The First Characterization of Fog Microphysics in the United Arab Emirates, an Arid Region on the Arabian Peninsula

Fog microphysics characteristics play an important role in fog life cycle, radiation and visibility. Measurements of fog microphysics are important in understanding these processes and improving existing model parameterization schemes. From 28 January to 17 February 2021, we carried out the first measurements of fog microphysics in the arid region of the United Arab Emirates and present initial results in this paper. Five events occurred, the longest lasting about 4 hr 28 min and the remaining lasting 2 hr or less. In this study, the meteorological conditions at synoptic scale associated with the formation of fog, are presented as well as the microphysics characteristics of fog. Maximum cloud droplet count ranged from 80 to 700 cc, Liquid Water Content (LWC) from 0.06 to 0.58 g m(-3) and the Median Volume Diameter reached 25 mu m in all the cases. The estimated Liquid Water Path reached a maximum of 100 g m(-2). The size distribution was bimodal for all visibility classes of mist, fog and thick fog with modes at around 4.5 mu m and between 22 and 25 mu m. The droplet size distribution broadened as the mature phase progressed, demonstrating a decrease in droplets in the 6-16 mu m range and an increase in droplets above 19 mu m. During the mature stage, droplet growth was observed with no associated decrease in LWC, suggesting growth by collision and coalescence was occurring. However, LWC did increase concurrently suggesting droplet growth by condensation was also taking place. Plain Language Summary During the cold season of each year, desert regions experience the formation of radiation fog due to the rapid cooling of the desert surface at night. Cities built on the fringes of major desert regions, such as the United Arab Emirates (UAE) cities, witness very poor visibility due to seasonal fog formation. The fog impacts air and road traffic and disrupts the daily life. Fog microphysics characteristics play a key role in fog life cycle, radiation and visibility. Measurements of fog microphysics are important to understand and improve existing model parameterization schemes for a more accurate forecast of fog occurrence and life cycle. We carried out the first fog microphysics measurements over the UAE, an arid region on the Arabian Peninsula, where fog is a frequent occurrence in winter months. Results from this study are the first of their kind in the region and are expected to advance the current knowledge on fog dynamics and characteristics.

Automated summary

The study conducted the first measurements of fog microphysics in the arid region of the United Arab Emirates, revealing the meteorological conditions and microphysics characteristics of fog formation. The results showed a bimodal size distribution of cloud droplets, with droplet growth observed during the mature stage, suggesting both collision/coalescence and condensation processes were involved in droplet growth.