Equivalent reflectivity factor

From Glossary of Meteorology

equivalent reflectivity factor

The radar reflectivity factor of a target consisting of water drops small compared with the radar wavelength, which would produce the same reflectivity as that of a target with unknown properties.

Mathematically, the equivalent radar reflectivity factor, Ze, of a given target is defined by
where η is the reflectivity of the target, λ is the radar wavelength, and 0.93 is the dielectric factor appropriate for water. This definition is based on the Rayleigh scattering approximation, which requires that the water drops have diameters no larger than about one-tenth the radar wavelength. For targets consisting of sufficiently small water drops, it follows from the definition that the reflectivity factor, Z, and Ze are equivalent. But if the composition and sizes of the particles are unknown, it is appropriate to regard the reflectivity factor that is determined from the measured reflectivity as Ze.
See radar equation.