Estimation of exit velocity of volcanic plume from analysis of vortex structures

•We proposed a simple method for estimating the exit velocities of volcanic eruptions.•A model of a vortex ring proposed in experimental jet engineering was used.•Validity of the model for volcanic plumes was checked using 3-D numerical simulation.•At Sakurajima eruption, the exit velocity was estimated as 40 m/s up to 80 s.

We propose a simple method for estimating the exit velocities of volcanic eruptions from the observation of volcanic plumes. For this purpose, we used a model of a vortex ring of an experimental jet, which was developed in the engineering field. To validate the model for the vortex structures of volcanic plumes, we applied it to plumes generated in 3-D numerical simulations. In 11 cases where exit velocity (66.8-200.5 m/s) is given as a boundary condition, we successfully estimated it with 7% underestimation by analyzing the size and motion of the leading vortex ring that forms at the plume front. Using the same procedure, we could also estimate the exit velocity by analyzing the trailing vortices that develop behind the vortex ring (14% underestimation). From these results, we conclude that: (i) the model of the vortex ring proposed by the jet engineering studies is appropriate for the vortex ring at the front of simulated volcanic plumes, and (ii) the model is also applicable to the trailing vortices of the plumes. These conclusions indicate that we can estimate the time evolution of the exit velocity for a series of eruptions from observations of the vortex structures of the actual volcanic plumes. By applying that method to an eruption of Sakurajima volcano on February 15, 2011, we found that following an increase during the first 10 s of the eruption, the exit velocity remained constant at >40 m/s up to 80 s after the onset of the eruption. Our method will be useful in understanding the time evolution of eruptive events, such as the transitional behavior from stable column to column collapse, from observations of volcanic plumes.