Hygroscopic Seeding Scheme Incorporated in a Bulk Microphysics Model

Abstract

The United Arab Emirates (UAE) launched the UAE Research Program (UAEREP) for Rain Enhancement Science in order to promote scientific advancement and the development of new technology. In the project “Advanced Study on Precipitation Enhancement in Arid and Semi-Arid Regions,” which was one of the three projects awarded the UAEREP prize in 2016, the authors plan to apply the cloud seeding model (Hashimoto and Murakami, 2016) to the assessment of seedability and to the evaluation of cloud seeding effects, after necessary modifications (Hashimoto, Murakami and Haginoya, 2017) and new developments.

New hygroscopic seeding scheme is incorporated in the cloud seeding model. This scheme is developed based on the look-up table representing the number concentration of nucleated droplets as a function of updraft velocity and number concentrations of two types of cloud condensation nuclei (CCN); ammonium-sulfate (AS) and sodium chloride (SC) particles. To establish the look-up table, we carried out a large number of sensitivity experiments with different updraft velocities and initial number concentrations of the two types of CCN, by using a bin microphysics air parcel model. This model has a three-dimensional bin space that is constituted by the masses of pure water and the two types of CCN to represent a variety of physico-chemical characteristics of droplets and CCN and to simulate droplet nucleation in a competitive situation between the two types of CCN for available water vapor. As the result of sensitivity experiments, we found three regimes for droplet nucleation; 1) AS particles dominate the droplet nucleation, 2) the droplet nucleation is suppressed, compared to other regimes, and largely influenced by the ratio of number concentrations of AS and SC particles, 3) SC particles dominate the droplet nucleation. Borders among these regimes change depending on the updraft velocity. The new hygroscopic seeding scheme is able to simulate the droplet nucleation seamlessly covering these regimes by the use of the look-up table.

Using the cloud seeding model with this scheme, numerical simulations of hygroscopic seeding are conducted in a three-dimensional domain under the simplified conditions; horizontally uniform profiles of horizontal wind, temperature, humidity and number concentration of AS particles, in a tree-dimensional domain. The domain is horizontally 12 km x 16 km wide. The top of the domain is 5 km height. The layer from 1 to 3 km height is initially moistened much more than upper and lower layers, as a potential cloud layer. Giving a warm bubble just under the moistened layer initiates uplifting of the air to form a cloud. In addition, a seeding airplane, which is represented by a moving point releasing SC particles at a constant seeding rate, flies below cloud base but above the warm bubble. Both of the seeded SC particles and the background AS particles are transported to cloud base to form cloud droplets and compete for available water vapor. Compared to the no-seeding simulation, the seeded cloud produces more rainwater. The amount of produced rainwater depends on the seeding rate and the dilution of SC particles during the transportation from the seeding point to the cloud base.

We plan to extend the numerical experiment of hygroscopic seeding to realistic conditions of the atmosphere in arid and semi-arid regions of the United Arab Emirates.

Acknowledgement

This study was supported by the Ministry of Presidential Affairs and the NCMS, the UAE, under the project in the UAEREP for Rain Enhancement Science, “Advanced Study on Precipitation Enhancement in Arid and Semi-Arid Regions.”

References

Hashimoto, A. and M. Murakami, 2016: Numerical simulations of glaciogenic cloud seeding with dry ice pellets and liquid carbon dioxide under simplified conditions. SOLA, 12, 20-23. doi:10.2151/sola.2016-005.

Hashimoto, A., M. Murakami and S. Haginoya, 2017: First application of JMA-NHM to meteorological simulation over the United Arab Emirates. SOLA, 13, 146-150, in print, doi:10.2151/sola.2017-027.