Abstract
The eddy-covariance technique measures carbon, water, and energy fluxes between the land surface and the atmosphere at several hundreds of sites globally. Collections of standardised and homogenised flux estimates such as the LaThuile, Fluxnet2015, National Ecological Observatory Network (NEON), Integrated Carbon Observation System (ICOS), AsiaFlux, and Terrestrial Ecosystem Research Network (TERN) / OzFlux data sets are invaluable to study land surface processes and vegetation functioning at the ecosystem scale. Space-borne measurements give complementary information on the state of the land surface in the surroundings of the towers. They aid the interpretation of the fluxes and support the training and validation of ecosystem models. However, insufficient quality, frequent and/or long gaps are recurrent problems in applying the remotely sensed data and may considerably affect the scientific conclusions drawn from them. Here, we describe a standardised procedure to extract, quality filter, and gap-fill Earth observation data from the MODIS instruments and the Landsat satellites. The methods consistently process surface reflectance in individual spectral bands, derived vegetation indices and land surface temperature. A geometrical correction estimates the magnitude of land surface temperature as if seen from nadir or 40° off-nadir. We offer to the community pre-processed Earth observation data in a radius of 2 km around 338 flux sites based on the MCD43A4/A2, MxD11A1 MODIS products and Landsat collection~1 Tier1 and Tier2 products. The data sets we provide can widely facilitate the integration of activities in the fields of eddy-covariance, remote sensing and modelling.
Highlights
Collections of standardised and homogenised flux estimates such as the LaThuile, Fluxnet2015, National Ecological Observatory Network (NEON), Integrated Carbon Observation System (ICOS), AsiaFlux, and Terrestrial Ecosystem Research Network (TERN) / OzFlux data sets are invaluable to study land surface processes and 5 vegetation functioning at the ecosystem scale
We offer to the community pre-processed Earth observation data in a radius of 2 km around 338 flux sites based on the MCD43A4/A2, MxD11A1 Moderate Resolution Imaging Spectroradiometer (MODIS) products and Landsat collection 1 Tier1 and Tier2 products
In order to partly account for variability in the observed land surface temperature (LST) that is related to changing observation geometry (Rasmussen et al, 2011; Guillevic et al, 2013; Ermida et al, 2014), a correction approach developed by Ermida et al (2018) estimates an LST offset as if the instrument would measure from directly above a site
Summary
The installation and maintenance of instrumental infrastructure at eddy-covariance (EC) sites worldwide require considerable financial and logistical efforts and labour force. We apply them to official data products from the Moderate Resolution Imaging 40 Spectroradiometer (MODIS) instruments and the sensors on board the Landsat satellites Both MODIS and Landsat have long observational coverage with a high temporal overlap with most freely available EC records. We 45 process EO data sets of both surface reflectance and land surface temperature (LST) for a limited area around a given flux site For both the quality control and the gap-filling, the approaches aim to be generalisable across all sites without accounting for specific local conditions, yet flexible enough to accurately reproduce phenological behaviour and characteristic features such as disturbances or fast transitions in managed ecosystems. 50 Observation geometries need special attention as the MODIS instruments measure in a wide swath to obtain high temporal coverage They scan across their track from right to left with view zenith angles up to 65 degree from nadir.
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