Analysis of S-band polarimetric radar observations of a hail-producing supercell

The Zhu-Macao polarimetric radar data are used to analyze the structure and microphysics of a supercell near Zhuhai during 07:00-10:00 UTC 20 April 2015 (hereafter "4.20" supercell case). The storm occurred near the surface cold front and the low-level shear line, where the CAPE was 1300 J/kg and the wind shear between 0 and 6 km was greater than 20 m/s. The total Richardson number was 18, which is close to the value for typical mid-latitude supercell and the relative helicity of the storm was 2.9 s^-1. A number of distinctive polarimetric signatures are found in the storm. The most remarkable signatures of hails were low differential reflectivity (ZDR) (-1 to 0.5 dB) and high reflectivity at horizontal polarizations (ZH) (>50 dBz). Because of tumbling, the ZDR of large hail area was close to 0. The cross-correlation coefficient (CC) was lower than 0.9 in the rain-hail mixtures and the hails had a negligible influence on the specific differential phase (KDP) (the slope of linear regression between ZH and KDP was 0.05). Because of the mixed phase particles and the liquid particles, a characteristic of enhanced ZDR and depressed CC was found near the updraft-perturbed melting layer. A shallow region of high ZDR (>3 dB) was found in the east edge of the forward-flank downdraft (FFD), called the "ZDR arc", which was associated with large, oblate raindrops. The size-sorting mechanism can explain this phenomenon. Compared to observations in North America, the higher environmental winds made the ZDR further away from the ZH center in the "4.20" supercell case. The ZDR columns were observed near the ZH center. The reason is that large raindrops were brought to the upper layer by updrafts and then losed orientation stability when they froze, thereby the ZDR values decreased significantly.