Joint Centre for Mesoscale Meteorology (JCMM)
JCMM Internal Report No.93:
Observations of oblate hail using dual polarisation radar and implications for hail detection schemes
by T J Smyth, T M Blackman and A J Illingworth
Coincidental radar measurements together with in situ ground truth observations of large, oblate hail has allowed new insights into the fall mode of oblate hailstones. The effect of the fall mode upon hail detection algorithms using multiparameter radar is investigated and discussed as is the vexed problem of extracting realistic rainfall rates in regions where hail and rain coexist. Observations presented here show that oblate hail can be associated with non-zero values of differential reflectivity, ZDR which, together with the coincidental values of the linear depolarisation ratio, LDR, suggest some degree of alignment. Although large hail can dominate backscatter through the diameter to the sixth power dependence, even if it is aligned it makes a negligible contribution to specific differential phase, KDP. Rainfall rates extracted using KDPin the presence of hail are more robust than those using the contaminated ZH and ZDR measurements, but proposed hail detection algorithms which compare the observed values of KDP and ZH do seem error prone. The hailstones measured in this study were of sufficient dimensions to allow Mie scattering to occur as S-band (Lambda = 9.75cm). This served to complicate the interpretation of the differential phase measurements. A new hail detection algorithm has been formulated which compares the differential phase predicted from ZH and ZDR with that actually observed. This avoids the problem of estimating KDP from the derivative of a noisy differential phase profile.