GCOS Implementation Plan - WMO

Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC
(2010 Update)
better (modern sensors offer the possibility of systematically generating global products at a spatial
resolution of about 300 m).
FAPAR is recovered from a range of sensors by various algorithms using the visible and infrared parts
of the spectrum, and the accuracy and reliability of these products is not always properly documented.
Currently available products have been shown to exhibit significant differences, which detract from
their usefulness in downstream applications. The CEOS WGCV, in collaboration with GCOS and
GTOS, should lead the comparison and evaluation of these FAPAR products as well as the
benchmarking of the algorithms used to generate them. Reference sites making in situ observations
should be fully engaged in this process, and it would be desirable if these sites were collocated with
the terrestrial reference sites proposed in Action T3, provided that these sites offer a reasonable
degree of spatial homogeneity over spatial scales comparable to the resolution of the sensors. WGCV
is identifying a core set of sites and measurement campaigns, which should be supported by the
CEOS agencies and by national research budgets.
Relatively homogeneous sites, at scales comparable to the typical spatial resolution of modern
sensors, are preferable, but a detailed characterisation of the spatial variability of those sites is
required.
Action T29 [IP-04 T29] 98
Action: Establish a calibration/validation network of in situ reference sites for FAPAR and LAI and
conduct systematic, comprehensive evaluation campaigns to understand and resolve differences
between the products and increase their accuracy.
Who: Parties’ national and regional research centres, in cooperation with space agencies coordinated
by CEOS WGCV, GCOS and GTOS.
Time-Frame: Network operational by 2012.
Performance Indicator: Data available to analysis centres.
Annual Cost Implications: 1-10M US$ (40% in non-Annex-I Parties).
ECV – Leaf Area Index (LAI)
LAI measures the amount of plant leaf material in an ecosystem. It is typically expressed as a nondimensional
value giving the number of square metres of leaf material per square metre of ground.
This variable plays important roles in models that represent processes, such as photosynthesis,
respiration and rain interception, that couple vegetation to the climate system through the radiation,
carbon, and water cycles. Hence, LAI appears as a key variable in many models describing
vegetation-atmosphere interactions.
LAI can be estimated in situ by destructive sampling or with the help of commercially available
dedicated instruments. It is routinely measured at a number of research sites dealing with surface
climate, ecological, or agricultural issues. CEOS WGCV is playing a coordinating role in this work.
Benchmarking and consistency checking are required for the global archive of LAI measurements.
For reasons set out below, in some parts of the globe (e.g., in the humid Tropics), LAI can only be
measured by in situ methods. However, the measurement network is sparse in many regions of the
world. It should be maintained and ideally expanded to become much more representative of the
diversity of ecosystem conditions. The development and maintenance of reference sites to address
this inadequacy should be addressed, as explained elsewhere in this document. Building on existing
networks, such as FLUXNET, LAInet and BIGFOOT, is a possible way to improve this situation (see
Action T3). The CEOS WGCV has begun to coordinate this through the creation of a centralized
database, an activity that should continue.
The retrieval of reliable LAI estimates from space remains difficult. When the canopy cover is sparse,
reflectance measurements are dominated by soil properties, and the accuracy of the LAI is low. When
LAI values exceed 3 or 4, optical measurements lose their sensitivity to changes in LAI (signal
saturation. Also, since the LAI measured by satellites is usually inferred from spectral reflectances in
the visible and infrared spectrum, it is de facto coupled to FAPAR estimates, even though both
variables are in principle independent and play quite different roles in the climate system.
Nonetheless, regular global LAI estimates from space are currently being produced, and this effort
98 See Action T3.
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