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,**/  .  ++  ,**/  +*  +3 !"The Kinematic Features of Volcanic Clouds: A Series of Small Eruptions
from +/ to +2, September ,**., at Asama Volcano, JapanAkihiko TERADAῌ, Yoshiaki IDAῌῌ, Sei IIJIMAῌῌῌ, Mitsuhiro YOSHIMOTOῌῌῌῌand Taketo SHIMANOῌῌῌῌῌ
Some fundamental features of ascending volcanic clouds have been revealed using images of the clouds thatwere automatically recorded by video cameras for some eruptions at Asama volcano on +/ to +2 September, ,**..
According to the analysis of +1 volcanic clouds that are nearly isolated and of a symmetrical shape, the radius ofa 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 canbe a constant that is identiﬁed with the entrainment constant. The entrainment constants determined in this way
have a mean value of about *.,. in good agreement with those obtained from previous laboratory experiments,
even if the values are greater than *.,/ or less than *.,* for -/ῌ of the analyzed volcanic clouds. During theascent of a cloud the height squared is nearly proportional to the time and the product of the radius and the ascentvelocity is almost constant. These empirical relations are consistent with well-known characters of a thermal thatmoves in incompressible uniform surroundings. Coupling these kinematic features of the volcanic clouds with theScorer’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 notchange signiﬁcantly during their ascent process. The cloud on +1 September has a relatively great density
contrast and small total buoyancy, probably reﬂecting hot ash particles in it supplied by a Strombolian eruption atthat time. The volcanic cloud discharged at ++ : /., September +/ contained ash of ,,/**῍-,1** tons or less withthe mean temperature of -+*῍-0* K or higher, and the volcanic cloud discharged at 2 : -2, September +2 contained
ash less than 2,-**῍3,+** tons with the mean temperature higher than -+* K or higher.
Key words : Asama volcano, thermal, volcanic cloud, buoyancy, entrainment#*0*῍*2+* $%&'()$*$ +* +, 2 -.
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Institute of Seismology and Volcanology, Graduate
School of Science, Hokkaido University, North +*West 2, Kita-ku, Sapporo *0*῍*2+*, Japan.
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ABCH/0/01IJ!0234
Graduate School of Life and Science, University ofHyogo, ,+01 Syosya, Himeji 01+῍,,*+, Japan.
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Karuizawa Weather Station, Japan MeteorologicalAgency, ++/+῍, Oiwake, Karuizawa, Kitasaku, Nagano-23῍*++/, Japan.
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Earthquake Research Institute, University of Tokyo,+῍+῍+ Yayoi, Bunkyo-ku, Tokyo ++-῍**-,, Japan.
#32*῍2/10 ^_C`a)bc*de .+Y$/0Y$fgf23=>?@The Center of Northeast Asian Studies, Tohoku
University, .+ Kawauchi, Aoba-ku, Sendai 32*῍2/01,Japan.
Corresponding author : Akihiko Terada
e-mail : terada@eos.hokudai.ac.jp