Study Uses Large-eddy Simulation to Demonstrate Importance of Mixed-phase Processes for Rain Enhancement in UAE
Cloud seeding aims to accelerate the hydrometeor growth and consequently convert water contained inside clouds to snow or rainfall more efficiently than it would naturally. Due to its importance in tackling water scarcity, seeding clouds by the release of hygroscopic or glaciogenic aerosol particles in convective clouds to enhance precipitation has received increasing scientific as well as commercial interest. While glaciogenic seeding in orographic clouds has consistently shown positive results and is relatively well understood, the effects of hygroscopic seeding remain uncertain and varied.
To gain a better understanding of the hygroscopic seeding effects on convective rainfall in atmospheric conditions typical for the summertime UAE, Professor Hannele Korhonen, the Director of the Climate Research Program at Finnish Meteorological Institute and a second cycle awardee of the UAE Research Program for Rain Enhancement Science (UAEREP), conducted a study employing a Large-eddy Simulation (LES) code together with a detailed aerosol–cloud microphysics model to investigate the conditions and processes conducive to seeding in the country. The study aimed to demonstrate that mixed-phase processes are important for the rain enhancement through hygroscopic seeding by incorporating seeding emissions in the model experiments.
The study involved model experiments in a semi-idealized setup describing clusters of moderately deep (cloud top up to 10 km) convective clouds based on summertime conditions observed in the country. To provide the initial conditions for the large-eddy simulations in this work, convective precipitation events were simulated by the numerical weather prediction (NWP) model. To select an appropriate case for the LES, days with observed precipitation were identified from ground observation station data delivered by the National Center of Meteorology (NCM). Potential cases were then screened using satellite observations to identify time and location of mixed-phase convective clouds likely to produce precipitation.
The study concluded that the effects of hygroscopic seeding go beyond the different pathways to invigorate the warm-phase collision–coalescence growth and impact the growth of ice particles in mixed-phase convective clouds, as the droplets formed on seeding aerosol are taken up by the updrafts. More specifically, the simulated hygroscopic seeding was seen to accelerate riming, thus enhancing the cold precipitation process and ultimately rainfall.
As the study is carried out in an area where mixed-phase processes are identified as the main source for rainfall in convective clouds, the model experiments highlight the importance of mixed-phase processes in mediating the effects of hygroscopic seeding on rainfall and offered compelling theoretical evidence in support to the hypothesized role of riming growth.
The study also suggest that a more realistic case-study setup is required for a better quantitative estimate of the seeding efficacy and to confirm its findings in a more general parameter space, especially in terms of the surface effects and meteorological conditions.
For more, see the following link:
https://doi.org/10.1175/JAMC-D-21-0183.1
