Roles of the Topographically-Affected Boundary Layer Low-Level Jet in the Moisture Transport Process of Nocturnal Rainstorms in Mountainous Areas around the Western Sichuan Basin

Abstract

Fifteen cases were selected from May to September 2010 to 2021 to study the moisture transport of nocturnal rainstorms in mountainous areas around the western Sichuan Basin (western SCB). Nocturnal rainstorms mainly occur along mountains in southwestern SCB, from 22 to 04 LST, peaking at 00 LST. The four following moisture transport paths (proportions) were obtained using the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT): the local path from southeastern SCB (51.9%), the southerly path from the South China Sea (35.1%), the northwestern path from western Tibetan Plateau (TP) (6.6%), and the easterly path from the East China Sea (6.4%). The persistent southwest monsoon is the large-scale circulation for nocturnal rainstorms in western SCB while the boundary layer low-level jet (BLJ) over southeastern SCB plays a key role in diurnal variations of moisture transport in western SCB. To discuss the roles of the topographically-affected BLJ in moisture transport, the wind field is decomposed into creeping flows (flows over mountains) and circumfluence flows (flows around mountains). At 22 LST, ageostrophic creeping flows turn southeast due to the clockwise rotation caused by inertial oscillations of the BLJ, transporting moisture directly into western SCB and converging because of the blocking effect of TP. Meanwhile, the opposite geostrophic circumfluence flows and ageostrophic circumfluence flows meet and accumulate at the “trumpet” topography in western SCB, leading to moisture convergence, and the moisture budget peaks at 22 LST, 2 h earlier than the rainstorm peak. The creeping flows force upward motions over western SCB, and the conflict between upward warm–moist flows and cold–dry downslope flows from TP triggers convection, which significantly strengthens the vertical velocity. The results confirm the existence of the BLJ over southeastern SCB both in the Eulerian and Lagrangian viewpoints and reveal the roles of the topographically-affected BLJ in the moisture transport process of nocturnal rainstorms in western SCB.