The Influence of Coastline on the Orientation of Squall Lines

South China is a high-incidence area of squall lines. Most squall lines in South China have an orientation of southwest-northeast. These squall lines usually form in southwest China and propagate to the coastal area of South China, where the coastline is generally orientated in also southwest-northeast direction. Observation shows that the orientations of some squall lines in South China tend to become parallel to the coastline while approaching to the coastline. This work tries to examine the possible relationship between the orientation of the coastline and the squall lines as they approach to the coastline in South China. By analyzing squall lines in South China from 2016 to 2017, we found that 76.54% of the squall lines were parallel to the coastline. The squall lines whose orientations have obvious changes when they approach to the coastline were chosen to examine the potential impact of coastline on the changes of the orientations of the squall lines. We found that the orientations of about 52% of these squall lines cases may have been mainly affected by coastline when those cases in which the changes of squall lines orientation could be caused by changes of background circulation were removed. Radar analyses and numerical experiments are being performed in order to examine the possible mechanism how the coastline may have changed the orientation of the approaching squall lines. It is speculated that land-sea-breeze and convergence associated with friction difference along the coastline could have played important roles in the changes of the orientations of the squall lines.