Differential reflectivity (Zdr)

In the case of amplitude (power) measurements, the larger horizontal axis of drops causes the power measured at horizontal polarization (of the electric field) to be larger than the power measured at vertical polarization. The ratio of the reflectivity factors Z_H/Z_V, expressed in dB, is called differential reflectivity (Z_dr).

• Zdr is generally positive in rain (that is, >1) and is usually less than 5 dB. When the rainfall rate is large, there are typically more large drops, so Z_dr is larger.
• Low Z_dr and high dBZ indicates the presence of hail, which may be tumbling with no preferred orientation.

Z_dr, because it is a ratio of powers, is not sensitive to the radar calibration, as long as the overall gain of the H and V channels is the same (or calibrated).