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A Modeling Study on the Development of a Bowing Structure and Associated Rear Inflow within a Squall Line over South China

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  • 1 Laboratory for Climate and Ocean–Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
  • | 2 Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
  • | 3 Laboratory for Climate and Ocean–Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
  • | 4 School of Atmospheric Sciences, Nanjing University, Nanjing, China
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Abstract

Through convection-permitting simulations, this study examines a large bowing structure within a squall line that occurred during the rainy season in South China. The bowing structure is closely associated with a local enhancement of (and balance between) the cold pool and the line-normal environmental low-level vertical shear. Rear inflow plays an essential role in the formation and evolution of this large bowing structure. It is found that the low-level rear inflow is largely a natural consequence of the baroclinically generated horizontal vorticity near the surface, while the midtropospheric rear inflow is forced by several pairs of bookend vortices. Vorticity budget and vortex-line analyses show that the bookend vortices form mainly through the tilting of horizontal vorticity. Consolidation of these pairs of bookend vortices forms a broad zone of contiguous rear inflow. The environmental flow and horizontal pressure gradient force associated with the midlevel pressure deficit induced by the rearward-tilting buoyant updrafts, on the other hand, are not primarily responsible for the formation of the rear inflow.

Corresponding author address: Dr. Zhiyong Meng, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China. E-mail: zymeng@pku.edu.cn

Abstract

Through convection-permitting simulations, this study examines a large bowing structure within a squall line that occurred during the rainy season in South China. The bowing structure is closely associated with a local enhancement of (and balance between) the cold pool and the line-normal environmental low-level vertical shear. Rear inflow plays an essential role in the formation and evolution of this large bowing structure. It is found that the low-level rear inflow is largely a natural consequence of the baroclinically generated horizontal vorticity near the surface, while the midtropospheric rear inflow is forced by several pairs of bookend vortices. Vorticity budget and vortex-line analyses show that the bookend vortices form mainly through the tilting of horizontal vorticity. Consolidation of these pairs of bookend vortices forms a broad zone of contiguous rear inflow. The environmental flow and horizontal pressure gradient force associated with the midlevel pressure deficit induced by the rearward-tilting buoyant updrafts, on the other hand, are not primarily responsible for the formation of the rear inflow.

Corresponding author address: Dr. Zhiyong Meng, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China. E-mail: zymeng@pku.edu.cn
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