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MTSAT Observation of the Eruption Cloud Occurred at the February ,, ,**3Eruption of Mt. Asama, Central Japan
Takayuki KANEKO῍, Hiroyuki DEN῍, Kenji TAKASAKI῍,῍῍, Atsushi YASUDA῍,Fukashi MAENO῍, Etsuro KOYAMA῍ and Setsuya NAKADA῍Mt. Asama (,/02 m a.s.l.) erupted on ,nd February ,**3 at + : /+ am (JST). Although it was a small-scaleeruption, involving a eruption plume rising ,*** m high from the summit, the ashes accidentally fell in the Tokyo
metropolitan area, due to strong seasonal wind, which draw public concern. We observed migration andenlargement processes of the eruption cloud, using infrared images from Multi-functional Transport Satellite(MTSAT), and analyzed its relationship to distribution of ash-fall deposits and meteorological conditions.
MTSAT is a gestational meteorological satellite of Japan Meteorological Agency and Civil Aviation Bureau andhas ﬁve observation channels in the visible to infrared regions (visible : + and infrared : .). Although theresolution is low (. km in infrared), the observation cycle is very high (every -* to 0* minutes), which is preferable
characteristics to eruption cloud studies. We used four MTSAT images taken on ,nd February at , : -* am, - :-* am, . : -* am and / : -* am. Images of brightness-temperature di#erence between two thermal infrared
channels (+*.-ῌ++.- mm and ++./ῌ+,./ mm) were applied for detecting distribution of the eruption clouds. The
results show that the eruption cloud migrated to the southeast along the line connecting between Mt. Asama andKatsuura-city on the Boso peninsula through the central part of Tokyo. The eruption cloud elongated toward thedirection of the migration and the total length increased with time. The speed of the head and tail of the eruption
cloud was estimated to be +-/ km/h and /+ km/h, respectively. This relative speed di#erence could cause
elongation of the total length. According to the meteorological data, at the altitude of .3**ῌ/1** m, wind was
blowing to the southeast at the speed of ++3 km/h, and at ,1**ῌ-+** m to the same direction at /* km/h. These
values roughly coincided with the estimated speed of the head and tail, respectively, indicating the di#erence in thewind speed was the main cause of the elongation. Distribution area of the ash-fall deposits on the ground did not
well much the area where the eruption cloud ﬂew over, but was located *ῌ.* km south of it. Unlike the higher
altitude, in the regions lower than ,*** m, wind was roughly blowing to the south at somewhat lower speed.
When falling ashes from the eruption cloud entered this region, they could be blown to the further south, which
could cause the wide and biased distribution of the ash-fall deposits, as observed.
Key words : MTSAT, Asama volcano, eruption cloud, infrared, remote sensing+ῌ ῐ ῎ ῑ ῏<=>>? @ABCDEFG?HIJJCK> L

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[TU\]K>S ^_`XDa\URbScdOe ++-ῌ**-,  +ῌ+ῌ+Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
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MODIS (MODerate resolution Imaging Spectro-radiometer) MTSAT (Multi-functional Transport SATelKeio University Shonan Fujisawa Campus, /-,,-Endo,Fujisawa, Kanagawa ,/,ῌ2/,*, Japan.
Corresponding author : Takayuki Kaneko
e-mail : kaneko@eri.u-tokyo.ac.jp