Observational Analysis of the Characteristics of Meso-γ-scale Vortexes Associated with the Extreme Hourly Precipitation in the Pearl River Delta Region

In this study, an objective algorithm to identify meso-γ-scale vortexes (MVs) using radial velocity observations from an S-band radar is developed. Then, for the 237 Extreme Hourly Precipitation (EXHP; > 75 mm) records at the surface weather stations in the Pearl River Delta (PRD) during five warm seasons, the properties and environmental conditions of the EXHP-associated MVs are analyzed. Further, the spatiotemporal distributions of the MV, instantaneous rain rate, and EXHP are illustrated for three events with the most abundant EXHP records. The major findings are as follows. About 42% EXHP records are accompanied by 57 MVs, including 84% of weak shear intensity, 12% of weak mesocyclone intensity, and 4% of moderate mesocyclone intensity, with the rotational speeds between 5 and 12 m s⁻¹, 12 and 16 m s⁻¹, and 16 and 21 m s⁻¹, respectively. The duration and core depth of the MVs are highly correlated (coefficient of 0.67) with averages of 39 min and 699 m, respectively. The hourly rainfall accumulation of an EXHP tends to increase with the influencing duration of MVs on the EXHP, while a majority of MVs might result from the EXHP-associated forcing such as condensational latent heating. Relative to the EXHP events with MVs in the United States, those in the PRD feature smaller environmental 0–3 km storm relative helicity (SRH) and 0–1 km vertical wind shear (VWS). However, compared to the non-rotational EXHP in the PRD, the rotational EXHP are associated with significantly higher 0–1 km VWS, 0–3 km SRH, humidity, and larger convective available potential energy. In the three selected events, rainstorms exhibit an irregular shape, a quasi-circular shape, and a quasi-banded shape of strong echo, respectively. The MVs are often located inside the strong radar reflectivity region, and some are next to its bow-shaped portion. Those longer-lived MVs with stronger rotation are collocated with the extreme 6-min rainfall accumulation (≥ 10 mm) in space and time, suggesting presence of positive feedback between low-level rotation and short-term rain rate. In the event influenced by a tropical storm, four MVs appear at almost the same location in succession and move along roughly the same path, forming an MV back-building process.