For spherical hydrometeors, what is the approximate value of ZDR?

For spherical hydrometeors, the approximate value of differential reflectivity (ZDR) is around 0 dB. This is because ZDR is a measure of the difference in reflectivity between two orthogonal polarizations (horizontal and vertical). In the case of spherical particles, such as raindrops, the radar returns from both polarizations are nearly equal since these particles reflect the radar energy uniformly in all directions. As a result, there is minimal difference in the reflectivity values, leading to a ZDR value that is close to 0 dB.

When considering other types of hydrometeors, like oblate or irregular shapes (e.g., ice crystals or hail), the ZDR values can deviate from 0 dB because these shapes influence how the radar energy is scattered. However, for the specific context of spherical hydrometeors, the ZDR remaining near 0 dB is indicative of their uniformity in reflection characteristics.