Vertical Structure and Evolution of the Stratus-to-Fog-to-Stratus Transition: first high-resolution UAV-LCPIS Measurements over the Yellow Sea coast

Abstract. This study presents, for the first time, high‑resolution concurrent vertical profiles of temperature, humidity, water vapour, and fog/cloud droplet microphysics during a stratus‑fog‑stratus transition over the Yellow Sea (15–16 May 2025). The primary observational platform relied on a novel Lightweight Cloud Particle Imager sonde (LCPIS) deployed on an unmanned aerial vehicle (UAV) system. The event progressed through three distinct stages: pre‑fog stratus, dense fog (minimum visibility ~100  m), and lifting to stratus. Pre‑fog stratus featured a shallow mixed layer (Ri  <  0.25) capped by a strong inversion (Ri  >  0.25), trapping moisture below 300  m. During the fog transition, the inversion base stabilized at 270  m with a lapse rate of 5.0  °C·(100 m)⁻¹, while the saturated layer extended to the surface. Within the fog layer, the Richardson number (Ri) was negative, indicating vigorous turbulent mixing, while Ri >  0.25 near the fog top formed a dynamically stable cap. After dissipation, Ri turned positive and evaporated moisture re‑formed stratus. Peak droplet number concentration (NC) increased from 50–100 cm^-3 (pre‑fog) to 100–120  cm^-3 (fog), liquid water content (LWC) doubled from 0.1–0.2 to 0.2–0.45  cm^-3, and mean diameter ranged 10–20  μm. Microphysical peaks occurred at ~200  m in both pre‑fog and fog stages. Key transition indicators include downward extension of the saturated layer to the surface, rise of the zero dew‑point depression height, and NC stabilization with LWC increase. This study provides the first synchronized vertical observations of thermodynamic, water vapour and microphysical parameters during a stratus-fog-stratus transition, offering a robust observational basis for improving sea fog forecasting.