Mechanism of the Influence of Topography on the Initial Upscaling of the South China Squall Line: A Numerical Simulation Study

In this study, the National Center for Environmental Prediction and the National Center for Atmospheric Research 1°×1° reanalysis data and the Weather Research and Forecasting (WRF) 4.0 mesoscale numerical model were used to simulate the upscale process of a squall line in South China on April 13, 2016. To examine the role of topography in the upscaling process of the squall line, a number of sensitivity tests was designed to comprehensively examine the influence of Nanling on the upscale growth of the squall line and associated mechanism. The WRF model effectively simulated the changes before and after the squall line crossed the mountain and the precipitation distribution. The convection after the mountain is stronger than that before the mountain, and the horizontal scale grows faster; however, the terrain sensitivity tests at different heights demonstrated that a suitable terrain height is extremely beneficial to storm development. The topography affects the scale and organization of the squall line, and the high topography disperses the convection in the north of Guangdong. Terrain can indirectly affect the distribution of convective cells and the strength of convective cells in the squall line by changing the horizontal flow field, water vapor field, vertical movement, and vertical wind shear at the lower level. The absence of terrain obstruction is beneficial to the jet stream and transporting water vapor toward the northward direction is more convenient. A certain terrain height is beneficial to the vertical movement of the low level; however, the terrain is extremely high to facilitate the vertical movement of the upper level; furthermore, the low level is more likely to be primarily detoured. When the terrain exceeds a certain height, the low-level convergence field is correspondingly weakened.