GCOS Implementation Plan - WMO

Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC
(2010 Update)
transmission of solar radiation in the soil-vegetation system. It is both a forcing variable controlling the
climate and a sensitive indicator of environmental degradation. Albedo varies in space and time as a
result of both natural processes (e.g., changes in solar position, snow cover, and vegetation growth)
and human activities (e.g., clearing and planting forests, sowing and harvesting crops, burning
rangeland, etc.).
Land surface albedo is sometimes monitored in situ on a national basis, but these observations are
not coordinated in a global network. Space agencies have been generating global albedo products, in
particular, from the MODIS and MISR sensors from 2000 onwards and from Meteosat since the midnineties.
Future products should be expected to deliver at least the same level of quality, for instance
on the basis of the VIIRS instrument and current or future geostationary platforms, although the
current lack of commitment to launch or to continue to operate multi-angular sensors seriously
hampers the feasibility of delivering high-quality albedo products in the future.
Daily-average surface albedo values have been derived experimentally from a single geostationary
satellite using a state of the art algorithm designed at the European Commission Joint Research
Centre (JRC). In response to a CGMS action, EUMETSAT has started to reprocess archives of
observations from the Meteosat instrument, using that algorithm, in order to generate historical time
series of broadband albedo over long periods of time. A separate preliminary investigation by
EUMETSAT has also shown the feasibility of generating this ECV on a near-global basis, using
multiple geostationary satellites. The Japanese Meteorological Agency has recently expressed
interest in coordinating the reprocessing of existing historical archives to generate such a global
product, over as long a period as possible, in the context of the Sustained Coordinated Processing of
Environmental Satellite Data for Climate Monitoring (SCOPE-CM) initiative.
Mono-angular multi-spectral sensors on polar-orbiting platforms usefully complement this monitoring
system by providing more comprehensive spatial coverage. However, the accuracy of these estimates
needs to be assessed, in particular, with respect to their sensitivity to perturbing factors because the
algorithms used to generate albedo products from these systems typically rely on the accumulation of
data over two weeks or more, when surface properties can change appreciably, e.g., with the
occurrence or disappearance of snow on the ground.
The generic term 'albedo' often refers to a variety of different geophysical variables, which correspond
to different definitions and measurements. Climate models typically require the ratio of the outgoing
flux of radiation over the incoming flux (known as the bihemispherical reflectance factor), in a couple
of broad spectral bands, while actual measurements and currently available derived products
correspond to hemispherical conical reflectance and directional hemispherical reflectance factors in a
limited set of narrow spectral bands. Such products often depend on specific assumptions on the
state of the atmosphere or on the relative contributions of diffuse and direct radiation. As of this
writing, existing products generated by different instruments or space agencies at spatial resolutions
ranging from 1 to 5 km lack consistency and exhibit small but consistent biases that need to be
resolved. This calls for comprehensive evaluation of the corresponding algorithms, the comparison of
these albedo estimates with in situ and other measurements, and the benchmarking and crosscomparison
of these products. Progress along these lines will consolidate confidence in the
algorithms and justify the reprocessing of existing archives to generate long and coherent time series
of global albedo products at the best available resolution. A fully characterised and broadly accepted
global albedo product will be very valuable not only for climate studies but also as a reference for
further studies and as a benchmark for other sensors and instruments.
Some research groups running land surface process models have already begun to assimilate these
new satellite-derived albedo products into their schemes and have noted improvements in the models’
performance. Further collaboration between the scientific communities involved is expected to result
in improved methods and data for assimilation and reanalysis purposes. This goal will also require
extensive algorithm benchmarking and product validation activities, as well as concerted efforts to
archive and make available such standardised albedo products to the weather, climate, and other
scientific communities in a form readily usable by these models.
Ocean albedo is discussed in the Ocean Colour ECV section.
120