| 1. |
- Groenendijk, M., et al.
(författare)
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Seasonal variation of photosynthetic model parameters and leaf area index from global Fluxnet eddy covariance data
- 2011
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Ingår i: Journal of Geophysical Research. - 2156-2202. ; 116, s. 04027-04027
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Tidskriftsartikel (refereegranskat)abstract
- Global vegetation models require the photosynthetic parameters, maximum carboxylation capacity (V(cm)), and quantum yield (alpha) to parameterize their plant functional types (PFTs). The purpose of this work is to determine how much the scaling of the parameters from leaf to ecosystem level through a seasonally varying leaf area index (LAI) explains the parameter variation within and between PFTs. Using Fluxnet data, we simulate a seasonally variable LAI(F) for a large range of sites, comparable to the LAI(M) derived from MODIS. There are discrepancies when LAI(F) reach zero levels and LAI(M) still provides a small positive value. We find that temperature is the most common constraint for LAI(F) in 55% of the simulations, while global radiation and vapor pressure deficit are the key constraints for 18% and 27% of the simulations, respectively, while large differences in this forcing still exist when looking at specific PFTs. Despite these differences, the annual photosynthesis simulations are comparable when using LAI(F) or LAIM (r(2) = 0.89). We investigated further the seasonal variation of ecosystem-scale parameters derived with LAI(F). V(cm) has the largest seasonal variation. This holds for all vegetation types and climates. The parameter alpha is less variable. By including ecosystem-scale parameter seasonality we can explain a considerable part of the ecosystem-scale parameter variation between PFTs. The remaining unexplained leaf-scale PFT variation still needs further work, including elucidating the precise role of leaf and soil level nitrogen.
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| 2. |
- Gioli, B, et al.
(författare)
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Comparison between tower and aircraft-based eddy covariance fluxes in five European regions
- 2004
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 127:1-2, s. 1-16
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Tidskriftsartikel (refereegranskat)abstract
- Airborne eddy covariance measurements provide a unique opportunity to directly measure surface energy, mass and momentum fluxes at the regional scale. This offers the possibility to complement the data that are obtained by the ground-based eddy covariance networks and to validate estimates of the surface fluxes that can be obtained by means of satellite products and models. The overall accuracy and the reliability of airborne eddy covariance measurements have already been assessed in the past for different platforms. More recently an international collaboration between several research laboratories and a European aeronautical manufacturer led to the development of a new small environmental research aircraft, called the Sky Arrow ERA (Environmental Research Aircraft). This aircraft has been used in the framework of the European Research Project RECAB (Regional Assessment and Modelling of the Carbon Balance in Europe), that is part of the CarboEurope projects cluster, to measure surface mass and energy exchange at five different European locations. An extensive comparison between airborne and ground-based flux data at seven flux measurement sites, showed the overall matching between airborne and tower data. While friction velocity and latent heat flux estimates made by airborne and tower data were comparable at all sites and under whatever conditions, substantial and consistent underestimation of CO2 (28% on average) and sensible heat fluxes (35% on average) was observed. Differences in the aircraft and tower footprint and flux divergence with height explained most of the discrepancies. (C) 2004 Elsevier B.V. All rights reserved.
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| 3. |
- Darbieu, C., et al.
(författare)
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Turbulence vertical structure of the boundary layer during the afternoon transition
- 2015
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 15:17, s. 10071-10086
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the decay of planetary boundary layer (PBL) turbulence in the afternoon, from the time the surface buoyancy flux starts to decrease until sunset. Dense observations of mean and turbulent parameters were acquired during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field experiment by several meteorological surface stations, sounding balloons, radars, lidars and two aircraft during the afternoon transition. We analysed a case study based on some of these observations and large-eddy simulation (LES) data focusing on the turbulent vertical structure throughout the afternoon transition. The decay of turbulence is quantified through the temporal and vertical evolution of (1) the turbulence kinetic energy (TKE), (2) the characteristic length scales of turbulence and (3) the shape of the turbulence spectra. A spectral analysis of LES data, airborne and surface measurements is performed in order to characterize the variation in the turbulent decay with height and study the distribution of turbulence over eddy size. This study highlights the LES ability to reproduce the turbulence evolution throughout the afternoon. LESs and observations agree that the afternoon transition can be divided in two phases: (1) a first phase during which the TKE decays at a low rate, with no significant change in turbulence characteristics, and (2) a second phase characterized by a larger TKE decay rate and a change in spectral shape, implying an evolution of eddy size distribution and energy cascade from low to high wave number. The changes observed either in TKE decay (during the first phase) or in the vertical wind spectra shape (during the second phase of the afternoon transition) occur first in the upper region of the PBL. The higher within the PBL, the stronger the spectra shape changes.
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| 4. |
- Yao, Yunjun, et al.
(författare)
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Estimation of high-resolution terrestrial evapotranspiration from Landsat data using a simple Taylor skill fusion method
- 2017
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 553, s. 508-526
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Tidskriftsartikel (refereegranskat)abstract
- Estimation of high-resolution terrestrial evapotranspiration (ET) from Landsat data is important in many climatic, hydrologic, and agricultural applications, as it can help bridging the gap between existing coarse-resolution ET products and point-based field measurements. However, there is large uncertainty among existing ET products from Landsat that limit their application. This study presents a simple Taylor skill fusion (STS) method that merges five Landsat-based ET products and directly measured ET from eddy covariance (EC) to improve the global estimation of terrestrial ET. The STS method uses a weighted average of the individual ET products and weights are determined by their Taylor skill scores (S). The validation with site-scale measurements at 206 EC flux towers showed large differences and uncertainties among the five ET products. The merged ET product exhibited the best performance with a decrease in the averaged root-mean-square error (RMSE) by 2–5 W/m2 when compared to the individual products. To evaluate the reliability of the STS method at the regional scale, the weights of the STS method for these five ET products were determined using EC ground-measurements. An example of regional ET mapping demonstrates that the STS-merged ET can effectively integrate the individual Landsat ET products. Our proposed method provides an improved high-resolution ET product for identifying agricultural crop water consumption and providing a diagnostic assessment for global land surface models.
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