UMass Boston

Albedo is defined as the ratio of upwelling to downwelling radiative flux at the surface. Downwelling flux may be written as the sum of a direct component and a diffuse component. Black-sky albedo (directional hemispherical reflectance) is defined as albedo in the absence of a diffuse component and is a function of solar zenith angle. White-sky albedo (bihemispherical reflectance) is defined as albedo in the absence of a direct component when the diffuse component is isotropic. Black-sky albedo and white-sky albedo mark the extreme cases of completely direct and completely diffuse illumination. Actual albedo is a value which is interpolated between these two as a function of the fraction of diffuse skylight which is itself a function of the aerosol optical depth [1], [4], [8]. The underlying assumption of an isotropic distribution of the diffuse skylight is approximate but avoids the expense of an exact calculation while capturing the major part of the phenomenon [2]. However, for large angles and bright surfaces it’s more appropriate to use the full anisotropic expression [8].

The MCD43A3 Albedo Product (MODIS/Terra Albedo Daily L3 Global 500m SIN Grid) provides both the white-sky albedos and the black-sky albedos (at local solar noon) for MODIS bands 1-7 as well as for three broad bands (0.3-0.7µm, 0.7-5.0µm, and 0.3-5.0µm). While the total energy reflected by the earth’s surface in the shortwave domain is characterized by the shortwave (0.3-5.0µm) broadband albedo, the visible (0.3-0.7µm) and near-infrared (0.7-5.0µm) broadband albedos are often also of interest due to the marked difference of the reflectance of vegetation in these two spectral regions. Liang et al. [3], Stroeve et al. [9] used observed spectra and numerical simulations to produce the conversion coefficients for MODIS used by the operational algorithm. One should keep in mind that spectral-to-broadband conversion is a function of atmospheric state to the extent that the spectral distribution of the solar downwelling flux depends on atmospheric properties and the solar zenith angle. The conversion coefficients computed by Liang et al. [3], Stroeve et al. [9] are derived for typical average cases. Variations of the exact results with aerosol optical depth and solar zenith angle are small but affect retrieval accuracies on the level of a few percent [4].

Local Attributes

Mandatory QA values for Collection 6:

          0 = processed, good quality (full BRDF inversions)

          1 = processed, see other QA (magnitude BRDF inversions)

Mandatory QA values for Collection 6.1:

         0 = processed, good quality (full BRDF inversions)

1 = processed, see other QA (magnitude BRDF inversions)

2 = processed, good quality (full BRDF inversions, only Band 6 is fill value due to non-functional or noisy detectors)

3 = processed, see other QA (magnitude BRDF inversions, only Band 6 is fill value due to non-functional or noisy detectors)

4 = processed, good quality (full BRDF inversions, only Band 5 is fill value due to non-functional or noisy detectors)

5 = processed, see other QA (magnitude BRDF inversions, only Band 5 is fill value due to non-functional or noisy detectors)

6 = processed, good quality (full BRDF inversions, both Band5 and Band 6 are fill value due to non-functional or noisy detectors)

7 = processed, see other QA (magnitude BRDF inversions, both Band 5 and Band 6 are fill value due to non-functional or noisy

References Cited

1. Lewis, P., and M. J. Barnsley, Influence of the sky radiance distribution on various formulations of the earth surface albedo, in Proc. Conf. Phys. Meas. Sign. Remote Sens., Val d'Isere, France, pp. 707-715, 1994.

2. Pinker, R. T., and I. Laszlo, Modelling surface solar irradiance for satellite applications on a global scale, J. Appl. Meteorol., 31, pp. 194-211, 1992.

3. Liang, S., A. H. Strahler, and C. W. Walthall, Retrieval of land surface albedo from satellite observations: A simulation study, J. Appl. Meteorol., 38, 712-725, 1999.

4. Schaaf, C. B., F. Gao, A. H. Strahler, W. Lucht, X. Li, T. Tsang, N. C. Strugnell, X. Zhang, Y. Jin, J.-P. Muller, P. Lewis, M. Barnsley, P. Hobson, M. Disney, G. Roberts, M. Dunderdale, C. Doll, R. d'Entremont, B. Hu, S. Liang, and J. L. Privette, First Operational BRDF, Albedo and Nadir Reflectance Products from MODIS, Remote Sens. Environ., 83, 135-148, 2002.

5. Lucht, W., C.B. Schaaf, and A.H. Strahler. An Algorithm for the retrieval of albedo from space using semi empirical BRDF models, IEEE Trans. Geosci. Remote Sens., 38, 977-998, 2000.

6. Lucht, W., Expected retrieval accuracies of bidirectional reflectance and albedo from EOS-MODIS and MISR angular sampling, J. Geophys. Res., 103, 8763-8778, 1998.

7. Lucht, W., and P. Lewis. Theoretical noise sensitivity of BRDF and albedo retrieval from the EOS-MODIS and MISR sensors with respect to angular sampling, Int. J. Remote Sensing, 21, 81-98, 2000.

8. Román, M. O., C. B. Schaaf, P. Lewis, F. Gao, G. P. Anderson, J. L. Privette, A. H. Strahler, C. E. Woodcock, M. Barnsley, Assessing the coupling between surface albedo derived from MODIS and the fraction of diffuse skylight over spatially-characterized landscapes, Remote Sensing of Environment, 114, 738-760,2010.

9. Stroeve, J., J. Box, F. Gao, S. Liang, A. Nolin, C. Schaaf, Accuracy Assessment of the MODIS 16-day Albedo Product for Snow: Comparisons with Greenland in situ Measurements, Remote Sens. Environ., 94, 46-60, doi:10.1016/j.rse.2004.09.001, 2005.

Science Data Sets

While the full MCD43A3 specification should be consulted for the most current description, the product includes two Science Data Sets (SDS) for each pixel in the tile. These SDSs are Albedo (containing both black-sky and white-sky values for 10 bands -- 7 spectral and 3 broad) and BRDF_Albedo_Band_Mandatory_Quality (containing 1 byte of quality flags).

The Albedo SDS is specified as:

•  Data Field Name: INT16 Albedo ("YDim", "XDim", "Num_Albedo_Bands", "Num_Albedos")

•  Description: Black-Sky Albedo (at local solar noon) and white-sky albedo for bands 1-7, and the vis, NIR and SW broadbands

• Data conversions:

               file data=(Albedo / scale_factor) + add_offset
               Albedo=(file data - add_offset)*scale_factor

The BRDF_Albedo_Band_Mandatory_Quality SDS is defined in the specification (V006 and V006.1)

Note that the V006 and V006.1 MODIS MCD43 products are retrieved daily and represent the best BRDF possible based on 16 days’ worth of inputs with the day of interest emphasized. Unlike the earlier reprocessed versions (where the date of the product signifies the first day of the retrieval period), and the Direct Broadcast version (where the date signifies the last day of the retrieval period), the date associated with each daily V006 and V006.1 retrieval is the center of the moving 16 day input window. This change is in response to user requests.

Local Attributes

In addition to the actual SDS data values produced at each pixel in a tile, each SDS is associated with a number of standard Local Attributes that apply to the data. For the Albedo SDS they include:

And for the BRDF_Albedo_Band_Mandatory_Quality SDS see specification (V006 and V006.1)

Global Attributes (Metadata)

In addition to the albedo and quality information (SDSs and Local Attributes) that is provided at a per-pixel level, each tile of the Level 3 MCD43A3 Parameters Product also includes three types of Global Attributes or Metadata which summarize the tile. These are Core, Archive, and Structural Metadata. Of particular interest to the user community are the Core Metadata QAFlags of SCIENCEQUALITYFLAG and SCIENCEQUALITYFLAGEXPLANATION and some of the Additional Attributes which identify the tile number or which summarize the quality of the product over that entire tile (see the full MCD43A1 specification (V006 and V006.1) for the complete listing).