Thermodynamic-microphysical coupling mechanisms of hydrometeor phase transitions during persistent wind turbine icing
摘要
An extreme winter icing event from 1 to 7 February 2024 in central-eastern China induced sequential hydrometeor phase transitions (freezing fog, freezing drizzle, intense freezing rain, and mixed-phase precipitation) at Hunan’s Gutaishan wind farm. To investigate the underlying physical mechanisms, this study utilized multisource observational data, ERA5 reanalysis, and turbine operational records. The combined influence of the South Branch Trough and the Western Pacific Subtropical High drove a prolonged convergence of southwesterly warm-moist advection with easterly cold air, maintaining a stable temperature inversion. During the freezing-fog stage, this cold-warm-cold inversion structure trapped near-surface moisture, leading to the accumulation of supercooled droplets that ultimately caused a shutdown of 50% of the operational turbines. The freezing-drizzle stage was governed by the warm-rain mechanism. Due to the absence of ice-phase hydrometeors in shallow clouds (cloud-top temperatures > -10 °C), purely liquid drizzle drops fell through the underlying cold layer to become supercooled, triggering a farm-wide shutdown. The intense freezing-rain stage followed the melting mechanism. Abundant ice-phase hydrometeors from deep clouds (cloud-top temperatures < -20 °C) melted completely within a pronounced warm layer (vertical temperature inversion strength of 10.0 °C km− 1) and subsequently became supercooled in the underlying cold layer, accelerating blade icing. Finally, the mixed-phase stage was driven by a melting-refreezing mechanism. Mid-level dry-cold air intrusion weakened the warm layer, causing descending solid hydrometeors to undergo only partial melting. These hydrometeors then formed mixed-phase precipitation (coexisting ice pellets and trace supercooled rain) within the deep cold layer, maintaining the farm-wide shutdown.