Near-source Doppler radar monitoring of tephra plumes at Etna

Abstract

Over the last twenty yearsMount Etna has produced more than one hundred explosive events ranging fromlight ash emissions to violent sub-plinian eruptions. Significant hazards arise from tephra plumes which directly threaten air traffic, and generate fallout affecting surrounding towns and infrastructures. We describe the first radar system, named VOLDORAD 2B, fully integrated into a volcano instrumental network dedicated to the continuous near-source monitoring of tephra emissions from Etna's summit craters. This 23.5 cmwavelength pulsed Doppler radar is operated in collaboration between the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC) and the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE) since 2009. Probed volumes inside the fixed, northward-pointing conical beam total about 1.5 km in length, covering the summit craters which produced all recent tephra plumes. The backscattered power, related to the amount of particles crossing the beam, and particle along-beamvelocities are recorded every 0.23 s, providing a proxy for the tephra mass eruption rate. Radar raw data are transmitted in real-time to the volcano monitoring center of INGV-OE in Catania and are used to automatically release alerts at onset and end of eruptive events. Processed radar parameters are also made available fromthe VOLDORAD database online (http://voldorad.opgc.fr/). In addition to eruptive crater discrimination by range gating, relative variations of eruption intensity can be tracked, including through overcast weather when other optical or infrared methods may fail to provide information. Short-lived dense ash emissions can be detected as illustrated for weak ash plumes from the Bocca Nuova and New South East craters in 2010. The comparison with thermal images suggests that the front mushroom of individual ash plumes holds the largest particles (coarse ash and small lapilli) and concentrations at least within the first hundred meters. For these short-lived ash plumes, the highest particle mass flux seems to occur typically within the first 10 s.Wealso analyze data fromthe first lava fountain generating an ash and lapilli plumeon 12 January 2011 that initiated a series of 25 paroxysmal episodes of the New South East Crater until April 2012. We illustrate the pulsating behavior of the lava fountain and showthat vertical velocities reached 250ms−1 (with brief peaks exceeding 300ms−1), leading to mean and maximumtephra fluxes (DRE) of 185 and 318m3 s−1 (with peaks exceeding 380 m3 s−1) respectively, and a total volume of pyroclasts emitted during the lava fountain phase of 1.3 × 106m3. Finally, we discuss capacities and limits of the instrument, alongwith future work aimed at providing source term inputs to tephra dispersal models in order to improve hazard assessment and risk mitigation at Etna.