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MISR Level 2 Top-of-Atmosphere/Cloud Products |
Also see the Statement dated February 16, 2001 for MISR Level 2 Top-of-Atmosphere/Cloud Products from February 16 to March 30, 2001.
This statement applies to MISR Level 2 TC Stereo for March 30, 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 have not done so already.
Many of the algorithms used in the stereo product retrievals have been developed specifically for the MISR instrument, and as such, are relatively untested. We expect to improve on these algorithms as we gain experience with the data. Trade-offs have been made at times to sacrifice accuracy or coverage for speed and vice versa.
In spite of all the warnings, the MISR Level 2 TC Stereo software which generated these products is believed to be functioning quite well except where noted below. This statement highlights major known problems with the products, as well as functionalities which are currently not implemented.
Note that the Classifiers and Albedo products, which contain cloud classification and TOA albedo respectively, have not been implemented. They are unavailable at this time.
REGISTRATION
Cloud motion calculations are quite sensitive to the quality of registration of
the D camera L1B2 ellipsoid-projected radiance products. Since Level 1 does not
yet utilize Reference Orbit Imagery (ROI) when performing registration
correction, the registration relies on a fairly static camera model. The camera
model changes periodically, and although the registration is typically much
better, the camera models in use only guarantee accuracy of 2 pixels or less in
the D cameras. Cloud height accuracy is nominally 562 m, corresponding to 1
pixel of accuracy in the A cameras. Under the best of conditions, the heights
often appear quantized. Further, they are occasionally made worse due to errors
in cloud motion caused by misregistration of the D cameras. A 2 pixel D camera
error translates to a 10-15 m/s error in the cloud motion vectors, which
propagates to an error of 1100 m in height. We expect the registration
reliability to improve significantly when the ROI is used, and we anticipate a
reduction in height and wind uncertainties of approximately a factor of 2. For
more details, including a link to a list of orbits with known registration
problems, see the
Registration Page.
DOMAIN ARTIFACTS
Cloud motion retrievals are made on 70.4 km domains. This may at times
result in discontinuities at domain boundaries for cloud heights.
STRIPES
Horizontal stripes may occasionally appear in the product for some
parameters. This is due to one or more missing lines of data in Level
1, and often shows up in Level 2 parameters as "No Retrieval" flag
values. For more details, read sections on
Gaps
and
Instrument Out-of-Sync in the Level 1 Statement.
BLUNDERS
Blunder detection has not been implemented. As a result, spikes may
occasionally appear in the cloud heights.
ALGORITHM UPDATES
The cloud motion and height retrievals have changed somewhat from the Level 2
Cloud Detection and Classification ATB (JPL D-11399, Rev. D). These changes
will be reflected in the next release of the document, Rev E. Highlights
include:
The histogramming of the cloud motion for each domain now involves the identification of clusters of points which may cross the histogram bin boundaries, resulting in a matrix which identifies the clusters. The clusters chosen for the two cloud layers must be local maxima in this matrix. The histogram now includes a center bin which is centered on 0.0 in each direction. We no longer concatenate bins with the same population together but rather choose the one with the smallest height range.
External meteorological inputs such as MODIS and NSIDC are not yet used. Instead, a static monthly climatology (the TASC dataset) is used.
PGE 8b, which includes cloud classifiers, the cloud shadow mask and topographic shadow mask, has not yet been developed.
PGE 8c, which includes local, restrictive, and expansive albedo, has been developed, but the product is not yet ready for public release.
Until we gain further validation of the Level 1 Radiometric Camera-by-camera Cloud Mask (RCCM), its usage has been turned off by Stereo processing. Therefore, the Stereoscopically Derived Cloud Mask relies solely on stereoscopically matched data to determine the presence of clouds.
