Albedo - a

Definition

The ratio of the light reflected to the light received by a body. Values for albedo range from 0 (pitch black) to 1 (perfect reflector). Surface albedo is often calculated for clear-sky conditions.

Information

Surface albedo is fundamental to set up surface energy balance, as it describes the ratio of the downwelling to upwelling shortwave radiative fluxes at the surface. The most important parameters influencing the earth´s climate and radiation budget are albedo and cloud. Surface albedo is prone to seasonal changes and interannual variability (Jacobs and van Pul 1990, Grant et al. 2000). Site specific surface albedo data have been obtained from in situ observations (Kondratyev 1972). These measurements do often have a high temporal resolution. In recent time space-borne measurements by means of remote-sensing techniques are becoming more and more important (e.g. Li and Garand, 1994). Nevertheless, these measuremnts should be calibrated by in situ observations. Therefore, the two types of measurements are complementary.

Current global data sets on surface albedo have a relative discrepancy of 20-30% in low- and mid-latitudes and over 50% in polar regions (Li et al. 1997). There exists essentially two types of global surface albedo data sets. One is obtained by combining global cover type information with the typical albedo values obtained for each cover type (Wilson and Henderson-Sellers, 1985). Such data sets generally do not contain sufficient temporal resolution and lack information on the interannual variability. The other is derived from clear-sky satellite measurements (Li and Garand 1994), which are hindered by the presence of clouds and insufficient knowledge of the atmospheric state. The major limitation for both types of data is the lack of spectral variation that is required by most models. (see als http://www.wmo.ch/web/gcos/terre/variable/albedo.html.)

Eco-hydrological models do require single albedo values as model input parameters. Global data sets are hence not very useful in this respect. Detailed data sets for single plant species or soil types have been intensively determined during the 60s and 70s and were published in various text books or reviews. Information in the PlaPaDa data set were collected from these sources and supplemented by recently published values. Albedo is most often depending for example on external driving forces such as sun angle, moisture content,season, plant stress etc. Therefore, PlaPaDa distinguishes between minimum and maximum values presented in literature. For the sake of statistical interpretation, cited values were taken as minimum and maximum value in cases where only one measurement value was given in literature (Breuer et al. 2002).

References

• Breuer L, Eckhardt K, Frede H-G, 2003. Plant parameter values for models in temperate climates. Ecol Model 169, 237-293.

• Grant IF, Prata AJ, Cechet RP, 2000. The impact of the diurnal variation of albedo on the remote sensing of the daily mean albedo of grassland. J Appl Met 39, 231-244.
• Jacobs AFG, van Pul WAJ, 1990. Seasonal changes in the albedo of a maize crop during two seasons. Agric For Met 49, 351-360.
  
• Kondratyev KY 1972. Radiation processes in the atmosphere, No. 309. Geneva, WMO.
• Li Z, Garand L, 1994. Estimation of Surface Albedo from Space: a Parameterization for Global Application. J Geophys Res 99, 8335-8350.
• Li Z, Moreau L, Arking A, 1997. On solar energy disposition, A perspective from observation and modeling. Bull Amer Meteor Soc 78, 53-70.
• Wilson MF, Henderson-Sellers A, 1985. A global archive of land cover and soils data for use in general circulation models. J Climatol 5, 119-143.

Links

Albedo database
Geographic Databases for Environmental Studies (Department of Data Base Research, Chiba University)

Radiation Products: Solar Radiation Budget
CCRS, Canadian Centre for Remote Sensing