For oblate hydrometeors, what is the expected ZDR?

In radar meteorology, the term ZDR (Differential Reflectivity) refers to the ratio of reflectivity factors measured in horizontal and vertical polarizations. When considering oblate hydrometeors, such as raindrops that are flattened or disk-shaped due to their shape and the influence of airflow around them, it is expected that ZDR values will be greater than 0 dB.

Oblate hydrometeors present a larger cross-sectional area for horizontal polarization than for vertical polarization because of their shape. This means that they reflect more power back to the radar in the horizontal plane compared to the vertical plane. As a result, the ZDR will yield positive values reflecting this difference in scattering properties.

In contrast, if the hydrometeors were perfectly spherical, ZDR would be approximately 0 dB since their scattering would be uniform in all directions. Values less than 0 or negative values are typically associated with hydrometeors that exhibit unusual shape characteristics or with certain types of debris or ice, which do not apply to the common scenario of oblate hydrometeors. Thus, the expectation of ZDR being greater than 0 dB aligns with the physical characteristics of these types of particles and their interaction with radar signals