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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; 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.

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; 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.

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.

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.