Influence of Synoptic Pattern on the Spatiotemporal Features and Diurnal Variation of Extremely Persistent Heavy Rainfall over the Sichuan Basin, China

The Sichuan Basin(SCB), China has a high incidence of extremely persistent heavy rainfall(EPHR) events. The EPHR events from 2009 to 2019 in the SCB were mainly concentrated over the northern and northwestern windward slopes and the central basin. They occurred from June to September, but especially in July, and peaked at 0300 LST. ERA5 reanalysis data and objective classification were used to investigate the synoptic patterns and their effects. There were three synoptic patterns during EPHR events, all accompanied by a Southwest Vortex. The location and intensity of the Southwest Vortex, thermal forcing of the Tibetan Plateau(TP), and low-level winds can greatly affect the intensity and spatial distribution of EPHR. When the Southwest Vortex was located in the western SCB and there were southerly low-level jets(LLJs), convergence and upslope wind would lead to EPHR over the northwestern or northern windward slopes. If there was no LLJ and the whole SCB was under the center of the Southwest Vortex, nocturnal EPHR was controlled by the internal circulation of the Southwest Vortex and the updraft generated by the thermal forcing of the TP, and the rainfall was weaker. The southeastern entrance of the SCB was a key area where the low-level wind dominated the nocturnal peak of EPHR. The nocturnal strengthened southeasterly wind in the key area is attributable to inertial oscillation, and the topographic friction plays an essential role in transporting momentum and moisture into the basin by generating easterly and northeasterly ageostrophic winds.