Influence of Synoptic Pattern and Low‐Level Wind Speed on Intensity and Diurnal Variations of Orographic Convection in Summer Over Pearl River Delta, South China

Abstract

Based on 5 years of operational Doppler radar data, the influences of atmospheric synoptic patterns and low‐level prevailing wind speed on the intensity and diurnal variations of summer orographic convection occurrence frequencies over Pearl River Delta, South China, have been investigated. Results show that the inland orographic convection and rainfall generally occurs under synoptic pattern characterized with the prevailing southwesterly wind within the lower troposphere. The summer orographic convection over the mountains in northeastern Pearl River Delta is not only controlled by the orographic thermal conditions but also the dynamic forcing with the increase of wind speed. Owing to the strong windward mechanical lifting and moisture transport associated with the strong ambient onshore winds, the number of convection occurrences characterized by a dominant diurnal afternoon peak occurs much more frequently in the high‐wind speed days. While due to the weak orographic mechanical lifting and moisture supply in the low‐wind speed days, the number of convection occurrences in the afternoon decreases considerably and two comparable peaks occur in the afternoon and early morning. The nighttime peak in the low‐wind speed days is mainly attributed to the nocturnal acceleration of the low‐level southwesterly wind associated with the inertial oscillation and the corresponded enhanced windward lifting effects.

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).…”

Diurnal cycle of precipitation and near-surface atmospheric conditions over the maritime continent: land–sea contrast and impacts of ambient winds in cloud-permitting simulations

“…Moderate increases in wind speed lead to topographic waves, which interfere with deep convection because of their alternating ascent and descent and decrease rainfall compared to weak winds. Rao et al (2019) suggested that higher wind speed conditions over south China can support a higher occurrence of convection and also stronger diurnal variation, although they did not directly examine the diurnal amplitude of precipitation. More recently, Zhu et al (2020) showed that both sea breeze circulations and afternoon rainfall over Hainan Island could become stronger under weaker ambient wind speed at the lower level.…”