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MISR Level 2 Aerosol/Surface Products |
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This statement applies to MISR Level 2 Aerosol/Surface Products for April 15, 2002, 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 have not done so already.
In spite of all the warnings, the MISR Level 2 Aerosol/Surface software which generated these products is believed to be functioning quite well except where noted below. This statement highlights major known problems and issues 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.
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).
CLOUD DETECTION STATUS
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, as
well as with the MISR-derived Radiometric Camera-by-camera Cloud Mask
(RCCM) and Stereoscopically Derived Cloud Mask (SDCM).
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.
EDGE-OF-SWATH ARTIFACTS OVER OCEAN
The retrieved optical depths over ocean at the edge of the MISR swath
occasionally appear brighter than the surrounding values. The cause of this
artifact is under investigation.
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.
EXPERIMENTAL AEROSOL ALGORITHM OVER HOMOGENEOUS SURFACES
A new algorithm which retrieves aerosol properties over homogeneous
surfaces is included. However, due to the experimental nature of this new
algorithm, results from this algorithm are included for diagnostic purposes
only. Affected fields in the aerosol product are ChisqHomog,
OptDepthHomogCalcPerBand, and ChisqHomogCalcPerBand.
SOME AEROSOL FIELDS NOT AVAILABLE
The following fields in the aerosol product are not currently computed, and
contain fill only: RegBestFitMixtureEqRefl; RegSfcRetrOptDepthUnc;
OptDepthDWCalcPerBand; OptDepthOTACalcPerBand; ChisqAbsCalcPerBand.
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.
QUILTING EFFECT IN LAND SURFACE REFLECTANCES
The land surface reflectances are reported on a 1.1 km x 1.1 km grid, while
the retrieved aerosol optical depths are computed on a coarser 17.6 km by
17.6 km grid. Values of aerosol optical depth on the coarser grid may
"jump" in magnitude from one region to a neighboring region, where
the aerosol distribution is not smooth across regions. Therefore, the
atmospheric correction using the coarser resolution aerosol data occasionally
results in a surface "quilting" effect which can be seen in the
land surface reflectances.
RELIABILITY OF LAND SURFACE REFLECTANCE VALUES DEPENDENT UPON AEROSOL
OPTICAL DEPTH MAGNITUDE
At the current time land surface retrievals, particularly for low albedo
surfaces, should be considered most reliable where aerosol optical depths
are low (< 0.2). Thus, it is recommended that users examine the
'RegMeanSpectralOptDepth' field 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.
