New insights into real-time detection of tephra grainsize, settling velocity and sedimentation rate

Abstract

Characterizing the size and settling velocity of pyroclastic fragments injected into the atmosphere during volcanic eruptions (i.e., tephra) is crucial to the forecasting of plume and cloud dispersal. Optical disdrometers have been integrated into volcano monitoring networks worldwide in order to best constrain these parameters in real time. Nonetheless, their accuracy during tephra fallout still needs to be assessed. A significant complication is the occurrence of particle aggregates that modify size and velocity distributions of falling tephra. We made the first use of the Thies Clima Laser Precipitation Monitor (LPM) for tephra-fallout detection at Sakurajima volcano (Japan), which is characterized by a lower size detection window with respect to more commonly used disdrometers (e.g., Parsivel2) and can more easily distinguish different falling objects. For the first time, individual particles have been distinguished from most aggregates based on disdrometer data, with the potential to provide useful grain-size information in real time. In case of negligible aggregation, LPM and collected sample-based estimates are in agreement for both grain-size and sedimentation rate. In case of significant aggregation, particle shape analyses and a dedicated drag equation are used to filter out aggregates from LPM data that also provide good agreement with collected tephra samples.

Tephra sedimentation and grainsize associated with pulsatory activity: the 2021 Tajogaite eruption of Cumbre Vieja (La Palma, Canary Islands, Spain)

“…The crossing time of each detected particle is a function of its settling velocity v, while the induced extinction of the laser is used to retrieve its size Dp. Full details of the instrument are available in Angulo-Martinez et al (2018) andFreret-Lorgeril et al (2022b). The LPM was located at three different sampling sites concomitantly for determining tephra accumulation rates with trays and acquiring videos of the eruption (Figure 1).…”

“…It should be noted that the LPM grainsize data were filtered following the procedure described in Freret-Lorgeril et al (2022b) to eliminate particles falling at the edge of the laser beam (i.e., margin fallers).…”

Volcanic lightning reveals umbrella cloud dynamics of the January 2022 Hunga Tonga-Hunga Ha’apai eruption

“…Eruptions at accessible volcanoes can be characterised through visible or infrared imagery (Self et al 1979;Patrick 2007;Tournigand et al 2017;Bombrun et al 2018), proximal seismoacoustic observations (Jolly et al 2017) and radar (Freret-Lorgeril et al 2018) and lidar (Scollo et al 2012) techniques. Additionally, eruption products and deposits can be rapidly characterised through sampling (Diaz Vecino et al 2022), field observations (Bonadonna et al 2011;Freret-Lorgeril et al 2022) and petrological analysis (Pankhurst et al 2022). Conversely, rapid observations of eruptions from volcanoes such as HTHH rely on far-field remote sensing techniques (McKee et al 2021a, b).…”