Turbulence and particle dynamics in volcanic clouds in humid atmospheres.

The eruption of Hunga Tonga–Hunga Ha’apai (HTHH), which began around 04:00 UTC on January 15, 2022, injected substantial amounts of water vapor above the stratosphere. The subsequent explosions occurred in an exceptionally humid environment. Although the massive umbrella cloud produced by the initial explosion obscured direct observation, volcanic lightning provided information about the later events. This prompts a reevaluation of how atmospheric and eruption-driven moisture affect plume dynamics and turbulent particle collisions within volcanic clouds, particularly under humidity conditions that greatly exceed those previously assumed. We conducted high-resolution simulations using a non-hydrostatic, stably stratified moist atmospheric model coupled with inertial particles. Our model incorporates a novel formulation that enables independent control of atmospheric and plume-source moisture. The simulations reproduce ring-shaped turbulent regions observed as lightning rings during the HTHH eruption pulses, and show that increased humidity compresses the turbulent regions inward and enhances upward transport. Additionally, the model captures oscillatory plume height behavior associated with gravity waves modulated by moisture. These findings shed light on a previously unexplored coupling between moisture, turbulence, and particles in volcanic eruptions. They also offer a framework for inferring internal plume properties and source parameters in successive eruptions obscured by pre-existing volcanic clouds.