Gravity Waves Associated with Frontal Rainfall as a Preconditioning Mechanism for Warm-Sector Heavy Rainfall in South China

This study investigates the convective preconditioning mechanisms for nocturnal coastal heavy rainfall in the warm sector of a quasi-stationary front in South China on 10–11 May 2014. We conducted cloud-permitting numerical simulations and found that, in addition to mesoscale lifting provided by double low-level jets, convective preconditioning played a vital role in this event. The preconditioning was characterized by mid-level moistening and destabilization with wave-like variation over the region, which was induced by gravity waves associated with low-level ascent and high-level descent (n=2 wave). These hydrostatic waves propagated southeastward at a speed of ~27 m/s. The low-level upward motions of the waves were strongest at approximately 3 km AGL, and greatly enhanced relative humidity through adiabatic cooling and vertical transport of water vapor. These gravity waves are mainly generated during the evolution of northern frontal rainfall forced by synoptic forcings. As frontal rainfall strengthened, low-level diabatic cooling from rainwater evaporation triggered n=2 waves, further enhancing relative humidity along wave path. This process is confirmed through vertical Flourier decomposition of the vertical temperature profile over the frontal region. A sensitivity experiment with an earlier initial condition failed to reproduce the preconditioning process by gravity waves and the occurrence of warm-sector heavy rainfall at the coast. The western part of frontal rainfall stimulated in the sensitivity experiment located farther north and developed more stable, resulting in a mismatch between the region of mid-level moistening and the strong lifting area associated with double low-level jets. Overall, this study sheds light on the key relationship between frontal and warm-sector heavy rainfall, bridged by gravity waves.