Observed and LES Simulated Turbulence Characteristics During Winter Fog Events: A WiFEX case study over IGI Airport, New Delhi.

Understanding the turbulent and microphysical dynamics of radiation fog is critical for forecasting low-visibility events over the Indo-Gangetic Plain. Here, we integrate high-frequency field observations from the WiFEX campaign with high-resolution WRF Large Eddy Simulations (LES) to examine the vertical structure and evolution of a dense fog episode over IGI Airport, New Delhi, during January 2023. Turbulence diagnostics reveal that fog initiation was not purely surface-driven but characterised by volumetric condensation within a shallow stratified layer (~20–100 m AGL), triggered by intermittent residual-layer shear turbulence. The LES captured this non-classical, aloft-initiated saturation with realistic cloud water fields, validating ceilometer backscatter profiles. During the mature phase, persistent stratification maintained a decoupled boundary layer, with elevated turbulence sustaining the upper fog layer and suppressed surface mixing prolonging fog longevity. Dissipation was governed by a dual-layer re-coupling of turbulence, driven by concurrent surface heating and residual-layer shear, which rapidly eroded the inversion and mixed out the fog. Model-observation synergy demonstrates that resolving the vertical asymmetry and intermittency of turbulence is essential for realistic fog simulation.