Dynamics of Strombolian ash plumes from thermal video: Motion, morphology, and air entrainment

Imaging volcanic plumes is essential to provide an observational basis for understanding and modeling plume dynamics. During June–July 2004, ∼150 Strombolian ash plumes were imaged at Stromboli volcano, Italy, with a forward looking infrared radiometer (FLIR) thermal video camera (30 Hz). Of these, 25–80 plumes were suited for different levels of quantitative analyses. In this study some simple analyses are applied to constrain basic parameters for the dynamics of Strombolian plumes during their initial ascent (∼130 m). Plume rise rates covered both gas thrust (>15 m s −1 ) and buoyant regimes (<15 m s −1 ), which in turn controlled lateral spreading rates and air entrainment rates. The half angle of lateral spreading of the plume front averaged 7.3(±1.6)° for gas thrust regimes and 13.5(±1.6)° for buoyant regimes, equating to mean air entrainment coefficients of 0.06–0.12 for gas thrust regimes and 0.22(±0.03) for buoyant regimes. These factors were also linked to plume morphologies, which included jets, starting plumes and thermals. A “rooted thermal” form was observed and presumed as an intermediary between starting plumes and discrete thermals. Plume rise could be approximated by a power law dependence with time. Rooted thermals spread and entrained air at rates approaching those of a discrete thermal but rose at a rate similar to that of a starting plume. Phenomena including helical motion and sedimentation were visible in the FLIR imagery. These results demonstrate that emergent plume behavior is progressive and highly transient. Furthermore, this study offers empirical reinforcement that entrainment dynamics are intrinsically different in (1) gas thrust versus buoyantly driven regimes and (2) plume fronts versus steady plumes.