Impact of wind on the condition for column collapse of volcanic plumes

The collapse of volcanic plumes has significant implications for eruption dynamics and associated hazards. We show how eruptive columns can collapse and generate pyroclastic density currents as a result of not only the source conditions, but also of the atmospheric environment. The ratio of the potential energy and the kinetic energy at the source quantified by the Richardson number, and the entrainment efficiency quantified by the radial entrainment coefficient have already been identified as key parameters in controlling the transition between a buoyant and collapsing plume. Here we quantify how this transition is affected by wind using scaling arguments in combination with a one-dimensional plume model. Air entrainment due to wind causes a volcanic plume to lower its density at a faster rate and therefore to favor buoyancy. We identify the conditions when wind entrainment becomes dominant over radial entrainment and quantify the effect of wind on column collapse. These findings are framed into a generalized regime diagram that also describes previous regime diagrams for the specific case of choked flows. Many observations confirm how bent-over plumes typically do not generate significant collapses. A quantitative comparison with the 1996 Ruapehu and the 2010 Eyjafjallajِkull eruptions shows that the likelihood of collapse is reduced even at moderate wind speeds relative to the exit velocity at the vent.