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

MOD43B1 BRDF/Albedo Model Parameters Product

The MOD43B1 BRDF/Albedo Model Parameters Product (MODIS/Terra BRDF/Albedo Model_1 16-Day L3 Global 1km SIN Grid) supplies the weighting parameters associated with the RossThickLiSparseReciprocal BRDF model that best describes the anisotropy of each pixel. These three parameters ( f_iso, f_vol, f_geo) are provided for each of the MODIS spectral bands as well as for three broad bands (0.3-0.7µm, 0.7-5.0µm, and 0.3-5.0µm). These parameters can be used in a forward version of the model to reconstruct the surface anisotropic effects and thus correct directional reflectances to a common view geometry (this is the procedure that is used to produce MOD43B4 Nadir BRDF-Adjusted Reflectances - NBAR) or to compute the integrated black-sky (at some solar zenith angle) and white-sky albedos (as is done for MOD43B3). Alternately, the parameters can be used with a simple polynomial to easily estimate the black-sky albedo with good accuracy for any desired solar zenith angle. The polynomial is as follows:

The appropriate constants are:

Term	Isotropic (iso)	RossThick (vol)	LiSparseR (geo)
g₀	1.0	-0.007574	-1.284909
g₁	0.0	-0.070987	-0.166314
g₂	0.0	0.307588	0.041840

Similarly, the white-sky albedo can be computed by using the equation:

and the estimates of the white-sky kernel integrals are:

Term Isotropic (iso) RossThick (vol) LiSparseR (geo)

White-sky
integral g 1.0 0.189184 -1.377622

In addition to the model parameters, the MOD43B1 BRDF/Albedo Model Parameters Product also provides extensive quality information. Band-averaged quality information and band-independent data are stored in the first 32-bit word, while band-specific quality information and inversion status information are stored in the second 32-bit word [1],[2],[3],[4].

Note: Reprocessed (V004) MODIS BRDF/Albedo products from Day 2000065 (5 March 2000) 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.

References Cited

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,and D. P. Roy, First Operational BRDF, Albedo and Nadir Reflectance Products from MODIS, Remote Sens. Environ., 83, 135-148, 2002.

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.

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

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.

Science Data Sets

While the full MOD43B4 specification should be consulted for the most current description, the product includes four Science Data Sets (SDS) for each pixel in the tile. These SDSs are BRDF_Albedo_Parameters (three model parameters representing fiso, fvol, fgeo for the RossThickLiSparseReciprocal BRDF model that are computed for bands 1-7 and three broad bands), the BRDF_Shape_Indicators (containing 8 computed shape factors), BRDF_Albedo_Quality (containing two 32-bit words of quality flags) and BRDF_Type (containing descriptors of the BRDF model used).

The BRDF_Albedo_Parameters SDS is specified as:

Data Field Name: INT16 BRDF_Albedo_Parameters ("YDim", "XDim", "Num_Land_Bands_Plus3", "Num_Parameters")

Description: BRDF and albedo model parameters (fiso, fvol, fgeo) for bands 1-7, and the vis, NIR and SW broadbands
Data conversions:
file data=(BRDF_Albedo_Parameters / scale_factor) + add_offset
BRDF_Albedo_Parameters=(file data - add_offset)*scale_factor

The BRDF_Shape_Indicators SDS is specified as:

Data Field Name: UINT8 BRDF_Shape_Indicators ("YDim", "XDim", "Num_Shape_Fields")

Description: Provides simple indicators of BRDF shape and properties
Num_Shape_Fields #1 is the red band ratio of nadir to 45 degree forward scattering at 45 degrees sun angle.

Num_Shape_Fields #2 is the NIR band ratio of nadir to 45 degree forward scattering at 45 degrees sun angle.

Num_Shape_Fields #3 is the red band ratio of 45 degree backward scattering to 45 degree forward scattering at 45 degrees sun angle.
(anisotropy index (here denoted as ANIX_red))

Num_Shape_Fields #4 is the NIR band ratio of 45 degree backward scattering to 45 degree forward scattering at 45 degrees sun angle. (denoted as ANIX_nir)

Num_Shape_Fields #5 is the red band ratio of white-sky albedo to BRDF isotropic parameter value.

Num_Shape_Fields #6 is the NIR band ratio of white-sky albedo to BRDF isotropic parameter value.

Num_Shape_Fields #7 is normalized difference anisotropy index (NDAX) at 45 degrees sun angle.

NDAX=(ANIX_red-ANIX_nir)/(ANIX_red+ANIX_nir)
Num_Shape_Fields #8 void, reserve for future use

(Nore: All shape indicators are only applied to the pixels with full inversion)

Data conversions:
file data=(BRDF_Shape_Indicators * scale_factor) + add_offset
BRDF_Shape_Indicators=(file data - add_offset)/scale_factor

The BRDF_Albedo_Quality SDS is specified as:

Data Field Name: UINT32 BRDF_Albedo_Quality ("YDim","XDim", "Num_QC_Words")

Description: BRDF and albedo quality information

Num_QC_Words #1 holds band-averaged quality and inversion status information and band-independent data (bits 00-01 are the MODLAND mandatory QA)

00-01 Mandatory QA
0 = processed, good quality
1 = processed, see other QA
2 = not processed due to cloud effects
3 = not processed due to other effects
02-03 Period used
0 = 16 days
1 = 32 days
04-07 land/water
0 = Shallow ocean
1 = Land (Nothing else but land)
2 = Ocean coastlines and lake shores
3 = Shallow inland water
4 = Ephemeral water
5 = Deep inland water
6 = Moderate or continental ocean
7 = Deep ocean
08-10 AM/MISR/PM
0 = AM
1 = AM/PM
2 = AM/PM/MISR
3 = AM/MISR
4 = PM
5 = PM/MISR
6 = MISR
11-15 Mean Szn of obs (overpass time)
0 = 0-5 degrees
1 = 5-10 degrees
2 = 10-15 degrees
3 = 15-20 degrees
4 = 20-25 degrees
5 = 25-30 degrees
6 = 30-35 degrees
7 = 35-40 degrees
8 = 40-45 degrees
9 = 45-50 degrees
10 = 50-55 degrees
11 = 55-60 degrees
12 = 60-65 degrees
13 = 65-70 degrees
14 = 70-75 degrees
15 = 75-80 degrees
16 = 80-90 degrees
16-17 snow/no snow
0 = no Snow
1 = Snow present
18-30 TBD
31 QAfill

Num_QC_Words #2 holds band-specific quality and inversion status information

00-03 Band1 Quality

0 = RMSE good, WoD(ref) good, WoD(WSA) good
1 = RMSE good, WoD(ref) good, WoD(WSA) moderate
2 = RMSE good, WoD(ref) moderate, WoD(WSA) good
3 = RMSE good, WoD(ref) moderate, WoD(WSA) moderate
4 = RMSE moderate, WoD(ref) good, WoD(WSA) good
5 = RMSE moderate, WoD(ref) good, WoD(WSA) moderate
6 = RMSE moderate, WoD(ref) moderate, WoD(WSA) good
7 = RMSE moderate, WoD(ref)moderate, WoD(WSA)moderate
8 = magnitude inversion (numobs >=7)
9 = magnitude inversion (numobs >3&<7)
10 = magnitude inversion (numobs <=3)
11 = Bus-in DB parameters (not currently used)
15 = Fill value

04-07 Band2 Quality (see explanation for band1)
08-11 Band3 Quality (see explanation for band1)
12-15 Band4 Quality (see explanation for band1)
16-19 Band5 Quality (see explanation for band1)
20-23 Band6 Quality (see explanation for band1)
24-27 Band7 Quality (see explanation for band1)
28-30 TBD
31 QAFill

(Note that while the RMSE indicates the fit of the full inversion, the Weights of Determination (WoD) give some indication of noise expected in the full retrieval and are dependent on the angular sampling of the available surface reflectances [4],[5],[6].)

The BRDF_Type SDS is specified as:

Data Field Name: UINT8 BRDF_Type ("YDim", "XDim", "Num_Type_Fields")

Description: Identifier for BRDF model used (RossThickLiSparseReciprocal).

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 BRDF_Albedo_Parameters SDS they include:

Name: Type: Value:

long_name HDF-STRING "BRDF_Albedo_Parameters"

units HDF-STRING "no units"

valid_range HDF-int16 0, 32766

_FillValue HDF-int16 32767

add_offset HDF-float64 0.0

add_offset_err HDF-float64 0.0

calibrated_nt HDF-int32 5

scale_factor HDF-float64 0.001

scale_factor_err HDF-float64 0.0

For the BRDF_Shape_Indicators SDS, they include:

Name: Type: Value:

long_name HDF-STRING "BRDF_Shape_Indicators"

units HDF-STRING "ratio or coefficient, no units"

valid_range HDF-uint8 0, 254

_FillValue HDF-uint8 255

add_offset HDF-float64 25.0

add_offset_err HDF-float64 0.0

calibrated_nt HDF-int32 5

scale_factor HDF-float64 0.04

scale_factor_err HDF-float64 0.0

For the BRDF_Albedo_Quality SDS, they include:

Name: Type: Value:

long_name HDF-STRING "BRDF_Albedo_Quality"

units HDF-STRING "concatenated flags"

valid_range HDF-uint32 0, 4294967294

_FillValue HDF-uint32 4294967295

For the BRDF_Type SDS, they include:

Name: Type: Value:

long_name HDF-STRING "BRDF_Type"

units HDF-STRING "model/type number"

valid_range HDF-uint8 0, 254

_FillValue HDF-uint8 255

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 MOD43B1 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 AdditionalAttributes which identify the tile number or which summarize the quality of the product over that entire tile (see the full MOD43B1 specification for the complete listing).