1,721,020 research outputs found
Geometry of the hemispherical radiometric footprint over plant canopies
Full text linkRadiometric measurements of hemispherical surface reflectance and long-wave irradiance are required to quantify the broadband albedo and the outgoing thermal radiation. These observations are typically integrated with eddy covariance measurements of sensible and latent heat fluxes to characterize the surface energy budget. While the aerodynamic footprint has been widely investigated, the geometry of the hemispherical radiometric footprint over plant canopies has been rarely tackled. In the present work, the size and shape of the hemispherical radiometric footprint are formalized for a bare surface and in presence of a vegetation cover. For this purpose, four idealized canopies are analyzed and the dependency of the radiometric footprint on leaf area index and canopy height is explored. Besides, the radiometric footprint is compared with the aerodynamic footprint in conditions of neutral stability. It was observed that almost 100% of the hemispherical radiometric signal originates within a distance of a few radiometer heights, while only about 50–80% of the cumulative aerodynamic signal is generated within a distance of about 20 sensor heights. In order to achieve comparable extensions of the footprint areas, hemispherical radiometric measurements should therefore be taken about 6–15 times higher than turbulent flux ones, depending on the vegetation type. The analysis also highlights that the size of the radiative footprint decreases at increasing leaf area index, whereas the aerodynamic footprint shows an opposite behavior. For the abovementioned reasons, this work may support the interpretation of energy flux measurements and the optimal design of eddy covariance stations located in heterogeneous sites
Footprints and fetches for fluxes over forest canopies with varying structure and density
No full textA stochastic trajectory model was used to estimate scalar flux footprints in neutral stability for canopies of varying leaf area distributions and leaf area indices. An analytical second-order closure model was used to predict mean wind speed, second moments and the dissipation rate of turbulent kinetic energy within a forest canopy. The influence of source vertical profile on the flux footprint was examined. The fetch is longer for surface sources than for sources at higher levels in the canopy. In order to measure all the flux components, and thus the total flux, with a desired accuracy, sources were located at the forest floor in the footprint function estimation. The footprint functions were calculated for five observation levels above the canopy top. It was found that at low observation
heights both canopy density and canopy structure affect the fetch. The higher above the canopy top the flux is measured, the more pronounced is the effect of the canopy structure. The forest fetch for flux measurements is strongly dependent on the required accuracy: The 90% flux fetch is greater by a factor of two or more compared to the 75% fetch. The upwind distance contributing 75% of flux is as large as 45 times the difference between canopy height and the observation height above the canopy top, being even larger for low observation levels
Patterns and trends of the dominant environmental controls of net biome productivity
Full text linkIn the last decades terrestrial ecosystems have reabsorbed on average more than one-quarter of anthropogenic emissions (Le Quéré et al., 2018). However, this large carbon sink is modulated by climate and is therefore highly variable in time and space. The magnitude and temporal changes in the sensitivity of terrestrial CO2 fluxes to climate drivers are key factors to determine future atmospheric CO2 concentration and climate trajectories. In the literature, there is so far a strong focus on the climatic controls of daily and long-term variability, while less is known about the key drivers at a seasonal timescale and about their variation over time (Wohlfahrt et al., 2008). This latter temporal scale is relevant to assess which climatic drivers dominate the seasonality of the fluxes and to understand which factors limit the CO2 exchange during the course of the year. Here, we investigate the global sensitivity of net terrestrial CO2 fluxes, derived from atmospheric inversion, to three key climate drivers (i.e. global radiation and temperature from WFDEI and soil water content from ERA-Interim) from weekly to seasonal temporal scales, in order to explore the short-term interdependence between climate and the terrestrial carbon budget. We observed that the CO2 exchange is controlled by temperature during the carbon uptake period over most of the land surface (from 55 % to 52 % of the total surface), while radiation is the most widespread dominant climate driver during the carbon release period (from 64 % to 70 % of the total surface). As expected, soil water content plays a key role in arid regions of the Southern Hemisphere during both the carbon uptake and the carbon release period. Looking at the decadal trend of these sensitivities (1985–2016) we observed that the importance of radiation as a driver is increasing over time, while we observed a decrease in sensitivity to temperature in Eurasia. Overall, we show that flux temporal variation due to a specific driver has been dominated by the temporal changes in ecosystem sensitivity (i.e. the response of ecosystem to climate) rather than to the temporal variability of the climate driver itself over the last decades. Ultimately, this analysis shows that the ecosystem response to climate is significantly changing both in space and in time, with potential repercussion on the future terrestrial CO2 sink and therefore on the role that land may play in climate trajectories
Patterns and controls of inter-annual variability in the terrestrial carbon budget
Full text linkThe terrestrial carbon
fluxes show the largest variability among the components of the global carbon
cycle and drive most of the temporal variations in the growth rate of
atmospheric CO2. Understanding the environmental controls and trends
of the terrestrial carbon budget is therefore essential to predict the future
trajectories of the CO2 airborne fraction and atmospheric
concentrations. In the present work, patterns and controls of the
inter-annual variability (IAV) of carbon net ecosystem exchange (NEE) have
been analysed using three different data streams: ecosystem-level
observations from the FLUXNET database (La Thuile and 2015 releases), the MPI-MTE (model tree
ensemble) bottom–up
product resulting from the global upscaling of site-level fluxes, and the
Jena CarboScope Inversion, a top–down estimate of surface fluxes obtained
from observed CO2 concentrations and an atmospheric transport model.
Consistencies and discrepancies in the temporal and spatial patterns and in
the climatic and physiological controls of IAV were investigated between the
three data sources. Results show that the global average of IAV at FLUXNET
sites, quantified as the standard deviation of annual NEE, peaks in arid
ecosystems and amounts to ∼ 120 gC m−2 y−1, almost 6
times more than the values calculated from the two global products (15 and
20 gC m−2 y−1 for MPI-MTE and the Jena Inversion,
respectively). Most of the temporal variability observed in the last three
decades of the MPI-MTE and Jena Inversion products is due to yearly
anomalies, whereas the temporal trends explain only about 15 and 20 % of
the variability, respectively. Both at the site level and on a global scale,
the IAV of NEE is driven by the gross primary productivity and in particular
by the cumulative carbon flux during the months when land acts as a sink.
Altogether these results offer a broad view on the magnitude, spatial
patterns and environmental drivers of IAV from a variety of data sources that
can be instrumental to improve our understanding of the terrestrial carbon
budget and to validate the predictions of land surface models
Canopy architecture and turbulence structure in a coniferous forest
No full textSynchronous sonic anemometric measurements at five heights within a mixed coniferous forest were used to test two different parameterisations ofcanopy architecture in the application of a second-order turbulence closure model. Inthe computation of the leaf drag area, the aerodynamic sheltering was replaced with anarchitectural sheltering, assumed to be analogous to the clumping index defined in radiativetransfer theory. Consequently, the ratio of leaf area density and sheltering factor was approximatedby the effective leaf area or the mean contact number, both obtained from the inversion of non-destructive optical measurements. The first parameter represents the equivalentrandomly dispersed leaf area in terms of shading, the second is the average number of leavesthat a straight line intercepts penetrating the canopy with a certain zenith angle. Theselection of this direction was determined by the analysis of the mean angle of the wind vectorduring sweep events. The drag coefficient values obtained from the inversion of themomentum flux equation, using the two proposed parameterisations, are in good agreement withvalues found in the literature. The predicted profiles of turbulence statistics reasonablymatch actual measurements, especially in the case of the mean contact numberparameterisation. The vertical profile of leaf drag area, obtained by forcing the turbulence modelto match the observed standard deviation of vertical velocity (w), is intermediatebetween the two empirical ones. Finally, the proposed canopy parameterisations were appliedto a Lagrangian transport model to predict vertical profiles of air temperature, H2O andCO2 concentration
Quantification, distribution and major predictors of soil N content along a range of forest ecosystems and climatic conditions in Italy
Full text link
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
