Microphysical and Electrical Characteristics of Fog in the United Arab Emirates

Fog is a prevalent weather phenomenon in several arid regions, including the Empty Quarter desert in the United Arab Emirates (UAE), located on the northeastern side of the Arabian Peninsula. Despite being primarily an arid country with desert landscapes dominating its terrain, most events causing visibility to drop below 1 km in the UAE are attributed to condensation processes rather than dust occurrences. We present in-situ measurements of fog microphysics from the Barakah Nuclear Power Plant (BNPP, a coastal site located at 23.968052°N, 52.267309°E) and atmospheric electric field measurements obtained during the Wind-blown Sand Experiment (WISE)-UAE field campaign conducted at Madinat Zayed (23.5761° N, 53.7242° E; elevation: 119 m).

Measurements of fog microphysics were conducted during the winter season of 2021 -2022 at the 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: a bimodal distribution in droplet number concentration (N_c), with modes at 4.5 µm and 23.2 µm . Despite the high proportion of fog smaller droplets associated with the fine mode, the greatest contribution to the liquid water content (LWC) comes essentially from medium to large droplets between 10 µm and 35 µm. The recalibration of existing visibility parameterization schemes revealed that the decrease (increase) in horizontal visibility with increasing (decreasing) LWC (FI, fog index) tends to be more gradual for the studied cases compared to standard visibility parameterization schemes. Additionally, the fog sedimentation velocity, estimated to be at a maximum of 1.85 cm s^-1, occurs predominantly in the LWC range of 100 - 200 mg m³, 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.

For the first time, the changes in the atmospheric electric field (E_z) during foggy conditions is studied in a hyper-arid region; the United Arab Emirates (UAE), using comprehensive measurements during the Wind-blown Sand Experiment (WISE)-UAE. The longer the fog persists, the more variable E_z becomes, primarily due to the fog's ability to absorb and redistribute the charges of the atmospheric small ions. This absorption alters the ion balance, affecting electrical conductivity within the atmosphere, which in turn leads to sustained alterations in E_z. A record high E_z value of 2571 V m^-1 was measured during a long-lasting fog event. E_z values returned to normal during the fog dissipation phase. The results of this work can be applied to develop techniques for fog harvesting and to improve fog forecasting by accounting for the effect of the electric field on fog lifetime and characteristics.