Cloud-Aerosol-Electrical Interactions for Rainfall Enhancement Experiment: CLOUDIX

There is compelling evidence that a strong coalescence process stimulates the development of a secondary ice process (SIP) in Cumulus Congestus (CuCg) clouds (Lawson et al. 2022). Recent numerical simulations by Korhonen et al. (2020) further demonstrate the importance of mixed-phase microphysics and riming in controlling the overall rainfall in modeled CuCg seeded with hygroscopic material over the UAE, which runs an operational cloud seeding program since 2002 under the UAE National Center of Meteorology (NCM). A summer 2019 field campaign collected measurements from CuCg clouds over the UAE with cloud base temperatures ranging from about 10°C to 12°C. Based on the analysis of over 300 cloud penetrations with the SPEC research Learjet, the sampled clouds did not appear to form a sufficiently strong collision-coalescence process to initiate the desired SIP (Wehbe et al. 2021; Lawson et al. 2022).

A follow-on “Cloud-Aerosol-Electrical Interactions for Rainfall Enhancement Experiment” (CLOUDIX) is underway in summer 2023. The primary objective of CLOUDIX is not to repeat the investigation of natural clouds, but to modify as many clouds as possible and look for enhanced coalescence and a SIP. A series of coordinated flight missions are being conducted between a UAE NCM King Air cloud seeding aircraft and the SPEC Learjet. The objective to modify the subcloud aerosol population is attempted via hygroscopic seeding and/or aerosol coagulation via electric charge in order to potentially enhance coalescence to the extent that the natural SIP is activated. The overarching goal is therefore to stimulate the natural SIP and potentially enhance precipitation by increasing the concentration of drops in the 15 to 35 µm size range, and possibly, reducing the concentration of drops with diameters less than 15 µm. Hygroscopic seeding with the appropriate material can potentially increase drops in the 15 to 35 µm size range. Reducing drops < 15 µm is much more challenging, but may be possible by creating an electrostatic charge in the central part of the updraft below cloud base Ambaum et al. (2022). The anticipated results will provide a contemporary dataset of in situ observations to cross-examine different cloud seeding materials with and without electric charge intervention.

References

Lawson, R. P., Bruintjes, R., Woods, S., & Gurganus, C. (2022). Coalescence and secondary ice development in cumulus congestus clouds. Journal of the Atmospheric Sciences, 79(4), 953-972.

Korhonen, H., Optimization of aerosol seeding in rain enhancement strategies (OASIS) (2020). Final Report to the UAE Program for Rain Enhancement Sciences. 10 pp. Available at: https://www.uaerep.ae/en/scientific-publications

Wehbe, Y., et al. (2021). Analysis of aerosol–cloud interactions and their implications for precipitation formation using aircraft observations over the United Arab Emirates. Atmospheric Chemistry and Physics, 21(16), 12543-12560.

Ambaum, M. H. P., Auerswald, T., Eaves, R., & Harrison, R. G. (2022). Enhanced attraction between drops carrying fluctuating charge distributions. Proceedings of the Royal Society A, 478(2257), 20210714.