Impact of initial water vapor on the upscale growing process of a squall line in South China

During the spring and summer seasons, in the South China region where abundant water vapor is present, squall lines can rapidly develop into larger scales within a short period of time. In order to explore the influence of water vapor content on the process of squall line scale growth in South China, using the WRF model, a numerical simulation was conducted for a squall line system in South China on 11 May 2020. We investigated the effect of the variation of water vapor at different levels on its intensity and structure, and discussed the growth mechanism of the squall line system. This squall line occurred with the presence of high-level jet and low-level wind shear complementing each other, within an unstable layer of "dry at the high level and wet at the low level". The simulation showed that, in the early stage of the squall line development, a large maximum convective available potential energy (MCAPE) was observed in the southern part of the convection and coastal warm areas, which is beneficial for the accumulation of unstable energy here. Meanwhile, with the high low-level wind shear, the linear structure of the convection was well maintained. Subsequently, the squall line propagated southward and merged with warm region convection, resulting a further scale growth. Water vapor experiments showed that the MCAPE values are primarily influenced by the moisture content in the low-level atmosphere. More low-level moisture content causes stronger thunderstorm high pressure. Additionally, the presence of higher MCAPE values and larger low-level vertical wind shear contribute to the growth of convective cells in the post-convective stage, prolonging their existence. Reducing the mid-level water vapor content results in a decrease in intense surface precipitation, a weakening of convective intensity, and a quick dissipation into individual convective cells. But when the squall line moves into the area with high MCAPE values, it once again develops into a linear structure. Therefore, an increase in low-level moisture or a decrease in mid-level moisture favors the genesis of convection. However, reducing mid-level moisture results in relatively drier air at mid-levels, making it difficult for the linear structure to be sustained. Further investigation into the internal structure of the squall line reveals that vertical motion and rear inflow also influence the scale growth of the squall line. In the convective system analyzed in this study, the strong rearward inflow enhances the upward motion and generates forward outflow, leading to severe surface wind. Strengthening low-level moisture not only increases the size of the stratiform cloud region at the rear of the convection, but also leads to a stronger upward motion sustaining vertically, which promotes prolonged convective activity. On the other hand, reducing mid-level moisture weakens convective intensity and lowers the height of the echo tops. During the development stage, the rearward inflow intensifies, and dry cold air descends rapidly, leading to the strengthening of the surface cold pool, and causing strong winds due to the forward outflow.