Vaisala IRIS - Input Velocity Corrections

5.5.1 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 eect of fallspeeds.
The eect of particle fallspeed depends on the sine of the elevation angle. For example at 0°
elevation, the fallspeeds do not aect 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=aZ
b
.
Since the particles are very dierent above and below the melting level, it is important to
use dierent 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 mm
6
/m
3
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.
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.
Chapter 5 – Configuring and Scheduling Optional IRIS Products
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