Adjusting threshold qualifiers
When optimizing thresholds for your application, it is recommended that you change only one parameter (level or criterion) at a time so that you can verify the effect.
The following table shows some tips for optimizing the levels for the default criteria.
| Qualifier | Adjustment | ||
|---|---|---|---|
| LOG | To optimize the LOG level, display dBT or dBZ and select the lowest value of the threshold that eliminates the display noise. If the LOG level is set too high you lose sensitivity. Note that if you average more pulses or ranges, then the threshold level can usually be reduced. The appropriate value is generally unique to each pulse width selection. | ||
| SQI | To optimize the SQI level, display velocity and select the lowest value of the
threshold that eliminates the display noise. If the SQI level is set too high you lose
sensitivity. In general, you should see a greater area covered by velocity than
reflectivity since the velocity is more sensitive. If you do not, you should reduce
your SQI threshold. Note that if you average more pulses or ranges, then the threshold
level can usually be reduced.
|
||
| CCOR | This is used to eliminate clutter targets that are very strong. It should not be set to eliminate all clutter targets on a clear day since this means that you are losing sensitivity. To optimize the CCOR threshold it is best to know your system coherency in terms of dB of clutter cancelation. Start at a value of 10 dB greater (closer to 0) than this. Now display a PPI of dBZ at an antenna elevation of approximately1 degree. The display should be relatively clean of any clutter targets since most are rejected. Now reduce the CCOR (more negative) to increase the number of clutter targets on the display until the number of clutter targets does not increase. The optimum value of the CCOR is approximately 5 dB more (closer to zero) than this point. For example, if the number of clutter targets is a maximum at -35 dB, then set the CCOR to approximately - 30 dB. Note that your clutter filter selection effects the result. | ||
| SIG | This should be done last. To optimize the SIG level, display the width W and select the lowest value of the threshold that eliminates the display noise. If the SIG level is set too high you lose sensitivity. If you average more pulses or ranges, then the threshold level can usually be reduced. | ||
| PMI | To optimize the PMI level, consider data acquired in the mode of (H+V) in PPP processing. Having first optimized the previous four Doppler qualifiers, inspect echo classification data of DB_HCLASS and seek for gates declared as “NoMet”, which are unlikely of meteorological origin. These bins may appear as “NoMet” data in your display, or DB_HCLASS data might be readily thresholded, depending on your color scales and the HydroClass configuration. In order to get the other data types thresholded in the same fashion, activate PMI as a thresholding mechanism in task configuration. The PMI threshold value to 0.45 implies the same strength of suppression to other selected data types as seen in DB_HCLASS. It is possible to recover more precipitation data, typically at edges of precipitation and the most far echoes (virga) by reducing the PMI threshold, as appropriate. In these customizations, the behavior of DB_HCLASS remains unchanged. |
Secondary SQI threshold
When thresholding dual pol, dBZ, and dBT reflectivity data with SQI , the comparison value for accepting those data is the secondary SQI threshold that is defined in a slope and offset from the primary user value. See Mf — Clutter filters.
The secondary threshold is more permissive (lower valued), and is traditionally used to qualify LOG data only in the Random Phase processing mode.
The secondary SQI threshold is applied uniformly in all processing modes when dual pol or reflectivity data are specified as being thresholded by SQI.
This gives you more freedom in applying an SQI threshold to your LOG data, because the cutoff value for dual pol and reflectivity can be chosen independently from the cutoff value for the other Doppler parameters. The full SQI test would not normally be applied to LOG data, because of the so-called "black hole" problem, which is the loss of LOG data within regions of high shear, even though, for instance, the reflectivity itself was strong. You can experiment with applying a secondary SQI threshold to help clean up the LOG data, without introducing any significant black holes.