Submitted by czr137 on
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Unstable retrievals may result in extremely large, unphysical values.  To detect the unstable retrievals that cannot be detected by examining Level 1 data for known causes of instabilties we rely on comparing individual values with average values.  Therefore this process requires the calculation of average values then comparing the values.

Binning the Values

Ozone and aerosol values are grouped by week into 10 degree latitude bins at each altitude on the retreival grid.

Iterative Checking

The values in those bins are then used to find average values.  If an outlying value is found in any bin, the entire scan is removed from the bins.  This process of averaging and checking individual values against the average is performed 3 times.

Averages

In each bin the mean and Standard Deviation (SD) are calculated.  The median and Median Absolute Deviation (MAD) are also found. The MAD follows the equation \begin{equation} MAD = median\left( \left|x-median(x)\right| \right) \end{equation}

Comparing for Outliers

The program then checks each value in each bin against and absolute limit, the mean and SD and the median and MAD.  

  • An absolute limit is set at 1 for both ozone and aerosol since they are in units of volume mixing ratios and cannot exceed 1
  • A limit is set at \begin{equation} mean_i \pm a_i SD_i \end{equation}
  • A limit is set at  \begin{equation} median_i \pm b_i MAD_i \end{equation}

For these limits the index "i" represents an altitude and the factors "a" and "b" are set to allow more variability at low altitudes where atmospheric dynamics are likely to create outlying values that are still accurate.

The following figures show the cumulative number of scans removed by statistical detection for the Aerosol and Ozone products: