The VT–Z relationships, and freezing level estimation are not perfect, but they can improve
the radial velocity estimates compared to performing no correction. The corrections
obtained are 1 ... 5 m/s (2 ... 10 knots), depending on the elevation angle. The corrections
have little eect (<1 m/s (<2 knots) correction) for elevation angles less than 5°.
Radial Velocity Input for NDOP- Unfolding for Vc
The NDOP product assumes that velocities are unfolded or dealiased. A Doppler radar has a
limit on the unambiguous velocity which is:
Vu = +– (Wavelength*PRF)/4
Radial velocities that exceed this are said to be folded. On a color display with blue
representing radial velocity toward the radar and red representing radial velocity away from
the radar, a fold appears as an adjacent blue-to-red color shift.
Unfolding can be performed in the signal processor itself (for example, using the dual PRF
technique), or can be done by IRIS when Vc is generated.
The unfolding for Vc is based on a VVP product which assumes that the wind field varies
linearly with distance from the radar. The VVP product used by Vc must have the special
name UNFOLD and must be available on the system where the unfolding is being performed.
This technique achieves 3 times unfolding. For example, for an S band system operating at
1000 Hz PRF (range 150 km (93.2 mi)), the unambiguous velocity is ±25 m/s ( ±50 knots).
With unfolding, the unambiguous velocity is ±75 m/s ( ±150 knots), which can handle most
extreme meteorological situations.
As with the fallspeed correction, the Vc unfolding correction can be made at ingest when
data are collected from the signal processor (selected in the TASK Configuration Menu) or
at reingest (selected in Setup).
The VVP unfolding technique works well if there is adequate echo coverage and that the
wind field is approximately linear in its variations. In the vicinity of strong shear lines or
fronts, this may not be the case.
If you are using dual-PRF velocity unfolding and it is adequate to prevent folding it is
recommended that you not use the VVP-based unfolding in Vc since you may harm the
data in some extreme cases.
IRIS Product and Display User Guide M211319EN-H
170 RESTRICTED
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