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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-88
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-88
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Apr 2019

Research article | 15 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Laboratory analysis of volcanic ash particles using a 2D video disdrometer

Sung-Ho Suh1, Masayuki Maki2, Masato Iguchi3, Dong-In Lee1, Akihiko Yamaji4, and Tatsuya Momotani4 Sung-Ho Suh et al.
  • 1Department of Environmental Atmospheric Sciences, Pukyong National University, Namgu, Busan, Republic of Korea
  • 2Research and Education Center for Natural Hazards, Kagoshima University, Korimoto, Kagoshima, Japan
  • 3Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Sakurajima, Kagoshima, Japan
  • 4Japan Weather Association, Higashi-Ikebukuro, Toshima-ku, Tokyo, Japan

Abstract. Radar variables of volcanic ash clouds are dependent on microphysical processes and can be expressed using physical parameters of volcanic ash particles, such as terminal velocity, axis ratio, and canting angle, which are necessary for quantitative ash-fall estimations. In this study, free-fall experiments of volcanic ash were accomplished using a two-dimensional video disdrometer under controlled conditions.

Samples containing a rotating symmetric axis were selected and divided into five types according to shape and orientation, i.e., oblate and prolate spheroids with horizontally and vertically oriented axes and spheres. The horizontally and vertically oriented particles were present in proportions of 75.5 % and 21.6 %, and oblate and prolate spheroids were in proportions of 76.2 % and 23.8 %, respectively. The most common shape type was a horizontally oriented oblate spheroid (57.3 %).

The terminal velocities were classified according to shape type. The terminal velocities of prolate spheroids (vertically oriented) particles were higher than those of oblate spheroids (horizontally). Terminal velocities were in the range 0.5 < volume–equivalent spherical particle diameter (D) < 1 mm for OH because of an increase in axis ratio and a sharp decrease in sample size from D < 0.7 mm. The axis ratios fell over a wide range, from 0 to 1.5, at D < 2 mm, but converged to 0.94 at D > 2 mm.

The histogram of canting angles followed unimodal and bimodal distributions with respect to horizontally and vertically oriented particles, respectively. The mean values were close to 0° and the standard deviation for the entire particle shape types was close to that of raindrops (10°) under calm atmospheric conditions.

Sung-Ho Suh et al.
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Short summary
This is a basic study on the aerodynamic features of basis parameters: terminal velocity, axis ratio, and canting angle, which are necessary to develop the quantitative ash fall estimation method based on weather radar. They were analyzed under controlled conditions from laboratory free fall experiments since the aerodynamic properties of the particle are highly dependent on external conditions. These results will help the development of quantitative ash estimation.
This is a basic study on the aerodynamic features of basis parameters: terminal velocity, axis...
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