The global economic consequences of the relatively small Eyjafjallajok̈ ull eruption in the spring of 2010 caught the world off guard. That the eruption cloud lasted for several months rather than weeks, efficiently disrupting air travel and the holiday plans of thousands of Northern Europeans, drew arguably more attention and a certainly garnered a highly emotional response. The unexpected longevity of this eruption cloud was touted to be the consequence of unusual ”perfect-storm-like” weather patterns that also conspired to produce the very dry conditions leading to the massive Russian fires later that summer. It was called ”an anomaly”. However, this anomaly nearly repeated itself the following year in the form of the 2011 Grimsvoẗ n eruption cloud. Indeed, in the geological record, possibly 45% of explosive eruptions produced long-lasting clouds similar to the 2010 Eyjafjallajok̈ ull event, which is clearly not so unusual.
A major reason that the behavior of the 2010 Eyjafjallajok̈ ull eruption cloud was surprising is that ”standard” models for how ash sedimentation works (i.e., heavy particles fall out of the cloud faster than light particles) are incomplete with significant consequences not just for assessing hazards to air traffic, but also for understanding, for example, the effect of volcanism on climate. Observations of the 2010 Eyjafjallajok̈ ull and 2011 Grimsvoẗ n umbrella clouds, as well as the structure of atmospheric aerosol clouds from the 1991 Mt Pinatubo event, suggest that an additional key process in addition to particle settling is the production of internal layering. I will use analog experiments on turbulent particle-laden umbrella clouds understood with simple models to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability leading to this layering. This “particle diffusive convection” strongly influences cloud longevity where volcanic umbrella clouds are enriched in fine ash. More generally, however, volcanic cloud residence times will depend on ash fluxes related to both individual particle settling and diffusive convection. I will discuss a new sedimentation model that includes both sedimentation processes captures real-time measurements of the rate of change of particle concentration in the 1982 El Chichon, 1991 Mt Pinatubo and 1992 Mt Spurr ash-clouds. Finally, although we have made progress to understanding how volcanic ash clouds ultimately work, there remain some fundamental problems that I will discuss, depending on time.