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Abstract:
Deliberate injection, or seeding, of large hygroscopic particles into clouds has been suggested as a potential approach for artificial enhancement of rainfall. The particles are expected to accelerate coalescence growth of droplets, leading to more rapid production of rain drops. However, in continental convective clouds rain formation is significantly controlled by mixed-phase microphysical processes. Accordingly, the interaction between hygroscopic seeding and ice particle growth has been highlighted in recent studies. In our previous work we used the large-eddy simulation model UCLALES-SALSA to study convective clouds in conditions typically observed at the United Arab Emirates (UAE). The results showed that hygroscopic seeding increased the rime fraction of ice particles, which was identified as the main source for subsequently increased surface rainfall. In our current work, we continue this line of research further, now focusing on the importance of secondary ice production. Secondary ice production is expected to promote cloud glaciation and growth of ice particles. By the token of our previous model studies, such changes would act to increase surface rainfall. Since field observations suggest the naturally occurring secondary ice production to be weak in clouds with continental characteristics, such as those at UAE, we hypothesize that hygroscopic seeding could trigger or enhance secondary ice production where it otherwise wouldn’t occur, providing a so far insufficiently studied pathway for rain enhancement. We will present results from UCLALES-SALSA model experiments, where our preliminary simulations indeed suggest an increased sensitivity of the clouds to seeding injection when secondary ice is present.
