1. Animation of ash concentration, this shows (visually) the presence of vortex tubes which appear to be shed from the bent-over plume. The frequency of shedding (Strouhal frequency) agrees well with the frequency shown in lab studies, e.g. Fric and Roshko, “Vortical structure in the wake of a transverse jet” JFM 279, 1-47 (1994)

2. Whirlwind -like structures
Under certain circumstances (depending upon upstream wind speed, for fixed heat source),
ash is transported downwards along relative vorticity “tubes”, similar to observations
of certain cases.
WIND
Ash
Sparks et al. Volcanic Plumes
Here is a case when the background wind speed is 10 m/s. Vortices can be seen in the pollutant concentration (see also next few slides). This is shown schematiccally in a diagram from Sparks “Volcanic Plumes”. These “whirlwinds” (the term used by the authrors in the report on the Surtsey eruption shown here) have also been seen in man-made events e.g. oil fires.
Surtsey volcano BAMS
Oil tank fire MWR