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As any beam of radiation passes through the atmosphere, it curves downward. This is caused not by the pull of gravity, but by the vertical variation in density of the atmosphere. Beams of radiation travel faster where the density is less. Normally, the beam doesn't curve nearly as much as the Earth itself curves, so the beam still gets farther away from the ground the farther it gets from the radar.
In extreme cases, a temperature inversion near the ground may be so strong that a radar beam which starts out moving upward is bent all the way down to the ground. This produces strong echo at large distances from the radar, and the phenomenon is called Anomalous Propagation. Generally, the Doppler velocity of AP echoes is zero, but the echoes themselves can move around because propagation characteristics of the atmosphere keep changing.
If you have data from more than one radar, AP is easy to identify, because the neighboring radars generally won't see the same thing. If you have just one radar but can look at different angles, you will find that AP tends to be confined to the lowest elevation angles. When the radar is pointed higher in the sky, it's unlikely that the beam can be bent backward all the way to the ground.
If you're stuck with just one radar and one elevation angle, you have to get more imaginative. The structure of AP echo tends to be "unusual", with sudden, sharp changes in intensity. Also, its motion tends to be erratic, although this can also be misleading. The best points of comparison are surface observations and satellite data.
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