zdr

ZDR is a polarimetric radar parameter used in meteorology, standing for differential reflectivity. It measures the difference between the reflectivity factors of vertically and horizontally polarized radar beams. The quantity is defined as ZDR = 10 log10(ZV / ZH), where ZV and ZH are the vertical and horizontal reflectivity factors, and it is typically expressed in decibels (dB).

Positive ZDR values indicate horizontally oriented, flattened precipitation particles, such as oblate raindrops, which produce stronger horizontal reflectivity. Negative ZDR values suggest vertically oriented or elongated particles, such as tall ice crystals or certain hail structures. Values near zero imply spherical particles or isotropic orientations. ZDR is commonly interpreted together with other polarimetric variables, such as correlation coefficient and differential phase, to classify precipitation and infer droplet size distributions.

In practice, ZDR supports a range of meteorological applications, including improved rainfall rate estimation, precipitation type discrimination (e.g., rain versus hail or snow), and hydrometeor identification. It is particularly informative for distinguishing large raindrops from smaller ones and for identifying mixed-phase precipitation. Limitations include sensitivity to signal attenuation, calibration accuracy, beam geometry effects, and complex mixtures of particle shapes, which can complicate interpretation, especially in melting layers or near the radar edge.