Anomalies On Doppler Wind Images

Velocity Aliasing

The maximum velocity that can be correctly displayed by a Doppler radar is known as the Nyquist velocity, and this is dependent on the wavelength and frequency of pulses emitted by the radar. Common Nyquist velocities for Bureau of Meteorology Doppler radars are 95km/hr and 140km/hr.

So what happens if the radial velocity is larger than the Nyquist velocity? In this situation we commonly see velocity aliasing, where the displayed velocity "wraps around" to the other end of the palette. For example, if the Nyquist velocity is 90km/hr, and the reflecting particle is moving directly away from the radar at 100km/hr, then the displayed velocity will actually be 80km/hr towards the radar. It is usual to see adjacent areas of extremes of the Doppler velocity palette (dark red next to dark blue) in areas where velocity aliasing is occurring, and it can sometimes be an indication of severe weather due to very strong winds.