Diurnal Off-Mountain Propagation of Rainfall and Low-Level Vertical Velocity over the Lee Side of the Yungui Plateau

Abstract Diurnal off-mountain propagation is a distinctive feature of rainfall over terrestrial areas, whereas the causes of this phenomenon are not well understood. Focused on the rainfall downstream of the Yungui Plateau (YGP), this study aims to examine whether the gravity waves stimulated by an assumed terrain-related thermal forcing could explain the feature. The results show that the rainfall diurnal phase propagates eastward at a speed of approximately 13 m s −1 over the lee side of YGP during warm seasons. The diurnal amplitude reaches its zonally maximum over the slope of the YGP and drops sharply downstream the terrain. The low-level vertical velocity exhibits similar diurnal characteristics. A linear model forced by a hollow heating is proposed to mimic the thermal forcing related to a mountain. Experiments with the model show that there are mainly two branches of waves around the terrain. One is over the upstream with upwind-tilting phase lines that move toward − z and − x directions. The other branch exists over the lee side of terrain with downwind-tilting phase lines that move toward − z and + x directions, which is considered to be relevant to the off-mountain propagation feature. The wave behavior over the YGP is then reproduced using the model. It is shown that the main features of the diurnal phase lag and the zonal amplitude distribution pattern of the low-level updraft could be captured by the model, suggesting an important role of the gravity wave in driving the diurnal propagation of vertical velocity and rainfall downstream large terrains. Significance Statement The purpose of this study is to better understand why diurnal off-mountain rainfall propagation exists over the downstream of Yungui Plateau (YGP). This is important because the off-mountain rainfall propagation influences the rainfall events over the lee side of mountains and can further influence people’s lives therein. Inspired by previous studies on coastal rainfall, we proposed a simplified linear gravity wave model forced by a topography-related heating function. The results showed that the thermally forced gravity waves could reproduce the main features of the propagating phase and amplitude pattern of upward motion disturbance over the lee side of the YGP. Our results highlight the importance of gravity waves stimulated by topography-related heating on the diurnal propagation features over the downstream of mountains.