Vaisala IRIS - Figure 36 Example of Radial Velocity against Azimuth Display

A Doppler radar can only measure the component of wind either towards or away from the
radar. This is called the "radial wind." However, by looking at the wind over all azimuths
around a full circle, the average wind speed and direction can be determined. This is shown
schematically in the following figure for the case of a South wind. The figure shows the
radial velocity at a range bin as a function of azimuth. When the radar points directly south,
the radial velocity measures the full speed of the wind (toward the radar). The IRIS
convention is that positive radial velocities are away from the radar. When the beam points
East or West, there is no component of the wind toward the radar, so the radial velocity is 0.
The divergence over the radar is estimated by taking the average of the radial velocity. Other
kinematic parameters can be estimated, such as the deformation and the axis of dilatation
of the wind.
0
North
Mean Wind
South
West East
+Vu
–Vu
East SouthWest North
Velocity vs. Azimuth Display — VAD for south wind
West
Figure 36 Example of Radial Velocity against Azimuth Display
The VVP algorithm assumes that the wind field varies linearly and computes the mean wind
speed and direction and divergence as a function of height. For each height interval, the
VVP algorithm performs a least squares fit of the observed radial velocity to linear windfield
model. Many thousands of points are used. The output is a line graph display of each of the
3 parameters against height with error bars to indicate the uncertainty in the least squares
fit.
The VVP Product Configuration menu requires no display configuration for the line graphs.
1. Select Type > VVP.
Chapter 3 – Configuring IRIS Products
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