Abstract:This study is concerned with a nocturnal convective process from the night of the 24th to the early morning of the 25th September 2017. The meso-β scale convective system on the south side of the frontal rainband perpendicular to the quasi-east-west frontal orientation gradually evolved into a bow echo, causing short-time severe precipitation in the middle and lower reaches of the Yangtze River, accompanied by Level 7 thunderstorm gale. From the perspective of the large-scale environment, there is no favorable thermodynamic conditions at night. Therefore it is difficult to make a forecast. In this study observations and numerical simulations are used to analyze the fomation mechanism. The radar observations show that there is a northeast-southwest meso-β scale convective belt, moving in the south-easterly direction, and there are new convective cells being triggered on its southwest side, forming the lateral back-building propagation. The new cells then merge into the main convective zone. Other new cells, generated ahead (southeast side) of the main convective zone, gradually develop into a northwest-southeast belt and move to the northeast, eventually making the original main northeast-southwest convective belt gradually strengthen and finally evolve into a bow echo. Although the high-resolution numerical model simulation results deviate from observations in intensity and time, the convective system evolution processes are very close to the observations. Therefore, the vertical vorticity equation is used to diagnose the mechanism. The results show that the vorticity tilt term plays an important role in the lateral back-building propagation. In the early stage of the convection development, new cells are generated on the southwest side under the effect of the vorticity tilt term and merge with the main echo. As the echoes continuously merge and strengthen, the divergence term becomes more important and the positive vorticity of the main echo increases significantly under both the vorticity tilt term and the divergence term. Besides, the vorticity vertical transport term propagates the positive vorticity upward, which is beneficial to the vertical extension of the main convection. New cells are triggered in front of the main echo due to the effect of the vorticity horizontal advection term. However, the vertical extension height is low, so it moves northeastward guided by the low-level wind. Its vorticity increases during the movement and it aligns into a northwest-southeast band, which finally leads to the conversion of the linear main echo into the bow echo. The formation of this bow echo differs significantly from the classical model which has the rear inflow jets in the rear part of the bow echo. On the contrary, this case is mainly influenced by the development of convective systems within the warm zone and has a significant frontal near-surface inflow.