Revisiting Summertime Nocturnal Rainfall Over the Western Sichuan Basin in Relation to Near-Surface Winds

Abstract

Near-surface winds (NSW) play a crucial role in linking topographical forcing and rainfall triggering. However, the NSW characteristics over the western Sichuan Basin (SCB) have been largely overlooked, limiting our understanding of the unique rainfall features in this complex terrain. This study aims to address this gap by analysing station-observed hourly rainfall and 10-m wind records during the summers from 2015 to 2022. We identified that a near-surface cyclonic circulation pattern exists over the western SCB on precipitation days (PDs). Specifically, a near-surface northerly-to-westerly wind shear was observed between the northern and southern parts of the western SCB during night-time on PDs. This wind shear forms as the wind over the northern part rotates from south-easterly in the afternoon to north-westerly at night, while wind over the southern part rotates from north-easterly in the afternoon to southwesterly at night. The intensity of this wind shear was significantly correlated with rainfall on the daily scale. Such a wind shear was also observed on the event scale, even when the dominant NSW patterns are different. The wind shear intensity increased gradually prior to the onset of rainfall events, providing favourable near-surface dynamic conditions for rainfall. The formation of the wind shear was found possibly related to the generation of a lee vortex downstream of the Yungui Plateau. The spatio-temporal characteristics of NSW identified in our results aim to enhance our understanding of the mechanisms driving nocturnal rainfall in the western SCB and to provide near-surface metrics for model improvement over complex terrains.