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Study Uses Hybrid Bin Microphysics Parcel Model to Investigate Impact of Hygroscopic Seeding on Precipitation Formation

Hygroscopic seeding accelerates precipitation formation by suppressing the activation of the natural, less hygroscopic particles, and or by promoting the formation of larger water drops directly. The seeding particles become activated at lower supersaturation due to their higher hygroscopicity and larger size compared to background cloud condensation nuclei (CCN).

Different techniques are used to estimate the droplet spectra just after the activation. In most bin microphysical models, only the number of the activated CCN particles is calculated. All activated droplets are combined into one bin, or distributed over multiple bins using a prescribed function.

To study how natural aerosol particles and different types of hygroscopic seeding materials affect the precipitation formation, Dr. Lulin Xue, a third cycle awardee of the UAE Research Program for Rain Enhancement Science (UAEREP) and the Chief Scientist at Hua Xin Chuang Zhi Science and Technology LLC, worked on a project that involved the development of a hybrid bin microphysical scheme to simulate the evolution of drop spectrum in an ascending air parcel.

The modeling efforts focused on simulating the effects of different seeding materials on clouds and precipitation over the UAE, including the ICE-70 (USA) and NCM flares, as well as the novel nanomaterial particles developed by Prof. Linda Zou’s 1st Cycle Project. The team incorporated Dr. Paul Lawson’s 2nd Cycle project aircraft measurements to realistically simulate the role of background aerosols during seeding operations over the UAE.

Carried out carried out near the Al Hajar Mountains in the UAE, the project introduced a novel parameter to describe the impact of different seeding particles on the evolution of the drop size distribution (DSD). The hybrid bin parcel model performed more than 100 numerical experiments to investigate how hygroscopic seeding can affect the DSD of convective clouds in various environmental conditions.

Their modeling results show that the nanomaterial seeding particles produce much stronger seeding effects compared to other seeding agents, especially in strong updraft conditions (~5 m/s), and can produce positive effects even in maritime influenced air masses where other seeding materials are ineffective.

The results of the study showed that the Ostwald-ripening effect has a substantial contribution to the broadening of the DSD near the cloud base. The efficiency of this effect increases as the updraft velocity decreases, whereas the efficiency of hygroscopic seeding is significant only if the size of the seeding particles falls in the coarse particle size range.


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