PPI Displays of Raw Doppler Data of Radial Wind

Operational measurement of Doppler weather radars consists of several PPI measurements (measurements with constant elevation angle and varying azimuth) at different elevation angles. The most simple visualization of these "volume" data is the projection of single PPI into the horizontal plane. The value in each pixel is then expressed by certain colour from a colour or gray scale. The distance from radar r and elevation angle of the corresponding PPI level a give the altitude of displayed target z = r sin(a).

Such visualization of a scalar field (as radar reflectivity or rain rate) is clear and intuitive. The same is not true for a vector field such as velocity and, thus, during PPI interpretation, we have to remember that Doppler radar does not measure velocity vector but only magnitude of its radial component.
 
Figure 11. Doppler velocity fields simulation
 
Simulations of PPI visualization of Doppler velocities for different observed wind fields were created to provide help with operational interpretation of Doppler data in the future. Simulations were done not only for horizontally uniform wind field but also for more complex fields, e.g. divergence, convergence or cyclonic and anticyclonic rotation. Simulation of aliased velocity and frontal discontinuity wind field were also created.

Determining a broad scale wind profile

Some principles for “smooth” flows:

  • Colour coding indicates whether radial velocities are inbound or outbound
  • The “zero isodop” (ZI) separates regions of inbound and outbound radial velocities. Along the ZI the total wind is either tangential or zero
  • With increasing range radar samples flow at higher and higher altitudes.
The zero isodop is a line connecting points of zero radial velocity and is a key feature visible on most Doppler velocity images. It separates a region of inbound velocity from a region of outbound velocity and passes through the origin (the radar).
 
 
Figure 12. Identifying the outbound and inbound velocities from the radar display
 
 
Figure 13. Using the ZI to determine wind direction

On the zero isodop the radial velocity is zero so, either wind is calm OR wind direction is perpendicular to radial.
 
 
Figure 14. Radial velocity on the radar display
Figure 15. Determination of wind direction.