Cause-Effect Relationship between Meso-γ-Scale Rotation and Extreme Short-Term Precipitation: Observational Analyses at Minute and Sub-Kilometer Scales

The cause-effect relationship between meso-γ-scale rotation and extreme short-term precipitation events remains elusive in mesoscale meteorological research. We aimed to elucidate this relationship by analyzing a rainstorm over the Pearl River Delta during the nocturnal hours of 15 May 2017 based on 6-min radar observations and 1-min rain gauge data. This rainstorm had a maximum hourly rainfall of 100.1 mm, with 26 stations recording hourly rainfall > 60 mm h.1 in 5 h. Extreme heavy precipitation was produced in association with a convergence zone along the southern side of a synoptic low-level shear line, where southwesterly warm, humid airflows with precipitable water of > 60 mm, little convection inhibition (< 10 J kg-1), and a low lifting condensation level (about 300 m) dominated. A meso-γ-scale vortex was quantitatively identified during the hour with the largest number of gauges observing extreme hourly rainfall. The vortex had a mean diameter of 6.1 km and a peak intensity of 3.1 × 10-3 s-1 during its lifetime of 54 min. The vortex initialized and remained inside the region of extreme rain rates (radar-retrieved rain rates > 100 mm h-1), reached its peak intensity after the peak of the collocated 6-min rainfall accumulation, and then weakened rapidly after the extreme rainfall region moved away. The radar-retrieved liquid water path was about five to seven times the ice water path and the specific differential phase (Kdp) below 0°C increased sharply downward during the lifetime of the vortex, suggesting the presence of active warm rain microphysical processes. These results indicate that the release of the latent heat of condensation induced by extreme rainfall could have contributed to the formation of the vortex in an environment with a weak 0.1-km vertical wind shear (about 4–5 m s-1) through enhanced lowlevel convergence, although the strengthening of low-level updrafts by rotational dynamic effects and short-term rainfall cannot be ruled out.