Abstract:Guangxi encountered a process of double-convective-bands from 20:00 BT 25 to 08:00 BT 26 March 2023. This process was significantly different from that of the double rain belts in the past, causing significant deviations in the subjective and objective forecasts. Based on multi-source observation data and ERA5 reanalysis data, the Rossby wave energy dispersion, moist potential vorticity and horizontal frontogenic forcing in this process are analyzed. The results show that this process occurred under the background of large-scale circulation adjustment. The two Rossby wave trains that originated from the polar vortex and the Black Sea in the mid- and high-latitude jointly promoted the gradual vertical rotation of the transversal trough in the northeast region, guiding the mid- and high-latitude cold air further southward. During this period, the south branch trough in the low latitude gradually moved eastward, providing the dynamic lifting above the cold cushion in Guangxi, and also promoting the convergence of cold and warm airs in the low troposphere in Guangxi. With the addition of cold air to south and the southerly winds advancing northward under the inertial oscillation, the convergence of warm and cold airs in Guangxi was enhanced. The enhancement of atmospheric moist baroclinicity led to the development of moist potential vorticity, resulting in conditional symmetric instability of stratification. The warm-moist air climbed from south to north reaching the conditional symmetric instability area near 700 hPa, and then combined with the positive vorticity advection in front of the upper-level trough to trigger elevated convection, resulting in the development of the north-branch convective band. Influenced by the special topography of Guangxi, the favorable configuration of the θse isoline and the flow formed a tensile deformation effect, leading to frontogenic forcing, which triggered the initial convection in the south branch of Northeast Vietnam. The large θse latitudinal gradient and strong vertical wind shear in the low level of the central Beibu Gulf resulted in strong moist baroclinicity, which promoted the organization and development of the south-branch convective system when it passed through, and formed bow echo due to the mid-level dry air entrainment. In order to capture the key information of the occurrence and development of elevated convection after the north-branch front, it is necessary to focus on the numerical model in forecasting the mid-level trough and the thermo dynamic conditions above the cold air cushion.