In situ terminal settling velocity measurements at Stromboli volcano: Input from physical characterization of ash

Abstract

9Ash particle terminal settling velocity is an important parameter to measure in order to 10 constrain the internal dynamics and dispersion of volcanic ash plumes and clouds that emplace 11 ash fall deposits from which source eruption conditions are often inferred. Whereas the total 12 Particle Size Distribution (PSD) is the main parameter to constrain terminal velocities, many 13 studies have empirically highlighted the need to consider shape descriptors such as the 14 sphericity to refine ash settling velocity as a function of size. During radar remote sensing 15 measurements of weak volcanic plumes erupted from Stromboli volcano in 2015, an optical 16 disdrometer was used to measure the size and settling velocities of falling ash particles over 17 time, while six ash fallout samples were collected at different distances from the vent. We focus 18 on the implications of the physical parameters of ash for settling velocity measurements and 19 modeling. Two-dimensional sizes and shapes are automatically characterized for a large 20 number of ash particles using an optical morpho-grainsizer MORPHOLOGI G3. Manually 21 sieved ash samples show sorted, relatively coarse PSDs spanning a few microns to 2000 μm 22with modal values between 180-355 μm. Although negligible in mass, a population of fine 23 particles below 100 μm form a distinct PSD with a mode around 5-20 μm. All size distributions 24 are offset compared to the indicated sieve limits. Accordingly, we use the diagonal of the upper 25 mesh sizes as the upper sieve limit. Morphologically, particles show decreasing average form 26 factors with increasing circle-equivalent diameter, the latter being equal to 0.92 times the 27 average size between the length and intermediate axes of ash particles. Average particle 28 densities measured by water pycnometry are 2755 ± 50 kg m -3 and increase slightly from 2645 29 to 2811 kg m -3 with decreasing particle size. The measured settling velocities under laboratory 30 conditions with no wind, < 3.6 m s -1 , are in agreement with the field velocities expected for 31 particles with sizes < 460 μm. The Ganser (1993) empirical model for particle settling velocity 32 is the most consistent with our disdrometer settling velocity results. Converting disdrometer 33 detected size into circle equivalent diameter shows similar PSDs between disdrometer 34 measurements and G3 analyses. This validates volcanological applications of the disdrometer 35 to monitor volcanic ash sizes and settling velocities in real-time with ideal field conditions. We 36 discuss ideal conditions and the measurement limitations. In addition to providing 37 sedimentation rates in-situ, calculated reflectivities can be compared with radar reflectivity 38 measurements inside ash plumes to infer first-order ash plume concentrations. Detailed PSDs 39 and shape parameters may be used to further refine radar-derived mass loading retrievals of the 40 ash plumes. 41 Highlights: 42• An optical disdrometer is used to measure ash sizes and settling velocities at Strombo...

Ash sedimentation by fingering and sediment thermals from wind-affected volcanic plumes

“…In addition, ground tarpaulins were placed next to the disdrometer to collect ash samples for each event recorded by the disdrometer. Freret-Lorgeril et al (2019) demonstrated the influence of ash particle shape on disdrometer terminal settling velocity and size measurements. Since the disdrometer is designed to measure the shape of spherical droplets for meteorological applications, the shape of settling ash particles were constrained and determined with a morpho-grainsizer Morphologi G3 ® by Freret-Lorgeril et al (2019).…”

“…The goal of our measurement campaign is to characterize quantitatively the sedimentation behavior of volcanic plumes with three techniques: (i) a 3 mm wavelength Doppler radar (95 GHz), designed to track ash in volcanic plumes ( Figure 1C); (ii) an optical disdrometer Parsivel 2 ( Figure 1C); and (iii) analyses of ground deposit samples acquired during the campaign (Freret-Lorgeril et al, 2019). In this study, it is important to note that each method detects a local Particle Size Distribution (PSD).…”

“…Disdrometers were originally designed for detecting hydrometeor precipitation. Nevertheless, Freret-Lorgeril et al (2019) and Kozono et al (2019) have highlighted the reliability of such instruments for detecting volcanic ash fallout with a high temporal resolution. Usually, disdrometer measurements are time-averaged either to avoid stochastic variations of measurements due to complex hydrometeor shapes (Löffer-Mang & Jürg, 2000) or to obtain stable quantitative data, e.g.…”

“…Since the disdrometer is designed to measure the shape of spherical droplets for meteorological applications, the shape of settling ash particles were constrained and determined with a morpho-grainsizer Morphologi G3 ® by Freret-Lorgeril et al (2019). Assuming particles fall parallel to their shortest axis, the disdrometer detects particle with circle-equivalent diameter DCE ≥ 0.230 mm (Freret-Lorgeril et al, 2019). The particle detection resolution (i.e.…”

Experimental High-Resolution Forecasting of Volcanic Ash Hazard at Sakurajima, Japan

“…The Sakurajima Volcanological Observatory (SVO), Disaster Prevention Research Institute (DPRI) of Kyoto University operates a Particle-Size-Velocity (parsivel) disdrometer network in order to monitor ashfall over the volcano. Parsivel disdrometers are laser-optical disdrometers used to measure the number, size and fall speed of hydrometeors and ash particles [15,22]. The OTT Parsivel 2 disdormeters used have operating limits of 0.2-25 mm for the particle size and 0.2-20 m s −1 for the velocity.…”