Abstract
AbstractWe investigate the benefits of assimilating in situ and satellite data of the fraction of photosynthetically active radiation (FAPAR) relative to eddy covariance flux measurements for the optimization of parameters of the ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystem) biosphere model. We focus on model parameters related to carbon fixation, respiration, and phenology. The study relies on two sites—Fontainebleau (deciduous broadleaf forest) and Puechabon (Mediterranean broadleaf evergreen forest)—where measurements of net carbon exchange (NEE) and latent heat (LE) fluxes are available at the same time as FAPAR products derived from ground measurements or derived from spaceborne observations at high (SPOT (Satellite Pour l′Observation de la Terre)) and medium (MERIS (MEdium Resolution Imaging Spectrometer)) spatial resolutions. We compare the different FAPAR products, analyze their consistency with the in situ fluxes, and then evaluate the potential benefits of jointly assimilating flux and FAPAR data. The assimilation of FAPAR data leads to a degradation of the model‐data agreement with respect to NEE at the two sites. It is caused by the change in leaf area required to fit the magnitude of the various FAPAR products. Assimilating daily NEE and LE fluxes, however, has a marginal impact on the simulated FAPAR. The results suggest that the main advantage of including FAPAR data is the ability to constrain the timing of leaf onset and senescence for deciduous ecosystems, which is best achieved by normalizing FAPAR time series. The joint assimilation of flux and FAPAR data leads to a model‐data improvement across all variables similar to when each data stream is used independently, corresponding, however, to different and likely improved parameter values.
Highlights
The terrestrial biosphere plays a key role in the control of the exchange of energy and matter between the land surface and the atmosphere [Pielke et al, 1998]
We evaluate the benefit of simultaneously assimilating fraction of photosynthetically active radiation (FAPAR) data, satellite-derived products at high and medium spatial resolution and ground-based measurements, along with flux tower measurements of both carbon and water fluxes using the state-of-the-art mechanistic terrestrial biosphere model ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystems [Krinner et al, 2005])
Our study focuses on two flux tower sites corresponding to deciduous broadleaf (Fontainebleau) and Mediterranean broadleaf evergreen (Puechabon) forests, for which net carbon exchange (NEE) and latent heat (LE) flux measurements are available at a half-hourly time step
Summary
The terrestrial biosphere plays a key role in the control of the exchange of energy and matter (in particular carbon and water) between the land surface and the atmosphere [Pielke et al, 1998]. Data assimilation techniques are increasingly used to reduce these uncertainties by improving the model parameters [Wang et al, 2001; Kaminski et al, 2013] while highlighting possible model deficiencies [Verbeeck et al, 2011; Kuppel et al, 2012; Keenan et al, 2013] In this context, in situ eddy covariance flux measurements have mainly been used to constrain the model parameters controlling the processes of carbon and water exchange [Wang et al, 2001; Braswell et al, 2005; Knorr and Kattge, 2005; Santaren et al, 2007; Moore et al, 2008; Williams et al, 2009; Groenendijk et al, 2011; Kuppel et al, 2014]. Eddy flux data alone may not be sufficient to disentangle different concurrent
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