An investigation of convective features and Z-R relationships for a local extreme precipitation event

Abstract

Considerable rainfall presented as two precipitation centers (PCs) were created by two meso-β-scale convective systems occurred at the urban edge of the Pearl River Delta over South China on 7 May 2017. In this study, precipitation features of rainy storms, vertical structures and properties of convection for two PCs (PC-1, PC-2), and preliminary correlation between minute precipitation rate (MPR) and convective intensities are investigated based on detailed observation. Results show that most of the convection (~99%) over two PCs was active below the freezing level, indicating the extreme-rain-producing convection was dominated by warm-rain processes. Discrepancies of convective and precipitation features existed between two PCs. High precipitation rate had associated with those convection with lower echo centroids and more concentrated occurrence frequency in vertical structure, which presented obviously in PC-2. Percentile above 95th of the MPR over PC-2 was remarkably higher than those over PC-1 according to the classified statistics of MPR in their key precipitation stages. The intense-to-extreme MPR (≥ 1.6 mm min−1) had more related to those moderate-to-severe convection (≥ 46 dBZ) at 0.5 km above ground level in this extreme precipitation event, implying that large-size raindrops in warm-rain processes might play an important role in producing high-efficient MPR. In addition, relatively-large biases presented in three frequently-used Z-R relationships when estimating precipitation rate for the rainfall core of PC-2. More suitable Z-R relationship was surveyed for the rainfall core in this event, in which constant values a and b were calibrated and fell at the edge of the recommended range. This suggested that suitable Z-R relationship remained possible to be developed for such an extreme precipitation event in operational nowcasting over South China.