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

Abstract

[1] 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 À1 ) and buoyant regimes (<15 m s À1 ), 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.

Chapter 9 Thermal, acoustic and seismic signals from pyroclastic density currents and Vulcanian explosions at Soufrière Hills Volcano, Montserrat

“…The gas-thrust phase feeds hot tephra into the plume and is responsible for the increase in the temperature within the thermal camera FOV. The buoyancy phase is, however, associated with the reduction in the temperature in the FOV as no new volcanic material is ejected, but, rather, the plume rises at constant velocity by convective air entrainment with a consequent reduction of its internal temperature (Wilson 1980;Patrick 2007;Marchetti et al 2009;Delle Donne & Ripepe 2012).…”

“…During the first phase, the volcanic plume is driven into the atmosphere by the gas thrust. Following this, the plume keeps rising at a relatively constant velocity by buoyancy, slowly expanding into the atmosphere while entraining air (Wilson 1980;Patrick 2007). The gas-thrust phase feeds hot tephra into the plume and is responsible for the increase in the temperature within the thermal camera FOV.…”

Thermal-image-derived dynamics of vertical ash plumes at Santiaguito volcano, Guatemala

“…On January 25, 2005, we imaged 25 of Caliente's plumes using thermal video, our aim being to quantify plume dynamics and thermal properties during emission and ascent. In doing so, we use the terminology of Turner (1962;1969) and Morton (1959) [and reviewed by Patrick (2007)] so that: (i) a jet is a high-velocity flow driven by initial momentum, (ii) a starting plume is a buoyant plume where the convecting plume front is fed by a steady feeder plume that develops below it, and (iii) a thermal is a detached vortex rising by buoyancy (Fig. 1).…”

Paroxysmal Explosions, Lava Fountains and Ash Plumes at Etna Volcano: Eruptive Processes and Hazard Implications

“…The uppermost portion is the umbrella region, where the volcanic plume reaches the zone of neutral buoyancy and spreads out laterally (Carey and Sparks, 1986;Bonadonna and Phillips, 2003). Within the buoyant umbrella region, plume rise rates are less than 15 m s −1 (Patrick, 2007;Patrick et al, 2007). Volcanic plume dynamics depend closely on the interaction with the atmospheric wind field.…”

“…The lowest portion is the gas-thrust region, and comprises the lava fountains (Sparks et al, 1997;Bonaccorso and Calvari, 2017). Measured ascent velocities in this region are normally greater than 15 m s −1 (Patrick, 2007;Patrick et al, 2007;Sahetapy-Engel and Harris, 2009), and for Etna vary between 33 and 125 m s −1 Carbone et al, 2015;Giuffrida et al, 2018), but values of 170, 227, and 405 m s −1 have been measured at Montserrat, Sakurajima, and Stromboli volcanoes, respectively (Clarke et al, 2002;Taddeucci et al, 2012;Tournigand et al, 2017). Above the gas-thrust is the convective region, where atmospheric mixing occurs (Sparks et al, 1997;Bombrun et al, 2018), causing a decrease in ascent velocity and lateral wind transport (Carey and Sparks, 1986).…”