Aerodynamic characteristics and genesis of aggregates at Sakurajima Volcano, Japan

Abstract

Aggregation of volcanic ash is known to significantly impact sedimentation from volcanic plumes. The study of particle aggregates during tephra fallout is crucial to increase our understanding of both ash aggregation and sedimentation. In this work, we describe key features of ash aggregates and ash sedimentation associated with eleven Vulcanian explosions at Sakurajima Volcano (Japan) based on state-of-the-art sampling techniques. We identified five types of aggregates of both Particle Cluster (PC) and Accretionary Pellet (AP) categories. In particular, we found that PCs and the first and third type of APs can coexist within the same eruption in rainy conditions. We also found that the aerodynamic properties of aggregates (e.g., terminal velocity and density) depend on their type. In addition, grainsize analysis revealed that characteristics of the grainsize distributions (GSDs) of tephra samples correlate with the typology of the aggregates identified. In fact, bimodal GSDs correlate with the presence of cored clusters (PC3) and liquid pellets (AP3), while unimodal GSDs correlate either with the occurrence of ash clusters (PC1) or with the large particles (coarse ash) coated by fine ash (PC2).

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

“…Our filtering strategy is in good agreement with observations of size and settling velocities obtained on falling aggregates by Diaz-Vecino et al 33 using a high-speed camera during the same measurement campaign. In fact, objects falling below the individual-particle trend in the LPM records form a trend which is in agreement with objects that are identified as ash clusters or PC1 19 , 29 .…”

“…3 d–f and S1 ). This bimodality was also observed in samples XII and XIII by laser diffraction 33 . Given that the LPM is not able to detect particles below 0.15 mm, GSDs BTS have been filtered down to this limit in order to make them comparable to GSDs LPM (black circles in Fig.…”

“…Red and blue dashed lines indicate the margin trend and the individual-particle trend, respectively. Diamonds indicate aggregate estimates retrieved based on high-speed camera analyses by Diaz-Vecino et al 33 (blue: Ash Cluster—PC1; black: Coated Particle—PC2; red: Cored Cluster—PC3). Vol% GSD of LPM records and obtained from BETTERSIZER analysis for events VIII, XII and XIII are shown in ( d ), ( e ) and ( f ), respectively.…”

“…The improved observation capabilities of the LPM have the potential to facilitate the discrimination of individual particles and particle aggregates during fallout. We tested the LPM at Sakurajima volcano (Japan), one of the most active volcanoes worldwide that has been erupting frequently since 1955 28 and whose fallout is frequently characterized by the occurrence of volcanic aggregates 22 , 29 – 33 . A network of 12 PS2 disdrometers (at the time of the field campaign; Fig.…”

“…Our main objective is to explore the potential of LPM to provide the size distribution of individual volcanic particles in real time and mitigate the impact of particle aggregation. Disdrometer records are here compared with data obtained independently on falling aggregates during the same measurement campaign by Diaz-Vecino et al 33 .

Figure 1 ( a ) Map of Sakurajima Volcano and disdrometer sampling sites (blue triangles: LPM sites; red dots: PS2 sites).

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