Vaisala IRIS - Page 183

For example, suppose the bin spacing and Range Filter Length values are selected so that
Clutter Length Scale corresponds to 3 range bins. Then any single isolated Clutter Bin would
be rejected, as would runs of 2 or 3 clutter bins. Runs of 4 or more clutter bins would not be
rejected because they are interpreted as real weather.
Regardless of the Clutter Length Scale, isolated clutter bins (single bins) are always
removed.
Dierencing (radial dierence example)
The next step is to perform the range dierencing of the radial velocities. The radial wind
shear algorithm computes the shear by taking the bin-to-bin dierence on a PPI surface.
If the dierencing algorithm encounters a blank bin, it skips out in range to the next valid bin
to take a dierence, provided the bin is within 0.5*Range Filter Length.
For example, if the Range Filter is set to correspond to 3 bins, the dierencing algorithm
dierences over a gap of one bin, but not over a gap of 2 or more bins. The velocity
dierences are placed as close as possible to the center of the dierence interval, to the
nearest bin that is less than or equal to half of the dierence interval.
Smooth the Dierences
Dierencing is inherently noisy. The next step is to smooth the computed dierences. This
not only reduces the noise, it also fills in small gaps of missing bins and cancels any
remaining isolated bins.
The smoothing algorithm uses a triangular window whose total width is equal to the Range
Filter Length value. The sum of the weights defined by the triangular filter is always 1 unless
a bin is thresholded.
In the previous figure, the Range Filter Length corresponds to 3 bins. Each velocity in the
filter range is multiplied by the corresponding weight and then summed. The mean is
obtained by dividing by the total weight. Thresholded bins are not weighted. If the total
weight is less than or equal to 0.5, the shear value is rejected. This eliminates isolated bins.
This approach can also fill gaps. A longer range filter rejects a longer run of isolated bins and
fills a longer gap.
Azimuth Shear Calculation
The following figure is a depiction of "B-Scan" space (a PPI surface). The coordinates are
range and azimuth. The radial shear is obtained by dierencing in range along a radial.
Chapter 5 – Configuring and Scheduling Optional IRIS Products
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