Mass Eruption Rates of Tephra Plumes During the 2011–2015 Lava Fountain Paroxysms at Mt. Etna From Doppler Radar Retrievals

Abstract

Real-time estimation of eruptive source parameters during explosive volcanic eruptions is a major challenge in terms of hazard evaluation and risk assessment as these inputs are essential for tephra dispersal models to forecast the impact of ash plumes and tephra deposits. In this aim, taking advantage of the 23.5 cm wavelength Doppler radar (VOLDORAD 2B) monitoring Etna volcano, we analyzed 47 paroxysms produced between 2011 and 2015, characterized by lava fountains generating tephra plumes that reached up to 15 km a.s.l. Range gating of the radar beam allows the identification of the active summit craters in real-time, no matter the meteorological conditions. The radar echoes help to mark (i) the onset of the paroxysm when unstable lava fountains, taking over Strombolian activity, continuously supply the developing tephra plume, then (ii) the transition to stable fountains (climax), and (iii) the end of the climax, therefore providing paroxysm durations. We developed a new methodology to retrieve in real-time a Mass Eruption Rate (MER) proxy from the radar echo power and maximum Doppler velocity measured near the emission source. The increase in MER proxies is found to precede by several minutes the time variations of plume heights inferred from visible and X-Band radar imagery. A calibration of the MER proxy against ascent models based on observed plume heights leads to radar-derived climax MER from 2.96 × 10 4 to 3.26 × 10 6 kg s −1. The Total Erupted Mass (TEM) of tephra was computed by integrating over beam volumes and paroxysm duration, allowing quantitative comparisons of the relative amounts of emitted tephra among the different paroxysms. When the climactic phase can be identified, it is found to frequently release 76% of the TEM. Calibrated TEMs are found to be larger than those retrieved by satellite and X-band radar observations, deposit analyses, ground-based infrared imagery, or dispersion modeling. Our methodology, potentially applicable to every Doppler radar, provides mass load parameters that represent a powerful all-weather tool for the quantitative monitoring and real-time hazard assessment of tephra plumes at Etna or any other volcano with radar monitoring.

Tephra From the 3 March 2015 Sustained Column Related to Explosive Lava Fountain Activity at Volcán Villarrica (Chile)

“…Lava fountain height, eruption column height and MER values calculated here are comparable, for instance, to the eruptive parameters obtained for the largest-scale lava fountain events at Mt. Etna (e.g., 24 September 1986, 5 January 1990, 22 July 1998 and the climax MER of paroxysmal events between 2011 and 2015; see Andronico et al, 2015;Freret-Lorgeril et al, 2018 for details). In addition, the erupted volume is very similar to that from the Kilauea Iki Crater eruption (1959), which was the highest historical lava fountain event registered in Hawaii (height up to 580 m and erupted a total non-DRE volume between 2.1 and 5.8 × 10 6 m 3 ; Stovall et al, 2011;Klawonn et al, 2014) and slightly larger than the DRE tephra volume estimated for 2012-13 Tolbachik fissure eruptions (about 4.0 × 10 5 m 3 ; Belousov et al, 2015).…”

Near-Real-Time Tephra Fallout Assessment at Mt. Etna, Italy

“…Automated detection of H by cameras could further reduce the time necessary to compute the isomass maps of the associated tephra deposits. Moreover, the use of only a few instruments for plume monitoring could be critical, as under some circumstances (for example at night, when the visible cameras cannot be used) they may not work properly; as such, a multi-instrument approach (e.g., radar [64], satellite [38], and cameras [65]) is favored. However, when using multiple instruments, some discrepancies among data could exist, and the role of the operator is still essential.…”

Multisensor Characterization of the Incandescent Jet Region of Lava Fountain-Fed Tephra Plumes

“…This Doppler radar measures both the radial velocity v r and the received backscattered power that characterizes the amount of detected tephra at high time resolution (i.e., 0.2 s). From the observation geometry, it is possible to convert v r into exit velocity v ex (i.e., v ex = 3.89 v r ) [16,17,28], whereas from the specifications of the L-band radar and the radar constant, the backscattered power can be transformed into the L-band reflectivity factor Z hh [15,17].…”

“…MER can be evaluated using a combination of infrasound measurements and thermal infrared (TIR) images [15]. An L-band ground-based Doppler radar (i.e., VOLDORAD 2B, [16,17]) has been used to study plume dynamics [16] and estimate the MER [17]. Furthermore, erupted mass can be estimated by X-band radars [18], capable of scanning the volcanic plumes at high spatial resolution (less than a few hundred meters) and with a relatively short temporal sampling.…”