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Study Uses New Dataset to Investigate Desert Precipitation

Precipitation generation requires a combination of conditions such as moisture availability, cloud processes that can grow droplets to large sizes, and conditions in which droplets can reach the surface without evaporating. Understanding the details of rainfall generation in these conditions is important for weather forecasting and for the evaluation of artificial interventions to increase rainfall.

Despite the presence of rain-bearing clouds, surface-reaching rainfall amounts remain scarce in the UAE. Cloud droplet number concentrations are typically substantial, likely due to the abundance of aerosol particles on which the water droplets form. Thus, the UAE offers an environment in which arid rainfall generation can be investigated, due to its proximity to the Arabian Gulf, but with contrasting areas of desert and mountains.

As the UAE’s rainfall regime is relatively understudied compared to other regions, a research team led by Giles Harrison, Professor of Atmospheric Physics at the University of Reading in the United Kingdom and a second cycle awardee by the UAE Research Program for Rain Enhancement Science (UAEREP), worked on a study using a new dataset to investigate precipitation processes over the UAE.

Taking place at Al Ain International Airport, where the annual rainfall is 76 mm, the study utilized two years (2018–2020) of the team’s ceilometer observations. The ground-based ceilometer data recorded and monitored precipitation events at a high spatial and temporal resolution. Observing these events from the surface allows for the path of falling droplets to be monitored and assessed for droplet evaporation effects. The team used ceilometer backscatter data to determine droplet fall speeds and sizes in order to characterize microphysical properties of the rainfall in the UAE.

Through studying both the evolution of water droplets from the cloud base down to the surface and the local circumstances, the team explored how successful precipitation depends on the initial size of the droplets and the thermodynamic profile below the cloud. They found that for 64 of the 105 rain events, the droplet diameters ranged from 0.60 to 3.75 mm, with a mean of 1.84 mm. The smaller droplets, higher cloud bases, reduced cloud depths, and colder cloud bases all acted to prevent successful precipitation, instead yielding virga (28 out of the 105 rain generating events).

The study demonstrates that a significant proportion of the potential precipitation events lead to only short-lived virga occurrences. Therefore, understanding the underlying processes statistically and physically can explain why and where rainfall reaches the surface, helping to take a more informed approach towards the interventions, such as cloud seeding and related technologies.

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