MODIS

Table of Contents

Introduction
Algorithm
Data Flow
MOD43B1 BRDF/Albedo Model Parameters Product
MOD43B3 Albedo Product
MOD43B4 NBAR Product
MOD43 Validation
MOD43C1 Albedo CMG
MOD43C2 BRDF CMG
MOD43C3 NBAR CMG
BRDF/Albedo Data
MODIS User Tools
Publications
Related Links

BRDF/Albedo@BU

MODIS BRDF/Albedo Product (MOD43B) User's Guide

Introduction

Due to its three dimensional structure, the Earth's surface scatters radiation anisotropically, especially at the shorter wavelengths that characterize solar irradiance. The Bidirectional Reflectance Distribution Function (BRDF) specifies the behavior of surface scattering as a function of illumination and view angles at a particular wavelength. The albedo of a surface describes the ratio of radiant energy scattered upward and away from the surface in all directions to the downwelling irradiance incident upon the surface. The completely diffuse bihemispherical (or white-sky) albedo can be derived through integration of the BRDF for the entire solar and viewing hemisphere, while the direct beam directional hemispherical (or black-sky) albedo can be calculated through integration of the BRDF for a particular illumination geometry. Actual albedos under particular atmospheric and illumination conditions can be estimated as a function of the diffuse skylight and a proportion between the black-sky and white-sky albedos. The MODerate resolution Imaging Spectroradiometer (MODIS) BRDF/Albedo Science Data Products provides white-sky albedos and black-sky albedos (at local solar noon) as both spectral and broadband quantities.

The MODIS BRDF/Albedo Product also provides Nadir BRDF-Adjusted Reflectances (NBAR) --- surface reflectances corrected to a common nadir view geometry at the mean solar zenith angle of the observation period. NBAR are also being provided in the initial public release (MOD43B4). These anisotropically-corrected surface reflectances can serve as important inputs for studies using vegetation indices and for land cover classification efforts (they are used directly as the primary input to MOD12Q, the MODIS Land Cover/ Land Cover Dynamics Product).

The BRDF specification itself is supplied to the scientific community as a separate product (MOD43B1) since it is useful in specifying a surface radiation scattering model for boundary layer parameterization of surface vegetation atmospheric transfer schemes in global climate models. With the model weighting parameters (fiso, fvol, fgeo) and a simple polynomial equation, black-sky albedo can be realistically estimated at any solar zenith angle a user may require. And, since the BRDF is an intrinsic property characterizing the structure of the surface, the parameters themselves may also provide biophysical information of interest.

Note: Reprocessed (V004) MODIS BRDF/Albedo products from Day 2000065 (5 March 2000 to present) have been assigned a "Validated (Level 1) Status". Users are urged to use the band specific quality flags to isolate the highest quality full inversion results for their own science applications. MOD43B and MOD43C products will continue to be evaluated and validated by the MODIS Science Team and MODIS Validation Scientists. Also note that the operational code continues to be evaluated, refined and updated. Data quality status can be assessed through the MODIS Land Data Operational Product Evaluation (LDOPE) site.

Algorithm

Every 16 days, the operational MODIS BRDF/Albedo algorithm makes use of multidate data and a semiempirical kernel-driven bidirectional reflectance model to determine a global set of parameters describing the BRDF of the land surface (MOD43B1). These one kilometer gridded parameters are then used to determine directional hemispherical reflectance ("black-sky albedo"), and bihemispherical reflectance ("white-sky albedo") for seven spectral bands (MODIS channels 1-7) and three broad bands (0.3-0.7µm, 0.7-5.0µm, and 0.3-5.0µm) at the mean solar zenith of local solar noon (MOD43B3). The nadir BRDF-adjusted surface reflectances (NBAR) for the seven spectral bands are also computed at the mean solar zenith angle of overpass during the 16-day period (MOD43B4).

The operational algorithm [1],[2], [3] relies on a combination of the RossThick-LiSparseReciprocal kernels [5] as the semiempirical model used to invert 16 day's worth of aggregated, atmospherically corrected, 1-km resolution, MODIS directional data and fit a BRDF to each land surface pixel. Broad band values are computed as well [4]. Combined data from the MODIS instruments on board both Aqua and Terra are also being produced (MCD43) and are under evaluation. The semiempirical kernel-driven BRDF model [6] represents the weighted sum of an iso-tropic parameter ( fiso ) and two functions (or kernels) of viewing and illumination geometry. One of these kernels (Kvol ) is derived from volume scattering radiative transfer models [7], while the other (Kgeo ) is derived from surface scattering and geometric shadow casting theory [8]. The BRDF parameters ( fiso , fvol , fgeo ) computed in the operational product are the spectrally dependent weights of each of these kernels used in forming the overall reflectance:

      R = fiso + fvol Kvol + fgeo Kgeo
When insufficient reflectances (currently set to less than seven observations) or a poorly representative sampling of high quality reflectances (as indicated in the quality flags) are available for a full inversion, use is made of a database of archetypal BRDF parameters [9],[10] to supplement the data available and perform a magnitude inversion. This database is currently associated with a static land cover of the globe [11], a seasonal model and average MODIS-derived BRDF parameters for each landcover and season.

The MODIS BRDF/Albedo Science Data Products is provided in a Sinusoidal Grid (SIN) projection with standard tiles representing 1200 by 1200 pixels on the earth [12]. While the projection becomes increasingly sheared with distance from the Greenwich meridian, the equal area properties of SIN mean that it is a good data storage format and it is possible to convert each tile to other, more common projections through the use of any one of a number of commercial or public software packages. These Level-3 one-kilometer MODIS Land (MODLAND) products are being released in Hierarchical Data Format - Earth Observing System (HDF-EOS) for each of the 317 land tiles on the globe (see HDF-EOS FAQ).

Each operational product is associated with extensive quality assurance information so that users can reconstruct the processing methodology used for each tile or pixel if they choose. At a minimum, all MODIS Land products supply a per-pixel quality flag indicating whether the algorithm produced results or not for that pixel and if so, whether the result is of the highest quality and can be used without reservation or whether (due to some uncertainties in the processing) the user should check the extensive additional product-specific quality assurance to make sure the output is appropriate for their application. In addition, each tile of data is accompanied by extensive metadata that provide similar quality assessments of the entire tile. Note that the per-pixel data and the quality information are computed for all land and coastal areas and for shallow water regions (pixels that are within 5 km of land OR are less than 50 meters deep). The products and quality flags are not computed for moderate or deep water regions (pixels greater than 5km from land and with water depths greater than 50m). The EOS land-water mask (which is static at a 1km resolution for Level 1B products) is passed along through the production chain with the reprojection and aggregation of the reflectance data to Level 3 and is stored for the user's convenience as bit flags in the per-pixel quality information associated with each MOD43B product.

References Cited
  1. 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.


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


  3. Wanner, W., A.H. Strahler, B. Hu, P. Lewis, J.-P Muller, X. Li, C. Schaaf, and M.J. Barnsley, Global retrieval of bidirectional reflectance and albedo over land from EOS MODIS and MISR data: Theory and algorithm, J. Geophys. Res., 102, 17143-17161, 1997.


  4. 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.


  5. Wanner, W., X. Li, and A. H. Strahler, On the derivation of kernels for kernel-driven models of bidirectional reflectance, J. Geophys. Res., vol. 100, pp. 21077--21090, 1995.


  6. Roujean, J. L., M. Leroy, and P.Y. Deschamps, A directional reflectance model of the Earth's surface for the correction of remote sensing data, J. Geophys. Res., 20, 455-468, 1992.


  7. Ross, J., The radiation regime and architecture of plant stands, Dr. W. Junk, Norwell, MA, 392 pp, 1981.


  8. Li, X., and A. H. Strahler, Geometric-optical bidirectional reflectance modeling of the discrete crown vegetation canopy: Effects of crown shape and mutual shadowing, IEEE Trans. Geosci. Remote Sens., 30, 276-292, 1992.


  9. Strugnell, N. and W. Lucht, Continental-scale albedo inferred from AVHRR data, land cover class and field observations of typical BRDFs, J. Climate, in press, 2000.


  10. Strugnell, N., W. Lucht and C. Schaaf, A global albedo data set derived from AVHRR data for use in climate simulations, Geophys. Res. Let., in press, 2000.


  11. Loveland, T. R., Z. L. Zhu, D. O. Ohlen, J. F. Brown, B. C. Reed, and L. M. Yang, An analysis of the IGBP global land-cover characterization process, Photogram. Eng. Remote Sens., 65, 1021-1032, 1999.


  12. Wolfe, R. E., D. P. Roy, and E. Vermote, MODIS land data storage, gridding, and compositing methodology: Level 2 grid, IEEE Trans. Geosci. Remote Sens., 36, 1324-1338, 1998.


Data Flow

The Level 2G Surface Reflectance Product (MOD09) for MODIS provides daily, cloud-cleared, atmospherically-corrected surface reflectances. The data from channels 1-7 are aggregated together in to a one kilometer resolution and stored in Level 3 SIN tiles. This aggregation and binning occurs on a daily basis and results in the interim Level 3 product MODAGAGG. In the higher latitudes, as many as four of these aggregated observations will be retained at each pixel for each day. The data from sixteen days worth of MODAGAGG are then used as the primary input for the MOD43B BRDF/Albedo Product. The algorithm fits a BRDF model to these directional surface reflectances and the parameters of the model (RossThick-LiSparseReciprocal) are provided to the community as a Science Data Product (MOD43B1) and are used to compute the albedo measures provided in MOD43B3 and the NBAR values provided in MOD43B4. These 1-km albedos, BRDF parameters and NBARS are used to generate 0.05 degree resolution Climate Modeling Grid (CMG) Products in a global geographic lat/lon projection (MOD43C1, MOD43C2, MOD43C3).
Author:Crystal Schaaf
Last Updated:April 2004