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MISR Level 2 Aerosol/Surface Products |
This statement applies to MISR Level 2 Aerosol/Surface Products for February 16, 2001, and beyond until such a time as further improvements to MISR software are made.
An extensive review of product quality has not yet been performed. Please read the summary words of caution if you did not do so earlier.
In spite of all the warnings, the MISR Level 2 Aerosol/Surface software which generated these products is believed to be functioning nominally except where noted below. This statement highlights major known problems with the products, as well as functionalities which are currently not implemented.
ACP DEPENDENCY
The quality of the aerosol product depends upon the quality of the Ancillary
Climatology Product (ACP). The ACP contains information on aerosol particle
properties and mixtures of aerosols. Refer to the
ACP quality statement
for further information.
CLOUD DETECTION PROBLEMS
Cloud screening is performed prior to
the aerosol retrievals. However, some clouds are currently not being detected.
The user is cautioned to be aware of this. Cloud screening is currently
performed with algorithms which utilize the angle-to-angle differences in
radiances across MISR cameras. These algorithms are described in the Level 2
Aerosol Algorithm Theoretical Basis document. External cloud screens from
other MISR products are not being used at the present time. Future product
upgrades will include the use of external cloud screening information.
OPTICAL DEPTH UNCERTAINTIES
The optical depth uncertainties which are reported over land areas are too
high, based upon limited comparisons with ground data. This will be fixed in a
future release.
ARTIFACTS IN AEROSOL OPTICAL DEPTHS OVER LARGE SPATIAL AREAS
The retrieved aerosol optical depths occasionally contain artifacts which show
up as large, block-shaped areas with identical optical depth values. This is
due to a limitation of the current aerosol retrieval algorithm. A fix for this
problem is planned for future versions.
TASC DEPENDENCY
The MISR TASC (Terrestrial Atmosphere and Surface Climatology) dataset provides
information on the climatological conditions of the area being observed by the
MISR instrument. This information is used during the aerosol retrieval
process. The TASC dataset is gridded on a month-by-month temporal basis. In
the future, this information will be obtained from more timely sources, e.g.
the Data Assimilation Office (DAO).
ALGORITHM UPDATES
The aerosol retrieval algorithms described in the Algorithm Theoretical Basis
document have been modified and improved, based on initial analyses of the
data. The next release of the MISR Aerosol Retrieval Algorithm Theoretical
Basis document will include an updated description of these algorithms.
DDV NOT YET AVAILABLE
The algorithm to retrieve aerosol properties over areas of dense, dark
vegetation (DDV) is not implemented. Instead, the algorithm to retrieve
aerosol properties over land areas of heterogeneous reflectances is used.
AEROSOL DEPENDENCY
The land surface product relies on the aerosol product for atmospheric
correction information. Therefore, the quality of the land product depends
upon the quality of the aerosol product. Refer to the
aerosol section for further information.
At the current time land surface retrievals, particularly for low albedo surfaces, should be considered most reliable where aerosol optical depths are low (< 0.1). Thus, it is recommended that users examine the 'Regional Mean Optical Depth' parameter in the Aerosol Product as part of their assessments of the surface parameters. Continuous refinements in the quality of the aerosol retrievals over land are planned for future releases and these are expected to result in improvements in the land product quality at larger optical depths.
FILL VALUES IN LAND SURFACE REFLECTANCES
Land surface reflectances are computed separately for each MISR spectral band.
In some cases, the land retrievals succeed in one MISR band, but not another.
This can cause visualization problems when viewing a composite image of land
surface reflectances which contains spectral bands for both successful and
unsuccessful retrievals. As the land surface retrievals improve in the future,
this problem should lessen.
LAI/FPAR NOT YET AVAILABLE
The algorithms to compute leaf-area index (LAI) and fraction of
photosynthetically active radiation (FPAR) are not yet implemented.
OCEAN NOT YET AVAILABLE
The Ocean Surface product, which contains surface reflectance properties
over ocean, has not been implemented. It is unavailable at this time.
