Diurnal Variations of Low-Level Winds and Precipitation Response to Large-Scale Circulations during a Heavy Rainfall Event

Abstract

A succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

Diurnal Variation of Meiyu Rainfall in the Yangtze Plain During Atypical Meiyu Years

“…The subtropical high was weak during the atypical Meiyu periods and was located 10°of longitude to the east of the Yangtze Plain (Figures 13a-13c). The enhanced subtropical high during the nighttime would provide stronger geotropic winds and warmer conditions that is beneficial for the occurring of morning precipitations (Rao et al, 2019;Zeng et al, 2019). On the contrast, the subtropical high was strong during the normal Meiyu periods and located close to the southeast of the Yangtze Plain (Figures 13d-13f).…”

“…By contrast, the area of subtropical high during the normal Meiyu periods was much larger and located on the southeastern side of the Chinese mainland ( Figure 2a). In addition, the stronger subtropical high during the normal Meiyu periods would provide strong geotropic winds and warm conditions that promote wind diurnal amplitude in both dynamic and thermodynamic manners (Rao et al, 2019;Zeng et al, 2019). In addition, the average 500-hpa geopotential height in northern China during the atypical Meiyu periods was larger than that of normal periods (Figures 1a and 2a), indicating higher northern air pressure during the atypical Meiyu periods.…”

“…Journal of Geophysical Research: Atmospheres strong geotropic winds and warm conditions that promote wind diurnal amplitude in both dynamic and thermodynamic manners (Rao et al, 2019;Zeng et al, 2019). The enhanced wind diurnal amplitude during the normal Meiyu periods over the Yangtze Plain was thought to be beneficial for the morning rainfall peak (Xue et al, 2018).…”

“…From the perspective of the diurnal variation, the geopotential height decreased during the daytime as a result of solar heating (Figures 13c and 13f), whereas the geopotential heights increased during the nighttime due to atmospheric cooling (Figures 13a and 13d). The enhanced subtropical high during the nighttime would provide stronger geotropic winds and warmer conditions that is beneficial for the occurring of morning precipitations (Rao et al, 2019;Zeng et al, 2019).…”

Nocturnal-to-morning rains during the warm season in South China: characteristics and predictability

“…The rainfall in the early morning accounts for a large proportion of the whole day's rainfall, especially in the pre-monsoon-onset period (Jiang et al 2017). In particular, regulated by strong nocturnal low-level southwesterly winds in the warm sector, extreme rainfall can form over specific mountainous areas (Chen, Zhang, and Zhao, 2016;Zeng et al 2019;Zhong et al 2019a) and coastal urban areas Chen 2018, 2019;Wu et al 2020) during the nocturnalto-morning hours.…”