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<p>To explore the formation mechanism of severe dense fog (SDF), we compare the similarities and differences in surface meteorological conditions and boundary layer structures among SDF, dense fog (DF) and heavy haze (HH) events based on observations from tethered balloon soundings, microwave radiometers, wind profiler radars and conventional ground-based meteorological stations, as well as analysis fields from the European Center for Medium-Range Weather Forecasts. In addition, we investigate the roles and mechanisms of ultra low-level jets (ULLJs) and vertical wind shear in the formation of the SDF. The results indicate that during the formation and development stages of the SDF, the maximum surface cooling exceeds 4°C, whereas no obvious cooling was observed during DF and HH events. Additionally, during the formation and maintenance stages of the SDF, the boundary layer was characterized by strong temperature inversion, an “upper-level dry and lower-level wet” (UDLW) structure, the presence of an ULLJ, and pronounced vertical wind shear. Similar ULLJs (or strong wind speeds) and vertical wind shear also occurred during DF and HH events. The latter featured a deep and strong inversion, but without the UDLW structure. The above differences indicate the following formation mechanisms of the SDF. (1) On SDF days, northerly (non-northerly) winds prevailed above (below) the jet, forming an UDLW structure that favors surface radiative cooling and humidification. In contrast, on DF and HH days, southerly winds prevailed above the jet, which was not conducive to the formation of the UDLW structure and surface radiative cooling. (2) The SDF days featured vertical wind shear that resulted in a distinct “upper-level stable and lower-level turbulent” structure in the surface layer, whereas the boundary layer remained generally stable on HH days. Consequently, the key factor determining whether the HH evolves into the SDF is the vertical wind profile within the boundary layer, which governs the formation of the UDLW structure and further creates the cooling, moistening and turbulence conditions favorable for the formation and maintenance of SDF.</p>

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Formation mechanisms of severe dense fog regulated by ultra low-level jets: Observations of dry-air intrusion and vertical wind shear over Shouxian County, China

  • 期刊论文

  • 发表日期：

    2025年10月14日

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  • Chun’e Shi,
  • Jie Yang,
  • Hao Zhang,
  • Guanying Yang,
  • Jianping Zhou,
  • Hanfeng Hu,
  • Jun Yang

摘要

To explore the formation mechanism of severe dense fog (SDF), we compare the similarities and differences in surface meteorological conditions and boundary layer structures among SDF, dense fog (DF) and heavy haze (HH) events based on observations from tethered balloon soundings, microwave radiometers, wind profiler radars and conventional ground-based meteorological stations, as well as analysis fields from the European Center for Medium-Range Weather Forecasts. In addition, we investigate the roles and mechanisms of ultra low-level jets (ULLJs) and vertical wind shear in the formation of the SDF. The results indicate that during the formation and development stages of the SDF, the maximum surface cooling exceeds 4°C, whereas no obvious cooling was observed during DF and HH events. Additionally, during the formation and maintenance stages of the SDF, the boundary layer was characterized by strong temperature inversion, an “upper-level dry and lower-level wet” (UDLW) structure, the presence of an ULLJ, and pronounced vertical wind shear. Similar ULLJs (or strong wind speeds) and vertical wind shear also occurred during DF and HH events. The latter featured a deep and strong inversion, but without the UDLW structure. The above differences indicate the following formation mechanisms of the SDF. (1) On SDF days, northerly (non-northerly) winds prevailed above (below) the jet, forming an UDLW structure that favors surface radiative cooling and humidification. In contrast, on DF and HH days, southerly winds prevailed above the jet, which was not conducive to the formation of the UDLW structure and surface radiative cooling. (2) The SDF days featured vertical wind shear that resulted in a distinct “upper-level stable and lower-level turbulent” structure in the surface layer, whereas the boundary layer remained generally stable on HH days. Consequently, the key factor determining whether the HH evolves into the SDF is the vertical wind profile within the boundary layer, which governs the formation of the UDLW structure and further creates the cooling, moistening and turbulence conditions favorable for the formation and maintenance of SDF.