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<p>Understanding the turbulent and microphysical dynamics of radiation fog is critical for forecasting low-visibility events over the Indo-Gangetic Plain. Here, we combine high-frequency field observations from the Winter Fog Experiment with high-resolution WRF Large-Eddy Simulations (LES) to examine the vertical structure and evolution of a dense fog episode over Indira Gandhi International Airport, New Delhi, during January 6–7, 2023. Turbulence diagnostics reveal that fog initiation was not purely surface-driven but was characterized by volumetric condensation within a shallow stratified layer, predominantly spanning 20–130&#xa0;m above ground level (a.g.l.), whose depth varied over time and persisted throughout the fog life cycle, triggered by intermittent shear-induced turbulence in the residual layer. The LES reproduces this non-classical, aloft-assisted saturation with realistic cloud water fields and visibility evolution. During the mature phase, a persistent inversion maintained a semi-decoupled boundary layer, characterised by weak near-surface turbulence and enhanced turbulent kinetic energy near the fog top. This sustained aloft saturation suppressed surface mixing and extended fog duration. Dissipation was governed by progressive turbulence deepening and radiative–mechanical coupling: after sunrise, buoyant turbulence near the surface and shear-driven mixing aloft eroded the inversion, promoting vertical recoupling and rapid fog clearance.</p>

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Vertical Turbulence Asymmetry in a Winter Fog Event over New Delhi: Evidence from WiFEX and WRF–LES

  • 期刊论文

  • 发表日期：

    2026年4月15日

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  • Priyangshu Choudhury,
  • Sandeep Wagh,
  • Sachin D. Ghude

摘要

Understanding the turbulent and microphysical dynamics of radiation fog is critical for forecasting low-visibility events over the Indo-Gangetic Plain. Here, we combine high-frequency field observations from the Winter Fog Experiment with high-resolution WRF Large-Eddy Simulations (LES) to examine the vertical structure and evolution of a dense fog episode over Indira Gandhi International Airport, New Delhi, during January 6–7, 2023. Turbulence diagnostics reveal that fog initiation was not purely surface-driven but was characterized by volumetric condensation within a shallow stratified layer, predominantly spanning 20–130 m above ground level (a.g.l.), whose depth varied over time and persisted throughout the fog life cycle, triggered by intermittent shear-induced turbulence in the residual layer. The LES reproduces this non-classical, aloft-assisted saturation with realistic cloud water fields and visibility evolution. During the mature phase, a persistent inversion maintained a semi-decoupled boundary layer, characterised by weak near-surface turbulence and enhanced turbulent kinetic energy near the fog top. This sustained aloft saturation suppressed surface mixing and extended fog duration. Dissipation was governed by progressive turbulence deepening and radiative–mechanical coupling: after sunrise, buoyant turbulence near the surface and shear-driven mixing aloft eroded the inversion, promoting vertical recoupling and rapid fog clearance.