Near source Doppler radar monitoring of tephra plumes at Etna
Over the last twenty years Mount Etna has produced more than one hundred explosive events ranging from light 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 cm wavelength 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 beam velocities 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 from the 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. We also analyze data from the first lava fountain generating an ash and lapilli plume on 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 show that vertical velocities reached 250 m s 1 (with brief peaks exceeding 300 m s 1) leading to mean and maximum tephra fluxes (DRE) of 185 and 318 m3 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 x 106 m3. Finally, we discuss capacities and limits of the instrument, along with future work aimed at providing source term inputs to tephra dispersal models in order to improve hazard assessment and risk mitigation at Etna.