Hail Detection with Polarimetric Radar

Proposed algorithms for detecting hail with polarimetric radar are reviewed. Because hailstones are typically larger than raindrops and have a particular fall mode, they have pronounced impact on radar measurements of radar reflectivity (Z_H), differential reflectivity (Z_DR), linear depolarization ratio (LDR), correlation coefficient (ρ_HV), and differential propagation phase (Φ_DP). All proposed detection techniques are based on departures from the rain-only case, and nearly all proposed polarimetric algorithms readily detect large hail. For small hail, problems occur, for example, with reflectivity and differential reflectivity-based methods due to natural variability in raindrop size distributions. Potential problems will be illustrated with measurements from several hailstorms.

For "marginal" hail events, pattern relationships among polarimetric variables should provide improved hail detection. In hail shafts a negative correlation arises between reflectivity and differential reflectivity that is independent of the drop size distribution (DSD). Linear depolarization ratio and correlation coefficient measurements respond weakly to DSD variations, but they are particularly sensitive to large frozen hydrometeors that are wetted and have irregular shapes. The correlation coefficient can fall from 0.99 to less than 0.95 in hail shafts. Correlations below 0.8 have been observed with extreme events. Depending on antenna isolation, LDR measurements often increase by 5 dB or more. Also, for rain there is a consistency among Z_H, Z_DR, and Φ_DP. Inconsistencies within thunderstorms are indicative of frozen hydrometeors. The relative advantages and disadvantages of these alternative hail detection methods will be examined.