconductivite a ouvert des perspectives extraordinaires en termes d’energie, notamment au niveau du transport et du stockage. De grandes avancees technologiques ont eu lieu dans ce domaine des supraconducteurs, dans les hautes et dans les basses temperatures. Cet article presente les materiaux existants, dans les deux domaines de temperature, et leur environnement. Puis il detaille les applications principales existantes et celles que l'on peut envisager dans l'avenir.

Le materiau supraconducteur a la propriete, dans des conditions donnees de temperature et de densite de courant, de transporter du courant sans dissipation energetique. Cet article s'interesse au supraconducteur avec une structure multifilamentaire torsadee, qui stabilise l'etat supraconducteur et offre un fonctionnement sur et satisfaisant. Il explique l'origine des pertes en courant alternatif, puis aborde la transition et la protection du fil supraconducteur dans un dispositif supraconducteur.

Les supraconducteurs ont la propriete a une temperature donnee dite critique d'etre parfaitement conducteurs de l'electricite. L’etat supraconducteur se distingue de l’etat normal par de nombreuses et diverses proprietes presentees dans cet article. La decouverte des supraconducteurs a haute temperature critique, globalement superieure a 80K, ouvre des debouches commerciaux pour ces materiaux.

The current emphasis on global climate studies has led the scientific community to set up a number of sites for measuring the long-term biosphere-atmosphere net CO2 exchange ( net ecosystem exchange, NEE). Partitioning this flux into its elementary components, net assimilation (F-A), and respiration ( F-R), remains necessary in order to get a better understanding of biosphere functioning and design better surface exchange models. Noting that F-R and F-A have different isotopic signatures, we evaluate the potential of isotopic (CO2)-C-13 measurements in the air ( combined with CO2 flux and concentration measurements) to partition NEE into F-R and F-A on a routine basis. The study is conducted at a temperate coniferous forest where intensive isotopic measurements in air, soil, and biomass were performed in summer 1997. The multilayer soil-vegetation-atmosphere transfer model MuSICA is adapted to compute (CO2)-C-13 flux and concentration profiles. Using MuSICA as a "perfect'' simulator and taking advantage of the very dense spatiotemporal resolution of the isotopic data set ( 341 flasks over a 24-hour period) enable us to test each hypothesis and estimate the performance of the method. The partitioning works better in midafternoon when isotopic disequilibrium is strong. With only 15 flasks, i.e., two (CO2)-C-13 nighttime profiles (to estimate the isotopic signature of F-R) and five daytime measurements ( to perform the partitioning) we get mean daily estimates of F-R and F-A that agree with the model within 15 - 20%. However, knowledge of the mesophyll conductance seems crucial and may be a limitation to the method.

Biological particles dispersion scale in atmosphere is considerably influenced by many parameters, in particular, surface characteristics. A numerical model has been developed to allow a better understanding of the interactions between landscape structure and particles dispersion. This model can be seen as a tool for evaluating the risks of the environmental contamination around a transgenic crops.

A partir d'une enquěte aupres de 280 exurbains des Cantons de l'Est, on a trouve que pour les Montrealais qui comptent pour le tiers des repondants, la distance ne semble pas etre un obstacle et qu'ils preferent des localites eloignees et isolees alors que les exurbains locaux setablissent a proximite de leur ville d'origine. Cinq types d'exurbains ont ete identifies parmi deux courants principaux d'exurbanisation: le premier serait une phase tardive d'urbanisation, une continuation de l'etalement urbain; le deuxieme en serait plutot un d'anti-urbanisation, relie au processus de changement dans les valeurs de notre societe. From a survey of 280 exurbanites in the Eastern Townships, Quebec, it was found that Montrealers made up one-third of the sample. Distance from the city did not seem to be a hindrance to them, since they preferred isolated situations, whereas the local exurbanites were concentrated around their regional towns. Five types of exurbanites were identified among two main trends of exurbanization: the first is a late phase of urbanization, in which urban sprawl is continued; the second is part of a counter-urbanization process related to cultural change.

National audience

International audience

We applied a site evaluation approach combining Lagrangian Stochastic footprint modeling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.