Effects of quasi-stationary fronts in southern China on large-scale freezing rain in winter

Using daily ERA-Interim reanalysis data in global 0.125°×0.125° grids and freezing rain observations in January from 1979 to 2016, quasi-stationary fronts in southern China and their effects on the formation of large-scale freezing rain in the winter are analyzed based on composite analysis of several typical cases. The results are as follows:(1) Under the joint effects of the Kunming Quasi-stationary Front and South China Quasi-stationary Front, large-scale freezing rain over southern China often exhibits a zonal distribution pattern. (2) Three mechanisms are responsible for the intense temperature inversion, the vast area affected by the freezing rain and the abundance of water vapor in southern China. First, cold-air damming (CAD) along the eastern Tibetan Plateau usually leads to cold air mass accumulation to the east of Hengduan Mountains and north of Nanling Mountains, affecting vast areas of southern China. The CAD is mainly caused by cold advection and adiabatic cooling, although diabatic cooling can also be observed on high-altitude region. Second, the warm layer formed by warm advection at 700 hPa and 850 hPa can be widespread as well. Third, moisture advection from South China Sea and the Western Pacific at 850 hPa leads to abundant water vapor in southern China. (3) The joint effects of the East Asian trough at 500 hPa, the southern branch trough at 700 hPa, the anticyclone at 850 hPa and the Mongolian High near the surface provide necessary circulation conditions for the convergence of polar continental air mass, tropical oceanic air mass and tropical continental air mass in the low-level troposphere to the east of the Tibetan Plateau. (4) The cold air outbreak in East Asia moves southward along the eastern border of the Tibetan Plateau, forcing the near-surface warm moist air to rise and leading to the formation the South China Quasi-stationary Front. Once the cold air mass accumulates to a certain thickness, it would climb up along the estern slope of the Plateau in low latitudes and converge with the westerly winds of the southern branch, forming the Kunming Quasi-stationary Front. The complicated frontal structure in the Kunming Quasi-stationary front and South China Quasi-stationary front accompanied with the extensive and intense inversion is favorable for the formation of extensive freezing rain.