Characteristics of daytime and nighttime types of torrential precipitation processes in North China

The similarities and differences in environmental conditions between daytime and nighttime torrential precipitation processes in North China have not been fully clarified. Based on precipitation data collected at 981 surface meteorological stations and ERA5 reanalysis data, the spatial and temporal distribution characteristics and environmental conditions of daytime and nighttime types of torrential precipitation processes in North China during the period from May to September of 2013—2023 are comprehensively analyzed. The objective classification method of the obliquely rotated T-mode principal component analysis is used to classify the circulation situations of daytime and nighttime types of torrential precipitation processes, and the characteristics of environmental physical variables of their corresponding circulation situations are then compared and analyzed. Direct comparison reveals that the environmental conditions of the nighttime heavy precipitation process in North China are different from that of the daytime heavy precipitation process, which provides an important basis for deepening our understanding of the formation mechanism of nighttime heavy rainfall in North China. The results show that the nighttime type torrential precipitation processes in North China develop more often after midnight, and have more occurrences over more concentrated regions, while the daytime processes and the first half-night precipitation of the nighttime processes have stronger convection and they mainly occur in July and August. Moisture of nighttime type is richer than that of daytime type, while CAPE of daytime type is higher than that of nighttime type. The distributions of both 850-hPa and 500-hPa temperature difference and 850-hPa vertical velocity are similar between the two types. Low-level wind speed and 0—1 km vertical wind shear are significantly higher in the nighttime type than in the daytime type. Low troughs and vortices at the edge of the subtropical high are the main synoptic systems influencing torrential precipitation processes in North China. The distribution characteristics of physical variable of the environments in different types of circulation situations are somewhat different. Moisture conditions of cold vortex of daytime type and pre-ridge circulation of nighttime type are the worst. 0—6 km vertical wind shear (SHR₆) and 0—3 km vertical wind shear (SHR₃) are generally not strong. SHR₆ of daytime torrential precipitation processes is slightly stronger than that of nighttime type, and SHR₃ of nighttime torrential precipitation processes is slightly stronger than that of daytime type. The above results indicate that the nighttime heavy rainfall over North China is closely related to the East Asian summer monsoon, which is characterized by abundant water vapor, high θ_se value, appropriate CAPE value and strong wind speed in the lower atmosphere. The low-level wind field and SHR₃ distribution indicate that one of the dominant factors of nighttime heavy rainfall over North China is the diurnal variations of low level jet or strong wind speed.