Impacts of forest inhomogeneities in edge flow: a momentum and turbulent kinetic energy budget analysis

International audienceForest landscapes are often characterized by multiple forest edges modifying the airflow exchanges with the atmosphere. Most of our knowledge on edge flow comes from numerical and wind tunnel experiments from which the canopy was approximated horizontally homogeneous. To investigate the impacts of tree-scale inhomogeneities on the wind dynamics along a forest edge, large-eddy simulations have been performed over inhomogeneous canopies. The three-dimensional forest structure is prescribed in the model from high resolution helicopter-based LiDAR scans of the Falster island forest in Denmark. Several simulations were performed to study the effect of different spatial configurations of the canopy structure on the flow. The simulations show that inhomogeneities induce significant dispersive fluxes at the forest leading-edge. These fluxes result from the spatial-variability of the flow as it penetrates through the sparsest regions of the edge. We show how the inhomogeneities change the momentum and energy transfers along the edge by performing a detailed momentum and turbulent kinetic energy budget analysis. The magnitude of the dispersive fluxes is then compared following the degree of inhomogeneities of the canopy

Spatial and temporal variability of surface temperature of a forest canopy

Acquiring the surface temperature is important in various research fields needing to quantify the surface energy balance (e.g. meteorology, hydrology, climatology, etc.). For such task, high-spatial resolution satellite thermal infrared (TIR) measurements are promising, but due to constraints in the specifications of space missions, TIR satellite images are prone to temporal and spatial variability in the measurement acquisition in relation with the rapid feedback between the surface temperature and atmospheric turbulence. This variability in measurement acquisition may limit the data accuracy. During daytime, the fluctuations of surface temperature are governed by large-scale convective eddy motions in the outer atmosphere and local coherent eddies forming over vegetation canopies. To estimate the possible error related to these fluctuations, it is necessary to understand the extent of their length- and time-scales. With such motivation, an experiment was conducted in 2015 in the Landes forest in France. A TIR camera overlooking a maritime pine forest stand (15-20 m high) was set up to acquire the surface temperatures at high spatial and temporal resolutions (≈10 m, 10 Hz), together with wind velocity from a sonic anemometer. A scaling analysis of the measurements (Fourier, wavelets, etc.) is performed to identify the scaling regime of the surface temperature fluctuations following the atmospheric stability. We discuss these results in the context of satellite thermal imagery uncertainties

A LiDAR method of canopy structure retrieval for wind modeling of heterogeneous forests

The difficulty of obtaining accurate information about the canopy structure is a current limitation towards higher accuracy in numerical predictions of the wind field in forested terrain. The canopy structure in computational fluid dynamics is specified through the frontal area density and this information is required for each grid point in the three-dimensional computational domain. By using raw data from aerial LiDAR scans together with the Beer-Lambert law, we propose and test a method to calculate and grid highly variable and realistic frontal area density input. An extensive comparison with ground-based measurements of the vertically summed frontal area density (or plant area index) and tree height was used to optimize the method, both in terms of plant area index magnitude and spatial variability. The resolution of the scans was in general low (<2.5 reflections m(-2)). A decrease of the resolution produced an increasing systematic underestimation of the spatially averaged tree height, whereas the mean plant area index remained insensitive. The gridded frontal area density and terrain elevation were used at the lower boundary of wind simulations in a 5 km x 5 km area of a forested site. The results of the flow simulations were compared to wind measurements using a vertical array of sonic anemometers. A good correlation was found for the mean wind speed of two contrasting wind directions with different influences from the upstream forest. The results also predicted a high variability on the horizontal and vertical mean wind speed, in close correlation with the canopy structure. The method is a promising tool for several computational fluid dynamics applications requiring accurate predictions of the near-surface wind field. (C) 2014 The Authors. Published by Elsevier B.V

Inter-Comparison Campaign of Solar UVR Instruments under Clear Sky Conditions at Reunion Island (21 • S, 55 • E)

International audienceMeasurement of solar ultraviolet radiation (UVR) is important for the assessment of potential beneficial and adverse impacts on the biosphere, plants, animals, and humans. Excess solar UVR exposure in humans is associated with skin carcinogenesis and immunosuppression. Several factors influence solar UVR at the Earth's surface, such as latitude and cloud cover. Given the potential risks from solar UVR there is a need to measure solar UVR at different locations using effective instrumentation. Various instruments are available to measure solar UVR, but some are expensive and others are not portable, both restrictive variables for exposure assessments. Here, we compared solar UVR sensors commercialized at low or moderate cost to assess their performance and quality of measurements against a high-grade Bentham spectrometer. The inter-comparison campaign took place between March 2018 and February 2019 at Saint-Denis, La Réunion. Instruments evaluated included a Kipp&Zonen UVS-E-T radiometer, a Solar Light UV-Biometer, a SGLux UV-Cosine radiometer, and a Davis radiometer. Cloud fraction was considered using a SkyCamVision all-sky camera and the Tropospheric Ultraviolet Visible radiative transfer model was used to model clear-sky conditions. Overall, there was good reliability between the instruments over time, except for the Davis radiometer, which showed dependence on solar zenith angle. The Solar Light UV-Biometer and the Kipp&Zonen radiometer gave satisfactory results, while the low-cost SGLux radiometer performed better in clear sky conditions. Future studies should investigate temporal drift and stability over time