Which hydrometeor type typically results in negative ZDR values?

Negative ZDR values are indicative of hydrometeors that are asymmetrical in shape, particularly those that exhibit an oblate or flattened profile. In the context of radar meteorology, ZDR, or differential reflectivity, is a measure of the difference in reflectivity between horizontally and vertically polarized radar pulses.

When considering hydrometeor types, ice particles, such as snowflakes or ice crystals, generally have complex shapes that can lead to negative ZDR values. Specifically, ice aggregates often tend to be irregular and can resemble flat or elongated configurations, which can produce lower reflectivity in the horizontal polarization compared to vertical polarization.

In contrast, spherical particles would typically yield ZDR values close to zero since their symmetrical shapes result in nearly equal horizontal and vertical reflectivity. Oblate particles, while they can also exhibit negative ZDR, are more commonly associated with raindrops. Prolate shapes are more aligned with elongated particles like certain types of hail, which can produce positive ZDR values or less defined responses. Therefore, ice particles stand out as the primary type associated with negative ZDR readings, due to their irregular and often flat profiles that alter the reflectivity characteristics significantly.