In response to water shortages exacerbated by population growth and climate change, an increasing number of countries have invested in weather modification technologies over the past decade, including precipitation enhancement, or cloud seeding. The United Arab Emirates (UAE) has been a leader in exploring this technology within the arid Gulf region, launching a cloud seeding program in 2002. Two decades on, the country is now taking stock of its efforts and cloud seeding research could provide an opportunity for broader regional collaboration as Gulf countries seek to deal with the shared challenges presented by climate change.
How does cloud seeding work?
Cloud seeding aims to increase the amount of natural rainfall through airborne or ground-based interventions in the microphysical processes of specific cloud types. Targeting warm cloud types, hygroscopic cloud seeding entails introducing large hygroscopic (water-attracting) aerosol particles into growing clouds. These hygroscopic particles compete with the smaller-sized natural aerosol particles, thus increasing the cloud’s uptake of available liquid water so that it produces larger drops. These larger drops then trigger a droplet multiplication effect, termed the collision-coalescence process, which enhances rainfall generation.
International efforts and challenges
Evaluating the effectiveness of operational cloud seeding programs is critical to advancing weather modification research and providing policymakers with realistic techno-economic metrics. According to the most recent review on global precipitation enhancement activities conducted by the World Meteorological Organization (WMO) Expert Team on Weather Modification, cloud seeding from aircraft platforms is generally more effective than other techniques, such as ground-based generators, customized rockets, and artillery shells. Results from operational cloud seeding programs spanning several countries, including Australia, China, India, Israel, South Africa, Thailand, and the United States, record increases of between 10% and 30% in precipitation and cloud “lifetime.” Alternatively, several studies reported the limited efficacy of seeding experiments for drought relief, along with inconclusive results stemming from unreliable measurements and/or co-occurring microphysical and dynamical processes that are difficult to account for.
The complex variability of cloud properties in both space and time makes it difficult to accurately evaluate the impact of cloud seeding. In fact, several of the difficulties of carrying out randomized experiments on cloud seeding are similar to those involved in designing randomized clinical trials in the medical field. However, clouds are more transient and less accessible than human patients, making it particularly difficult to reproduce randomized seeding experiments.
To overcome the limitations of field experiments, long-term statistical analyses have been carried out to evaluate seeding impacts using control-target (unseeded-seeded) regression derived from historical rainfall records. However, such analyses rely exclusively on local rain gauge measurements that fail to capture potential changes in climate circulations that may influence local rainfall patterns, far beyond seeding effects. Hence, interpreting the results requires diagnosing the physical mechanisms associated with the statistical variability in seeded rainfall amounts.
Weather radars generate high-resolution and real-time estimates of cloud and precipitation properties above the surface by emitting electromagnetic signals and analyzing backscatters from intercepted hydrometeors. They return continuous volumetric scans of cloud systems that provide critical information on their microphysical and thermodynamic evolution throughout their lifetime. As such, weather radars have been a key instrument in several cloud seeding programs and are an important infrastructural prerequisite for conducting weather modification research. Investigating the effects of seeding by combing through both physical and statistical analyses is considered the most systematic approach to evaluating cloud seeding experiments.
The UAE cloud seeding program
In the past few decades, the Middle East has undergone substantial development that led to the establishment and expansion of large cities, such as Dubai and Abu Dhabi in the UAE. This growth, compounded by rising temperatures, strains water supplies, especially in arid regions, creating a need for improved monitoring and capture of available rainfall. Over the past two decades, the UAE has implemented an operational aircraft-based hygroscopic seeding program to compensate for its mean annual rainfall of less than 120 mm.
The UAE cloud seeding program, implemented by its National Center of Meteorology (NCM), follows the conventional approach of igniting hygroscopic flares composed of natural salts (primarily potassium chloride) at the base of convective clouds near the updraft core. Launched in 2002, the program targeted frequent summertime convection along its northeastern Hajjar Mountains bordering Oman. The program expanded over the years, and since 2010 it has been able to target suitable cloud candidates year round over the entire UAE.
A recent study constituted the first attempt to evaluate objectively the long-term impacts of the UAE’s ambitious cloud seeding program. The study implemented a hybrid methodology combining both statistical and physical analyses of long-term rain gauge records from before (1981-2002) and after the launch of the seeding program as well cloud properties from weather radar data. A posteriori historical target/control regression was carried out along with a search for change points in the time series to statistically decouple natural and seeded rainfall data signatures. Results indicated an average increase of 23% in rainfall associated with seeding with a statistically significant uptick in rainfall trends detected in 2011. Additionally, the radar-based physical analysis comparing an archive of storm properties between unseeded (87) and seeded (65) storms showed consistent and systematic enhancements in storm properties within 15 to 25 minutes of seeding. The overall results are in line with similar evaluations of operational hygroscopic seeding in other regions. This combined evidence provides important insights into the long-term impacts of cloud seeding operations over the UAE and its contribution to the nation’s water resources.
The UAE Research Program for Rain Enhancement Science: A catalyst for interdisciplinary research and innovation
Despite its promise, aircraft-based cloud seeding technology has not progressed as quickly as other research fields. The UAE Research Program for Rain Enhancement Science (UAEREP), an international merit review research initiative, was established in 2015 to stimulate and promote scientific advancement and the development of new technology in the field. To this end, the UAEREP provides managed grant assistance to projects targeting innovative research on cloud seeding and the broader field of rainfall enhancement.
The below schematic consolidates all 11 projects funded over the UAEREP’s four awarding cycles since 2015, including research areas as diverse as land-atmosphere feedback, aerosol-cloud-precipitation interactions, and warm, cold, and mixed-phase cloud microphysics. More importantly, the schematic highlights the interdisciplinary nature of these research projects, which are leveraging the latest advancements in material science and nanotechnology for the development of high-performance seeding materials, robotics, and unmanned aerial vehicle technology for more cost-effective and lower-risk seeding missions, and AI/machine learning tools for more efficient computing and early guidance systems. The full list of research output across these projects can be accessed here.