Abstract
Understanding rainfall in arid and water-scarce regions is central to the efficient use of water resources in agriculture, irrigation, and domestic food security. This work presents a new dataset with which to study precipitation processes in arid regions, utilising two years (2018–2020) of ceilometer observations made at Al Ain International Airport in the desert region of Al Ain, United Arab Emirates (UAE), where the annual rainfall is 76 mm. Ceilometer data provide a novel method by which to study both the evolution of water droplets from the cloud base down to the surface and the local circumstances required for rain to successfully reach the surface. In this work, we explore how successful precipitation depends on the initial size of the droplets and the thermodynamic profile below the cloud. 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. We find that smaller droplets, higher cloud bases, reduced cloud depths, and colder cloud bases all act to prevent successful precipitation, instead yielding virga (28 out of the 105 rain generating events). We identify how these multiple regional factors combine—specifically, we identify clouds deeper than 2.9 km, droplet diameters greater than 2 mm, and a midpoint below-cloud RH profile greater than 50%—to give successful rainfall, which may ultimately lead to more efficient rainfall enhancing measures, such as cloud seeding.
Highlights
The results show that the microphysical conditions within the cloud, the height and depth of the cloud, and the thermodynamic conditions beneath the cloud are all important in successful precipitation
The study described here presents a new collection of data over two years from the United Arab Emirates (UAE)—a region of the world where detailed microphysical observations of precipitation events are only beginning
The novel use of ceilometer data demonstrates an effective method to study the path of falling raindrops from the cloud base to the surface, which is not possible using more readily available data, such as from weather radar or surface-based precipitation gauge networks
Summary
The generation of precipitation in arid regions is, by definition, rare. It requires moisture availability, cloud processes that can grow droplets to large sizes, and conditions in which droplets can reach the surface without evaporating. For precipitation to occur at all requires a fortuitous combination of circumstances, all of which individually have a low probability, and may occur intermittently. Understanding details of rainfall generation in these conditions is important for weather forecasting and the evaluation of artificial interventions to increase rainfall. The United Arab Emirates (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
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