In-situ measurements of fog microphysics: Visibility parameterization and estimation of fog droplet sedimentation velocity

• Published in: Atmospheric research Vol. 309; p. 107570
• Main Authors: Nelli, Narendra, Francis, Diana, Abida, Rachid, Fonseca, Ricardo, Masson, Olivier, Bosc, Emmanuel
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.10.2024; Elsevier
• Subjects: Environmental Sciences; Fog droplet sedimentation; Fog Index; Fog microphysics; Fog monitor; Hyper-arid regions; Microwave radiometer; Physics; Radiation fog; Visibility; Fog microphysics; Radiation fog; Fog droplet sedimentation; Microwave radiometer; Fog monitor; Visibility; Fog Index; Hyper-arid regions
• ISSN: 0169-8095; 1873-2895
• DOI: 10.1016/j.atmosres.2024.107570

Measurements of fog microphysics were conducted during the winter season of 2021 ‐2022 at the Barakah Nuclear Power Plant (BNPP), located in the Western coastal region of the United Arab Emirates. Twelve fog events were observed during this period. The primary objective of this study is to detail the microphysical characteristics of these events and refine current visibility parameterization schemes based on in-situ measurements of fog microphysical properties. All observed fog events are found to share a common feature of a bimodal distribution in droplet number concentration (Nd), with modes at 4.5 μm and 23.2 μm. Despite the high proportion of smaller fog droplets associated with the fine mode, the greatest contribution to the liquid water content (LWC) comes essentially from medium to large droplets between 10 and 35 μm. The recalibration of existing visibility parameterization schemes revealed that the increase in horizontal visibility with increasing FI (fog index) tends to be more gradual for the studied cases compared to standard visibility parameterization schemes. Additionally, the fog droplet sedimentation velocity, estimated to be at a maximum of 1.85 cm s−1, occurs predominantly in the LWC range of 100–200 mg m−3, corresponding to a median volume diameter 24.8 μm. Our findings shed new light on the complexity of fog microphysics and its impact on visibility, underscoring their importance in refining weather models for accurate fog forecasting. •Observed consistent bimodal size distribution of fog droplets across 12 fog events, with a dominance of smaller droplets.•Smaller droplets (1-10 μm) formed initially and contributed majorly to Liquid Water Content (LWC).•Larger droplets (10-50 μm) developed later, contributing substantially to total condensed water.•Fog droplets sedimentation velocity can reach up to 1.85 cm s-1, correlated to a median volume diameter of 24.8 µm.