Abstract
A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.
摘要
2020 年 8 月 10 日至 18 日, 中国西南四川盆地发生持续性强降水事件, 除第一天外, 降水量呈现显著的夜间峰值和下午谷值的特征. 累积降水最大中心位于盆地西北部, 但夜雨增强首先主要出现在盆地西南部, 随后是盆地西北部. 研究结果表明, 异常强的西太平洋副热带高压西伸, 有利于盆地东南侧偏南低空急流的维持, 从而有利于持续的水汽输送和盆地异常高的可降水量分布. 盆地降水日变化与低空急流风速和风向的周期性变化密切相关, 这种周期性变化由惯性振荡和地形热力强迫共同引起. 夜间增强的降水大多由盆地西南部新生的湿对流带来. 夜间盆地西南部大气具有高的对流有效位能和更湿的近地层空气, 由高原西移到盆地的降水云有关的冷空气/偏北气流带来的冷空气与从南边而来的暖空气相遇有利于对流的触发. 暖湿气流在冷空气之上斜升, 同时有西南涡和准地转抬升的助力, 促进了对流不稳定能量的释放和强降水的产生.
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Acknowledgements
This study was supported by the National Natural Science Foundation of China (Grant Nos. 41775050, 91937301, 41775002, 42005008) and the Science Development Fund of Chinese of Academy of Meteorological Sciences (2020KJ022).
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Article Highlights
• The diurnal cycle of rainfall with higher nocturnal amount over the basin was closely related to the periodic oscillation of a LLJ.
• The periodic oscillation of the LLJ was caused by the combination of inertial oscillation and terrain thermal forcing.
• The nocturnal rainfall was enhanced by convective storms initiated over the southwest of the basin during evening hours.
• The convective initiation occurred as cold air from either previous precipitating clouds or a larger-scale northerly flow met a warm and humid current from the south.
This paper is a contribution to the special issue on Summer 2020: Record Rainfall in Asia—Mechanisms, Predictability and Impacts.
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Xia, R., Luo, Y., Zhang, DL. et al. On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10–18 August 2020. Adv. Atmos. Sci. 38, 2183–2200 (2021). https://doi.org/10.1007/s00376-021-1118-7
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DOI: https://doi.org/10.1007/s00376-021-1118-7