Abstract:In mid-December 2023, Shanxi Province experienced a weather event of low temperature, rain, snow and icing. On 13 December, wire icing occurred in Yuanqu County located in southern Shanxi, significantly disrupting people’s lives and production activities. Using conventional meteorological observations, Doppler weather radar data and the ERA5 reanalysis datasets, this paper investigates the formation mechanism of this serious disaster event caused by wire icing. The results show that this wire icing event in Yuanqu was of the mixed rime and glaze type, which was caused by the combined effects of short-duration freezing rain in the early stage and prolonged freezing fog in the subsequent period. Freezing fog played a more significant role than freezing rain. In the north frontal zone at 500 hPa, the horizontal trough in front of the high pressure ridge near the Ural Mountains and a deep cold vortex near the Sea of Okhotsk both remained stable with less movement. The short-wave trough ahead of the bottom of the horizontal trough moved rapidly eastward into Shanxi, while in the south frontal zone, the southwest jet stream at 700 hPa was strong. The cold trough in the northeast-southwest direction at 850 hPa was located over North China, and the surface inverted-trough in Hetao Area developed violently, interacting with the return-flow weather pattern. Besides, the southwest warm-humid airflow ascended along the low-level cold air cushion, providing a favorable large-scale circulation background for the formation of freezing rain and freezing fog in Yuanqu. The temperature advection configuration of “upper warming and lower cooling” in vertical direction led to a “cold-warm-cold temperature” structure in Yuanqu from low to high levels, with the mid-level temperature above 0℃ and the lower-level temperature below 0℃. The short-term freezing rain in Yuanqu, triggered by the short-wave trough, was attributed to the melting mechanism. The vertical humidity advection configuration of “wet above and dry at bottom”, coupled with sinking motion together made the lower atmosphere highly saturated. Additionally, temperature inversion continued in the lower atmosphere. These conditions collectively facilitated the long-term persistence of freezing fog in Yuanqu. Abundant supercooled water droplets that had two phases of rapid growth continuously impinged on the wire surfaces, causing ice accretion to become thickened progressively. Southeasterly airflow ascended along the windward slopes of the horn-shaped terrain, and also played a certain role in promoting the thickening of accretion on wires.