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

Abstract

Explosive basaltic eruptions eject a great amount of pyroclastic material into the atmosphere, forming columns rising to several kilometers above the eruptive vent and causing significant disruption to both proximal and distal communities. Here, we analyze data, collected by an X-band polarimetric weather radar and an L-band Doppler fixed-pointing radar, as well as by a thermal infrared (TIR) camera, in relation to lava fountain-fed tephra plumes at the Etna volcano in Italy. We clearly identify a jet, mainly composed of lapilli and bombs mixed with hot gas in the first portion of these volcanic plumes and here called the incandescent jet region (IJR). At Etna and due to the TIR camera configuration, the IJR typically corresponds to the region that saturates thermal images. We find that the IJR is correlated to a unique signature in polarimetric radar data as it represents a zone with a relatively high reflectivity and a low copolar correlation coefficient. Analyzing five recent Etna eruptions occurring in 2013 and 2015, we propose a jet region radar retrieval algorithm (JR3A), based on a decision-tree combining polarimetric X-band observables with L-band radar constraints, aiming at the IJR height detection during the explosive eruptions. The height of the IJR does not exactly correspond to the height of the lava fountain due to a different altitude, potentially reached by lapilli and blocks detected by the X-band weather radar. Nonetheless, it can be used as a proxy of the lava fountain height in order to obtain a first approximation of the exit velocity of the mixture and, therefore, of the mass eruption rate. The comparisons between the JR3A estimates of IJR heights with the corresponding values recovered from TIR imagery, show a fairly good agreement with differences of less than 20% in clear air conditions, whereas the difference between JR3A estimates of IJR height values and those derived from L-band radar data only are greater than 40%. The advantage of using an X-band polarimetric weather radar in an early warning system is that it provides information in all weather conditions. As a matter of fact, we show that JR3A retrievals can also be obtained in cloudy conditions when the TIR camera data cannot be processed.

Examples of Multi-Sensor Determination of Eruptive Source Parameters of Explosive Events at Mount Etna

“…Bombs and blocks were carried up to 5 km from the vent and a 2-cm thick tephra deposit with cm-sized lapilli up to 20 km from the vent was observed [27]. The 23 November 2013 paroxysmal episode has been well documented and already used in the past as a case study to test multi-sensor strategies [7,15,31,33].…”

“…One of these radars is a scanning microwave weather radar (MWR) working at a wavelength λ of 3 × 10 −2 m and located at Catania airport, 33 km south from Mount Etna (Figure 1a). The MWR is operated by the Italian Civil Protection and provides 12 different elevated scans up to a maximum distance of 80 km from the radar site and with space-time resolution of 100 m and 10 min [7,14,15,33]. In addition, we take advantage of the fixed-pointing VOLDORAD-2B (V2B), located at La Montagnola Station (2610 m a.s.l.)…”

“…However, due to intrinsic limitations (e.g., sensor detection limits, deposit exposure), individual strategies cannot provide a comprehensive characterization of all these ESPs. Recently, multi-sensor strategies have been developed to better constrain MER, H T, and TGSD from ground sampling, plume models, and available remote sensing systems [7,15,[30][31][32][33][34]. Ultimately, multi-sensor strategies are also being used for the real-time determination of column height during volcano monitoring activities [22].…”

“…Between 2011 and 2015, Etna has produced about 50 paroxysmal events associated with the emission of fountain-fed tephra plumes. All paroxysms are characterized by highly varying ESPs with plume heights ranging between 5.2 and 17.6 km above sea level [8,33,35,36]. Such variability of eruptive processes may challenge the development of multi-sensor strategies especially because they are mostly based on strong paroxysms such as the events on 23 November 2013 [6,7,27] and 3-5 December 2015 [14,37,38].…”

“…observed [27]. The 23 November 2013 paroxysmal episode has been well documente already used in the past as a case study to test multi-sensor strategies [7,15,31,33].…”