MODIS BRDF/Albedo Product (MOD43B) User's Guide
MOD43B3 Albedo Product
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]. 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].
The MOD43B3 Albedo Product (MODIS/Terra
Albedo 16-Day L3 Global 1km 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] used observed
spectra and numerical simulations to produce the conversion
coefficients for MODIS and MISR 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] 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].
In addition to the spectral and broadband albedo quantities
themselves, the MOD43B3 Albedo 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 [4],[5],[6],[7].
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
- 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.
- 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.
- 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.
- 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.
- 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.
While the full MOD43B3
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 Albedo_Quality (containing two 32-bit words 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 Albedo_Quality SDS is specified as:
Data Field Name: UINT32 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 Szn of Local Solar Noon
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].)
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:
| Name: | Type: | Value: |
| long_name | HDF-STRING | "Albedo" |
| units | HDF-STRING | "albedo, 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 |
And for the Albedo_Quality SDS, they include:
| Name: | Type: | Value: |
| long_name | HDF-STRING | "Albedo_Quality" |
| units | HDF-STRING | "concatenated flags" |
| valid_range | HDF-uint32 | 0, 4294967294 |
| _FillValue | HDF-uint32 | 4294967295 |
In addition to the albedo and quality information (SDSs and Local
Attributes) that is provided at a per-pixel level, each tile of a
Level 3 MOD43B3 Albedo 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 MOD43B3 specification for the complete listing).
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