Measurement report: Microphysical and optical characteristics of radiation fog – a study using in situ, remote sensing, and balloon techniques

Abstract. Based on in situ observations, remote sensing, and tethered balloon soundings, this study examines vertical profiles of microphysical and thermodynamic properties in radiative fog layers in the Strzyżów valley (Southeastern Poland). During three radiation fog events in September 2023, 74 soundings were performed, with 41 employing the OPC-N3 instrument to capture droplet spectra. All cases showed similar conditions, with liquid water path consistently above 15 g m−2, placing most observations within the thin fog regime. Effective droplet radius decreased with height (3–4.6 µm over 100 m), while larger droplets (18.5 µm) were near the ground. Fog dissipation occurred from both top and bottom. The mature stage showed peaks in liquid water content (LWC) and droplet number concentration (Nc) at about 80% of fog depth. Larger droplets (≥ 18.5 µm) were removed within minutes, affecting fog longevity. Equivalent adiabaticity (αeq) – the scaling of adiabatic lapse rate to match observed liquid water path (LWP) – ranged from 0 to 0.6, with one rare case of negative near-ground αeq. Instruments above and below the fog allowed estimation of effect of the fog's impact on radiation flux. The difference of total shortwave and longwave NET (downward − upward) radiation at ground level before and after dissipation reached 150 W m−2. A linear relationship was found between reduction in longwave radiation and LWP under optically thin conditions. Mean LWC in the fog core ranged from 0.2–0.4 g m−3, and Nc reached 300 cm−3. Near-surface effective radius was 8–10 µm, decreasing with height. Agreement between model outputs and observed fluxes supports the retrieved microphysical parameters.