eXe

Differential Reflectivity: ZDR

The differential reflectivity, Z_DR, is the ratio of the returned power from the horizontal and vertical signals and is a function of the particle's shape and orientation. It is independent of the absolute calibration and particle concentration, but is affected, as one might guess, by differential attenuation. It essentially gives the drop shape as a function of size.

Raindrops are not symmetrical, being flattened in the vertical. Hail, on the other hand, tumbles as it falls and looks symmetrical to the radar. Hail is often mixed with heavy rain when viewed over the huge size of a radar bin. Very large rain drops, and there does not have to be a lot of them, mixed with small hail will cause Z_DR values to increase. If there is more hail in the bin, values of Z_DR will drop towards zero, though in practice typical values are between 0.5 and 1.0. Z_DR values due to various precipitation types are:

Light rain (0.5-1.0 dB)
Moderate rain (1.5-2.0 dB)
Heavy rain (2.5-3.0 dB)
Rain/Hail (0.0-1.0 dB)
Dry snow (0.0-0.3 dB)
Wet snow (1.0-2.0 dB)
Ice crystals (2.0-3.0 dB)

There is some overlap evident. For example, moderate rain and wet snow have similar Z_DR values.