A new view of the dynamics, stability and longevity of volcanic clouds

Powerful explosive volcanic eruptions inject ash high into the atmosphere, which spreads to form umbrella clouds. Identifying key physical processes governing the dynamics, stability and longevity of umbrella clouds is central to assessing volcanic hazards as well as the nature of volcanic forcings on climate. Here we present a series of laboratory experiments producing turbulent particle-laden jets and subsequent axisymmetric intrusive gravity currents into a stratified environment. Our experiments reproduce many of the main dynamical regimes observed during the formation of an explosive volcanic column, and highlight new dynamics for the umbrella cloud. Theoretical predictions of column collapse from a simple model of a turbulent jet are in good agreement with experimental observations as well as previous studies. Depending on the flow intensity, the strength of the initial environmental density stratification and the particle concentration at the source, resulting umbrella clouds can, however, evolve through a series of new regimes as a result of the dynamics of particle sedimentation within these flows as well as from their bases. Using scaling theory we show that during cloud spreading, internal sedimentation drives the growth and intermittent overturn of thin, gravitationally unstable “particle boundary layers” (PBLs) as particle-rich plumes. This PBL-driven convection can have remarkable effects ranging from progressive dilution of clouds to their catastrophic overturn and collapse. In natural eruptions, whether the dynamics of PBLs play a major role in particle sedimentation depends on the grain size distribution inside the cloud and on eruption column height. In general, particles larger than ~ 60 μm-1 mm are expected to settle individually, whereas finer particles will accumulate PBLs resulting in the formation of armless mammatus clouds or dangerous gravity currents at much larger distances from the volcanic vent than ever considered before. Such dynamics is apparent in observations of numerous modern eruptions and is inferred from the deposits of historic and prehistoric eruptions for where there exist appropriate data. Consideration of the consequences of these phenomena for problems such as volcanic hazards to humans and climate change may, thus, be very important in the assessment of future eruptions.