Interplay Between Shear at the Base of Volcanic Clouds and Ash Sedimentation

Ash sedimentation models typically assume particles settle at their terminal velocity, with little attention thus far paid to the effect of shear, caused either by fast spreading of the volcanic cloud or wind drag. In particular, such shear may have consequences for ash-cloud fingers. These are downward-propagating columns of ash that form from settling-driven gravitational instabilities at the base of the volcanic cloud. The fingers are thought to enhance sedimentation of particles and have been witnessed at eruptions from numerous volcanoes such as Eyjafjallajökull (Iceland) and Mt. Etna (Italy). Although recent studies have started to develop theoretical frameworks for the formation of these structures and to characterise them through field studies and analogue experiments, there has been no consideration of how finger formation and the resultant sedimentation rate are affected by shear.

We present results from analogue experiments where a buoyant, particle-bearing, fresh-water gravity current propagates above a denser, sugar-solution environment. The propagation speed increases with the density difference between the two fluids. During spreading, particles settle out of the current, forming visible fingers produced by the settling-driven gravitational instability. By varying the initial density ratio between the current and environment and the particle size, we observe different behaviours depending on the relative timescales of spreading and settling. For fast spreading, shear inhibits sedimentation as particles remain suspended in the current. In contrast, slow spreading allows sufficient time for sedimentation and thus fingers are produced. The resultant loss of particles from the current increases the density difference, leading to faster spreading. This acceleration creates interfacial shear instabilities which modulate the gravitational instability, grouping fingers into larger-wavelength downwellings.

We systematically characterise the observed behaviour according to the occurrence and interaction of gravitational and shear instabilities and present our results in terms of dimensionless quantities including the ratio of spreading and settling timescales. We then use these parameters to scale our results to the natural system. This study will ultimately produce parameterisations of ash-finger enhanced settling in models of tephra dispersal, a crucial tool for hazard and risk assessments.