GOSIC Banner
 
Facilitating Access to Global Observing Systems Data and Information

GCOS Terrestrial ECV T8
Albedo & Reflectance Anisotropy

Definition: Albedo is the fraction of solar energy that is diffusely reflected back from Earth to space. Measurements of albedo are essential for climate research studies and investigations of the Earth’s energy budget. Different parts of the Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the Sun’s energy. Deserts, ice and clouds, however, have high albedos; they reflect a large portion of the incoming solar energy. The high albedo of ice helps to insulate the polar oceans from solar radiation. Over the whole surface of the Earth, about 30% of incoming solar energy is reflected back to space. Because a cloud usually has a higher albedo than the surface beneath it, clouds reflect more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. Hence, this ‘cloud albedo forcing’, taken by itself, tends to cause a cooling or ‘negative forcing’ of the Earth’s climate.

Introduction: Terrestrial albedo is a joint property of the land surface and of the overlying atmosphere; it controls the 'supply' side of the surface radiation balance and is required to estimate the net absorption and 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 mid-nineties. 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 cross-comparison 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.

Comparing albedo products measured in situ with those derived from space platforms is fraught with difficulties because of the large differences in scale and spatial and temporal resolution, differences in measurement protocols, and other practical issues. Nevertheless, such efforts must be pursued to ensure that existing products remain clearly linked to in situ measurements, to allow comparisons between similar products derived from different instruments, to evaluate the quality of new products as they become available, and to test the performance of algorithms after they have been updated.

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Observations: Surface albedo can be estimated from shortwave, broadband or multi-spectral radiometer measurements with good horizontal resolution. Current measurements of albedo and reflectance are obtained primarily using multi-spectral imagers such as AATSR, AVHRR, MODIS, MERIS, Vegetation and instruments on some geostationary satellites (such as MSG). Clouds, aerosols and atmospheric gases affect the achievable accuracy, which is currently marginal to acceptable, but should improve as progress is made in interpreting data from high resolution, multi-spectral instruments. Surface conditions (moisture, surface vegetation, snow cover etc.) strongly affect albedo and high quality ground truth data is necessary in support of satellite measurements. Better understanding of the reflectance properties of different surfaces and more accurate aerosol data (to correct atmospheric effects) are needed to improve surface reflectance measurements. As aerosol concentration increases within a cloud, more cloud droplets form. Since the total amount of condensed water in a cloud does not change much, the average droplet becomes smaller. This has two consequences: clouds with smaller droplets reflect more sunlight and such clouds last longer. Both effects increase the amount of sunlight that is reflected to space without reaching the surface. The Terra spacecraft is yielding greater knowledge of such cloud/aerosol effects, with MODIS and MISR providing data on cloud features, and ASTER providing complementary, high spatial resolution measurements. Terra’s data provide new insights into how clouds modulate the atmosphere and surface temperature. Further multi-directional and polarimetric instruments (e.g. POLDER) also provide measurements leading to better estimates of albedo. New sensors, such as GERB and SEVIRI on board the MSG missions (starting with Meteosat-8) are providing improved capabilities for measuring surface albedo. Improved sounder performance will yield more information on the infrared surface emissivity spectrum. Multi-spectral imaging sensors such as AVHRR/3, IVISSR and AWIFS will provide global visible, near-infrared and infrared imagery of clouds, ocean and land surfaces. CEOS has undertaken to improve the continuity of terrestrial climate monitoring through enhancements to the moderate-resolution historical record. AVHRR data reprocessing will be undertaken to ensure a consistent data set to contribute to historical albedo. CEOS will also work to enhance the quality of the Fundamental Climate Data Records generated from the AVHRR record. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement)

References:

Data, Product, Metadata and Information Access

[ECV Matrix Main Page] [About the ECV Matrix] [Reference Documents] [Contact] [Updated June 6, 2010]

Non-satellite or in-situ Satellite