QuikSCAT Wind Retrieval Ambiguity Selection Quality Assurance
The BYU quality assurance (QA) algorithm infers the quality of scatterometer
retrieved winds by evaluating the self-consistency of the selected wind flow
using a simple wind field model. A simple flag product is provided to assist
users in identifyin potential ambiguity selection problems in the JPL L2B wind
product. The QA flagging is done in two ways:
First, the selected wind is compared to a low order Karhunen Loeve
(KL) model fit on a region-by-region basis. Individual cells are flagged
where the directional or vector error exceeds thresholds. The rms vector
error and number of flagged cells are computed for each region.
Second, the directions for each region are binned and the resulting
histogram is inspected for multimodalities.
If a region exceeds a threshold in rms error and number of cells flagged and contains
a multimodal directional histogram, it is flagged as a possible region of ambiguity
selection error (ASE). Regions are also classified as "good", "fair", or "poor"
according to the number of cells flagged in the region by a set of constant thresholds.
Full documentation of the quality assurance algorithm is given here: QA
| Each wind vector cell (wvc) is given a level QA bit flag defined in the
adjacent table. Cells flagged with both the region ASE flag and the ASE
cell flag have a high likelhood of indicating the position of ambiguity
selection errors. The QA flag can be viewed as an integer number between
0 and 15 indicating increasing uncertainty of the selected wind flow. Because
the QA algorithm is effective in locating unnatural breaks in the wind flow,
areas with realistic fine-scale wind features (such as storms and fronts)
may be flagged while ambiguity selection errors resulting in consistent
wind flow may not.
| [q3,q2,q1,q0] || QA Flag |
| [x,x,x,1] || Noisy Vector Cell Flag |
| [x,x,1,x] || ASE Cell Flag |
| [0,0,x,x] || Region "Good" Rating |
| [0,1,x,x] || Region "Fair" Rating |
| [1,0,x,x] || Region "Poor" Rating |
| [1,1,x,x] || Region ASE Flag |
The QA bit flag files for the entire SeaWinds data set can be found at our anonymous
FTP Site. For simplicity, multiple flag files are combined
into .tar files. Each .tar file contains several hundred
qaflagxxxxx.dat.gz files ordered by rev number. The qaflagxxxxx.dat
files are in direct format, 1 byte per wvc, with L2B ordering: size 76x1624
(cross track first, then along track). The array matches the L2B file. The following
figure is an example of the QA bit flag on a region of ambiguity selection error.