Input Velocity Corrections

Corrected Radial Velocity Vc

IRIS corrected velocity (Vc) can include corrections for fallspeed and velocity folding. The fallspeed correction is based on VT–Z relationships above and below the melting level. The unfolding correction is based on a VVP product.

Vc can be generated either:

  • When the data are collected from the signal processor.

    This is configured in the TASK Configuration Menu.

    This is recommended for real time operation.

  • When RAW restored from tape or received over the network are re-ingested (to make ingest files).
  • This is configured in the Setup utility.

    Note that if Vc is already in the data when they are re-ingested, it is re-calculated from V. When Vc is generated, the uncorrected radial velocity V is still preserved.

    This is recommended for archive data or in systems where the communication bandwidth is limited so that the extra burden of transmitting Vc (as well as the standard radial velocity) is too much for the network.

Fallspeed Correction for Vc

A key assumption is that the vertical airmotions are weak as compared to the horizontal airmotions. This means that the radial winds are assumed to be caused by the horizontal wind only. However, while vertical airmotions may be weak, the fallspeeds of the hydrometeors (of order 1 to 10 m/s (2 to 20 knots) for rain) can make a significant contribution to the radial velocity. Therefore it is necessary to correct the radial velocities for the effect of fallspeeds.

The effect of particle fallspeed depends on the sine of the elevation angle. For example at 0° elevation, the fallspeeds do not affect the radial velocity. At 30° elevation angle (a typical maximum elevation in a volume scan), then half of the fallspeed would be observed (sine 30 = 0.5). Thus a 10 m/s (20 knots) fallspeed (hail and rain mixed) would contribute 5 m/s (10 knots) to the radial wind which is significant.

The fallspeed correction in Vc is made using a VT–Z relationship (terminal fallspeed - reflectivity). These take the general form of VT=aZb.

Since the particles are very different above and below the melting level, it is important to use different VT–Z relationships for these 2 cases. The default relationships used in IRIS are:

  • Above the melting level (snow and graupel) VT = 0.8 Z 0.06
  • Below the melting level (rain) VT = 2.70 Z 0.11

Here Z is in mm6/m3 and VT is in m/s.1

These relationships are entered by your system manager in the Setup utility. In addition, the average height of the freezing level is input for each month of the year.

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 effect (<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.
1 For a discussion of VT–Z relationships refer for example to a text on radar meteorology such as Battan, Louis, J., 1973: Radar Observation of the Atmosphere, University of Chicago Press, p 132.