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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 identifying 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 documentation.

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 likelihood 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.
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.