The effect of fine particles on ash cloud and plume dynamics

Powerful explosive volcanic eruptions produce turbulent, buoyant plumes that inject ash high into the atmosphere, which spread as gravity currents to form umbrella clouds. The transport, dispersion and sedimentation of ash within these flows are key physical processes influencing the dynamics, stability and longevity of umbrella clouds. Understanding the underlying mechanics at work is, consequently, crucial for assessing volcanic hazards as well as the nature of volcanic forcings on climate. State-of-the-art models applied to predict the evolution of the plumes and umbrella clouds must include effects related to both particle-particle and particle-fluid momentum exchanges, which are complex and poorly understood. Particles suspended in both the plume and umbrella cloud modify the intensity of the turbulence. This has a knock-on effect on the entrainment of fluid into the flows, and hence the mass and buoyancy fluxes that ultimately drive the spread of the cloud. Accordingly, we investigate the effect of polydisperse particle distributions on the plume and cloud dynamics. In particular, we look at how the presence of fine particles (ash, pumice or aerosol particles) affect the longevity of the umbrella cloud, and how re-entrainment of particles of all sizes leads to instability of the cloud. Our investigation involves laboratory experiments where a particle/water mixture is injected into a density-stratified environment. The results of our experiments are used to inform a simple axisymmetric forced-plume model. Despite its simplicity, the model is capable of predicting the onset of instability as a function of the source conditions. Early dynamics of umbrella cloud formation as the spreading of a particle-laden plume at it's neutrally buoyant level.

Experimental plume showing turbulent structure and onset of sedimentation.