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 observations from a Doppler weather radar is built. Then, for the 237 Extreme Hourly Precipitation (EXHP; > 75 mm) records observed at the densely distributed surface weather stations in the Pearl River Delta (PRD) during five warm seasons, the associated MVs’ properties and environmental dynamic and thermodynamic parameters are statistically analyzed. Further, the three events with the most abundant EXHP records are analyzed to illustrate the spatiotemporal distributions of the MV, instantaneous rain rate, and strong radar echo. Major findings are as follows. About 41.8% of the EXHP records are accompanied with MVs. Of the total 57 MVs, about 84% are of weak shear intensity (rotational speed <12 m/s), 12% of weak mesocyclone intensity, and 4% of moderate mesocyclone intensity. The MVs have an average duration of about 39 minutes and an average core thickness of 699 meters, with the correlation coefficient of 0.67 between the duration and core thickness. Relative to the EXHP events with MVs in the United States, those in the PRD have smaller 0—3 km storm relative helicity (SRH) and 0—1 km vertical wind shear (VWS). However, compared to the EXHP without MVs in the PRD, the EXHP with MVs has significantly higher 0—1 km VWS and 0—3 km SRH, which provides better environmental dynamic conditions for the formation of MVs. Meanwhile, the EXHP with MV tend to occur in an environment with larger moisture amounts and higher convective available potential energy, which provides environmental instability and moisture conditions for strong convective systems.. In the three events with the largest (top 3) number of EXHP records, the EXHP-producing convective storms are featured with substantially different morphologies, i.e., a meso-β-scale irregular shape, a meso-γ-scale quasi-circular shape, and a meso-β-scale quasi-banded shape, respectively. The MVs are often located inside the strong radar reflectivity region, many of which are near the strongest echo core and some of which are next to the bow-shaped part of the strong echo. Those MVs that last longer and of relatively stronger rotation are highly correlated with the extreme 6-min rainfall accumulation (≥10 mm) in space and time, suggesting that the positive feedback between low-level rotation and short-term rain rate may be functioning. In the event influenced by a tropical storm, there was an MV back-building process in which three MVs appeared at almost the same location in succession and moved along roughly the same path.