浅間火山で頻発した小噴火の噴煙運動の特徴 : 2004年9月15-18日噴火(<特集>2004年浅間山噴火(2))

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Some fundamental features of ascending volcanic clouds have been revealed using images of the clouds that were automatically recorded by video cameras for some eruptions at Asama volcano on 15 to 18 September, 2004. According to the analysis of 17 volcanic clouds that are nearly isolated and of a symmetrical shape, the radius of a cloud increases linearly with increasing height, meeting self-similarity of ascending motion nearly up to its maximum height. If the height is measured from a suitable virtual origin, the ratio of the radius to the height can be a constant that is identified with the entrainment constant. The entrainment constants determined in this way have a mean value of about 0.24 in good agreement with those obtained from previous laboratory experiments, even if the values are greater than 0.25 or less than 0.20 for 35% of the analyzed volcanic clouds. During the ascent of a cloud the height squared is nearly proportional to the time and the product of the radius and the ascent velocity is almost constant. These empirical relations are consistent with well-known characters of a thermal that moves in incompressible uniform surroundings. Coupling these kinematic features of the volcanic clouds with the Scorer's relation and the equation of state, we evaluate the total buoyancy, the total mass, the density and the mean temperature of the clouds that are regarded as thermals. The total buoyancy of most volcanic clouds did not change significantly during their ascent process. The cloud on 17 September has a relatively great density contrast and small total buoyancy, probably reflecting hot ash particles in it supplied by a Strombolian eruption at that time. The volcanic cloud discharged at 11:54, September 15 contained ash of 2,500-3,700 tons or less with the mean temperature of 310-360K or higher, and the volcanic cloud discharged at 8:38, September 18 contained ash less than 8,300-9,100 tons with the mean temperature higher than 310K or higher.
特定非営利活動法人日本火山学会の論文
2005-12-30

浅間火山で頻発した小噴火の噴煙運動の特徴 : 2004年9月15-18日噴火(<特集>2004年浅間山噴火(2))

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