The Impact of Kunming Quasi-Stationary Front on Yunnan in 2023

Utilizing surface observation data and ERA5 reanalysis data, this study analyzed the impact of Kunming quasistationary front (QSF) on Yunnan in 2023. Typical cases were selected to comparatively analyze the frontal weather, frontal structure, and circulation characteristics of the westward-advancing, maintaining, and eastward-retreating Kunming quasistationary fronts. The results indicate that Yunnan was influenced by 27 Kunming quasi-stationary front processes in 2023, including 14 westward-advancing events lasting 106 d, 5 maintaining events lasting 32 d, and 8 eastward-retreating events lasting 27 d. Westward-advancing events occurred throughout the year, maintaining events primarily occurred in spring and autumn, and eastward-retreating events mainly occurred in early winter, spring, and late autumn. Generally speaking, the westwardadvancing Kunming quasi-stationary front often brings strong cooling, localized snowfall, and cold waves to central and eastern Yunnan. The maintenance of the Kunming quasistationary front means persistent low temperatures, overcast, and rainy weather with limited sunshine in eastern Yunnan. And the eastward-retreating type often causes abrupt changes in the weather conditions in eastern Yunnan, transitioning from overcast to cloudy or clear. The isotherms within the frontal zone of the westward-advancing type are densely packed and perpendicular to the ground. Among the 3 types, the westward-advancing type is characterized with the highest upward height, the thickest cold air and the strongest easterly wind behind the front, which is followed by the maintaining type, with the eastward-retreating type as the weakest. All 3 types exhibit clockwise secondary circulation behind the front, with the westward-advancing type being the strongest. The following conditions are favorable for the westward advancement of Kunming quasi-stationary front: The 500 hPa East Asian trough is oriented northeastsouthwest, with strong cold advection behind the trough; the southern branch trough is located in the Arabian Sea with weak warm advection ahead; a shear line exists between Sichuan and Yunnan at 700 hPa; an anticyclone is located to the east of the Sichuan Basin at 800 hPa; and the surface cold high is near (110°E, 40°N). The following conditions are favorable for the maintaining of Kunming quasi-stationary front: The 500 hPa East Asian trough is quasi-north-south oriented, with weak cold advection behind the trough; the southern branch trough is located in the northern Bay of Bengal with moderate warm advection ahead; the Sichuan-Yunnan shear line at 700 hPa is not prominent; the anticyclone at 800 hPa is located to the north of the Sichuan Basin; and the surface cold high is to the east of 110°E and to the north of 40°N. The eastward retreating of Kunming quasi-stationary front is likely to be triggered by the following conditions: The 500 hPa East Asian trough is to the east of 140°E, with weak cold advection behind the trough; the southern branch trough deepens, with strong warm advection ahead of the trough; there is not Sichuan-Yunnan shear line at 700 hPa; the anticyclone at 800 hPa is positioned further south; and the surface cold high is weak.