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

Abstract. This study investigates the vertical profiles of microphysical and thermodynamic properties within radiative fog layers in the Strzyżów valley (Southeastern Poland), based on in-situ, remote sensing and tethered balloon soundings data. Across three case studies of radiation fog that occurred in September 2023, 74 soundings were performed, with 41 employing the OPC-N3 instrument to capture droplet spectra. The results indicated similar weather conditions in all cases, with a liquid water path consistently above 15 g·m⁻², although no transition to dense fog was observed. The effective droplet radius decreased with height (between 3–4.6 µm for 100 m), with larger droplets (≥18.5 µm) concentrated near the ground.

The fog dissipated both from the top and bottom, with the mature fog stage marked by peak liquid water content (LWC) and the droplet number concentration (N_c) near 80 % of the fog height. Theoretical calculations of droplet terminal velocity (for droplets ≥18.5 µm) indicate that larger droplets are removed from fog layers within minutes, affecting the longevity of the fog. Equivalent adiabaticity values (α_eq – the ratio by which the adiabatic lapse rate of the mixing ratio needs to be multiplied to give the same amount of liquid water path as observed in a specific cloud) ranged between 0 and 0.6. Except in one instance where negative values α_eq were observed near the ground, a phenomenon scarcely reported in existing fog studies.

Having instruments measuring radiation at two different heights, it was possible to estimate the effect of fog on reducing the total shortwave and longwave (NET) radiation at ground level by 150 W·m⁻² (just before the fog disappearing and after). The measured dependence of the reduction of longwave radiation by fog depends linearly on the amount of liquid water path.

As a result of the measurements, average values of liquid water content and droplet number concentrations were obtained for the observed optically thin fogs in the valley area. Mean LWC in the fog layer core was found between 0.2–0.4 g·m⁻³, with N_c up to 300 cm⁻³. The effective radius (8–10 µm) exhibited a linear height-dependent decrease, with radiation model closures yielding minimal biases, supporting the accuracy of radiation assessments within fog environments.