Diurnal Off-Mountain Propagation of Rainfall and Low-Level Vertical Velocity over the Leeside of the Yungui Plateau: A Gravity Wave Mechanism

Diurnal rainfall off-mountain propagation is a distinctive feature of rainfall diurnal cycle over terrestrial areas, whereas the cause of this phenomenon is still controversial. This study aims to document the diurnal cycle of rainfall and low-level circulations over the leeside of the Yungui Plateau using hourly rain gauge records and reanalysis data, and to develop a linear gravity wave model to explain the diurnal features. It was found that nocturnal to early morning rainfall dominated the leeside of the Yungui Plateau during the warm seasons, and the diurnal phase of rainfall propagated eastward at a speed of approximately 13 m s^-1. The low-level circulations also displayed evident diurnal oscillation, which can contribute to the triggering of rainfall events during the night, and influence the decay stage of rainfall events. The low-level vertical velocity exhibited a similar diurnal signal propagating eastward. A simplified linear gravity wave model forced by a topography-related heating function was proposed and analytically solved to explain the diurnal features. It was found that increase of background wind (mountain radius) could enhance (suppress) the propagation speed downstream of a mountain, and that magnitude and vertical scale of heating mainly influenced the amplitude of wave field. The main features of diurnal oscillation and propagation signals downstream of the Yungui Plateau was then analytically reproduced using the model. The results demonstrate that gravity waves stimulated by a topography-related thermal forcing may play a key role in forming the diurnal features of rainfall and low-level circulations downstream of the mountains.