Characteristics of Particle Size Distributions of Falling Volcanic Ash Measured by Optical Disdrometers at the Sakurajima Volcano, Japan

Masayuki Maki,Masato Iguchi,Ren Takaoka

doi:10.3390/atmos12050601

Masayuki Maki, Masato Iguchi + Show 1 more

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https://doi.org/10.3390/atmos12050601

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Journal: Atmosphere	Publication Date: May 6, 2021
License type: CC BY 4.0

Affiliation: Kagoshima University, Kyoto University

Abstract

In the present study, we analyzed the particle size distribution (PSD) of falling volcanic ash particles measured using optical disdrometers during six explosive eruptions of the Sakurajima volcano in Kagoshima Prefecture, Japan. Assuming the gamma PSD model, which is commonly used in radar meteorology, we examined the relationships between each of the gamma PSD parameters (the intercept parameter, the slope parameter, and the shape parameter) calculated by the complete moment method. It was shown that there were good correlations between each of the gamma PSD parameters, which might be one of the characteristics of falling volcanic ash particles. We found from the normalized gamma PSD analysis that the normalized intercept parameter and mass-weighted mean diameter are suitable for estimating the ash fall rate. We also derived empirical power law relationships between pairs of integrated PSD parameters: the ash fall rate, the volcanic ash mass concentration, the reflectivity factor, and the total number of ash particles per unit volume. The results of the present study provide essential information for studying microphysical processes in volcanic ash clouds, developing a method for quantitative ash fall estimation using weather radar, and improving ash transport and sedimentation models.

Highlights

The physical properties and chemical components of ash particles emitted by explosive volcanic eruptions represent useful basic data for investigating the mode, scale, and mechanisms of eruptions
We examined the characteristics of the particle size distribution (PSD) of volcanic ash particles and the relationships among PSD parameters
We used a total of 166 PSD samples collected by Parsivel2 during six explosive eruptions of the Sakurajima volcano in 2018

Summary

Introduction

The physical properties and chemical components of ash particles emitted by explosive volcanic eruptions represent useful basic data for investigating the mode, scale, and mechanisms of eruptions. Volcanic sediment data analysis has facilitated many types of research, such as the detection, tracking, and prediction of ash fall to prevent volcanic disasters [6,7,8], estimation of the content fraction of particles smaller than 63 μm (which affect aircraft operation) from total grain size distribution [7], and investigation of variation in tephra features with distance from the volcano crater using the 100-year eruption records of volcanoes along the western coast of North America [8]. Investigating ground sediments deposited following volcanic eruptions is an important method in volcanology research, and for volcanic disaster prevention, sediment data acquired on the ground are inevitably time-integrated, they are usually accumulated hours or days after an eruption. Conventional geological methods for investigating sediments are limited in that the properties and sedimentary environment of volcanic ash particles change depending on weather conditions, such as wind and rain, during and after sedimentation [9], such that sediment data are often unsuitable for remote sensing and numerical forecasting of ash fall distribution and transportation, which change from moment to moment

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