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

The similarities and differences of environmental conditions between daytime and nighttime torrential precipitation processes in North China have not been fully clarified. Based on the precipitation data from 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 then the characteristics of environmental physical variables of their corresponding circulation situations are compared and analyzed. Through direct comparison, the environmental conditions of the nighttime heavy precipitation process in North China are found to be different from the daytime heavy precipitation process, which provide an important basis for deepening the understanding of the formation mechanism of nighttime heavy rainfall in North China. The results show that the nighttime type of torrential precipitation processes in North China develop more often after midnight, and have more torrential precipitation amount and concentrated regions with torrential precipitation, while the daytime processes and the first half-night precipitation of the nighttime processes have stronger convection, both of which 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 influencing synoptic systems on torrential precipitation processes in North China. The distribution characteristics of physical variable of environments in different types of circulation situations are somewhat different. Moisture 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 θse 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.