The Calibration of Differential Reflectivity

The dawn of modern weather radar polarimetry was ushered in by the seminal paper of Thomas Seliga and V.N. Bringi in 1976 entitled, “The use of radar differential reflectivity measurements at orthogonal polarizations for measuring precipitation”. It was know that large drops resemble oblate spheroids in shape and they reasoned that the reflectivity of such drops should yield higher reflectivity for horizontal polarization measurements as compared to vertical polarization measurements; and thus differential reflectivity, Z_dr, was born. Indeed this conjecture was shown true by Seliga, Bring and Al-Khatib in their 1979 paper, “Differential Reflectivity Measurements in Rain: First Experiments”. However, these papers left the community with the conundrum of just how to calibrate a radar measured Z_dr to a tenth of a decibel, especially considering that the dynamic range of precipitation of interest can be 90 dB and larger.