Other
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esasp.474e...7k&link_type=abstract
Proceedings of the SPECTRA Workshop - The concept of a space-borne Earth Observation Mission addressing the terrestrial componen
Other
Scientific paper
Under the Kyoto-Protocol of the UN climate convention, there is a considerable scope for using natural carbon sinks for compensating CO2 emissions. Those sinks, produced by the land biosphere, must therefore be quantified and monitored efficiently, and at a rather fine spatial scale. The most reliable method to date for quantifying net fluxes of carbon across the land surface relies on inverse modelling of atmospheric tracer transport and achieves a spatial resolution of typically ~1000 km (Kaminski et al., 2001a). Other available data only refer to specific sites (e.g. Valentini et al., 2000), and are therefore difficult to extrapolate in space. Therefore, the demands of reporting land-atmosphere carbon exchanges at the national level can neither be met by inverse atmospheric modelling, nor by direct flux measurements alone. A possible solution to this problem lies in combining inverse atmospheric modelling with models of terrestrial vegetation activity, and with assimilation of satellite data into such models (Knorr &Schulz, 2001). These models can be validated against site data, driven by spatially extensive satellite remote sensing information at a rather high spatial resolution, and be inverted against atmospheric tracer data. This approach has been applied to both a simple (Kaminski et al., 2001b) and a process-based biosphere model (Rayner et al., 2001). To make optimal use of remotely sensed information to provide carbon fluxes at the required high spatial resolution, a three-stage inverse modelling scheme is proposed that uses the adjoint of a process-based model of vegetation-atmosphere carbon fluxes: (1) Site-data of carbon and energy fluxes are used to constrain model parameters and estimate uncertainty ranges (scale ~1-100 m); (2) an assimilation scheme for FAPAR or other biophysical parameters from global-coverage satellite data is used to insure consistency with space observations (scale ~1-10 km); (3) a further assimilation and inverse atmospheric transport scheme uses atmospheric CO2 and O2:N2 measurements such that the results are fully consistent with the atmospheric carbon budget (scale ~1000 km). Within this scheme, the SPECTRA instrument would be ideally suited to provide the basis for extrapolating site-specific measurements of step (1) to the remote-sensing based data assimilation in step (2). It would provide an in-depth analysis of the spectral signatures at the flux sites, and could thus be used for optimal validation of BRDF models used for relating remotely sensed information to biophysical quantities over larger areas. Leaving out the largescale, inverse atmospheric modelling step (3), SPECTRA data could also be used for providing a test case for combined assimilation of site-specific CO2 flux and spectral reflectance data into a process-based biosphere model. Such a scheme would be needed to provide vital information for relating spectral, biophysical and biogeochemical information needed for producing reliable, high-resolution information on carbon sources and sinks of land surfaces.
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