Observations of oblate hail using dual polarization radar and implicationsfor hail-detection schemes

Coincidental radar measurements and in situ ground-truth observations of la rge, oblate hail has allowed new insights into the fall mode of oblate hail stones. The effect of the fall mode upon hail-detection algorithms using mu lti-parameter radar is investigated and discussed, as is the vexed problem of extracting realistic rainfall rates in regions where hail and rain coexi st. Observations presented here show that oblate hail can be associated wit h nonzero values of differential reflectivity, Z(DR), which, together with the coincidental values of the linear depolarization ratio, suggest some de gree of alignment. Although large hail can dominate backscatter through the diameter-to-the-sixth-power dependence, even if it is aligned it makes a n egligible contribution to specific differential phase, K-DP. Rainfall rates extracted using K-DP in the presence of hail are more robust than those us ing the contaminated reflectivity in the horizontal polarization, Z(H), and Z(DR) measurements, but proposed hail-detection algorithms which compare t he observed values of K-DP andZ(H) do seem error prone. The hailstones meas ured in this study were of sufficient dimensions to allow Mie scattering to occur at S-band (Wavelength lambda = 9.75 cm). This served to complicate t he interpretation of the differential-phase measurements. A new hail-detect ion algorithm has been formulated which compares the differential phase pre dicted from Z(H) and Z(DR) With that actually observed. This avoids the pro blem of estimating K-DP from the derivative of a noisy differential-phase p rofile.