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MTSAT Observation of the Eruption Cloud Occurred at the February ,, ,**3
Eruption 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-scale
eruption, 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 and
Katsuura-city on the Boso peninsula through the central part of Tokyo. The eruption cloud elongated toward the
direction 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

,**3 6 , 7 , 8MNOPQRSTU=> VWXYZ

[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

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Fig. +. (a) Sketch of the eruption column on , February at , : *. am (JST), as inferred from theWeb-camera footage (Maechannet,). (b) Impact craters on the inner wall of Maekake-yama(Contrast-enhanced image. Original photo was taken by E. Koyama on , February). (c) Striae
on the snow cover surface of the northern side of the summit generated by ballistic projectiles
during eruption. Two directions can be recognized in the striae, as indicated by black and whitearrows (Contrast-enhanced image. Original photo was taken by E. Koyama on , February).
This area is that boxed in (a). (d) ALOS AVNIR-, image of Asama (1 February) (ῌ JAXAEORC). Depositional area of the ash fall is extending to the southeast (indicated by arrow). (e)Sketch map of the summit area. This area is that boxed in (d). (f) The -.1 m parabola antennaof the MTSAT receiving system installed at Earthquake Research Institute, University of Tokyo.

121

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perature Di#erence) = 95> ?@ABC

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|.º= T  /* km  34RS -+/ km!<»LLV=¼ Fig. ,c   V+.½W= <+T +32 km b,, km =UFig. ,. Brightness temperature di#erence (TK) images
between two thermal infrared channels (+*.-ῌ++.- mm
and ++./ῌ+,./ mm) of MTSAT. (a) , : -* am (JST),(b) - : -* am, (c) . : -* am, (d) / : -* am. Insets arethose with contrast stretching focused on identifying
the head of the eruption cloud on the sea (nocorrection was made for the di#erence between landand sea areas).

./01 20345671 89 :; <=1>

124

Fig. -. Migration and elongation of the eruption cloud as detected by MTSAT images. Dottedlines show the distribution limit of the ash-fall deposits (Maeno et al. submitted). Solid line
indicates the depositional axis of the ash-fall deposits (extrapolation from the distribution at
the southeastern foot of the volcano).

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125

1"23456/ῌ+  

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(0+ km/h)

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Fig. /. Wind direction (solid lines) and speed at the timeof eruption (- : ** am), provided by NOAA “Fourtimes daily data of the NCEP/NCAR reanalysis”(http : //www.cdc.noaa.gov/data/gridded/data.ncep.
reanalysis.html) based on Kalnay et al. (+330).
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Fig. 0. Schematic illustration of migration and elongation processes of the eruption cloud.
Arrows show the wind speed at .3**ῌ/1** m (/** hPa), ,1**ῌ-+** m (1** hPa), +-**ῌ+0**m (2/* hPa) and * m (+*** hPa) adopted from Fig. /.

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H«J(¬­g"K,®¯0(°±<AL¸' ¹@Aº»¼@°½0ῌ ῑ ῐ ῍Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W.,
Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G.,
Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W.,
Higgins, W., Janowiak, J., Mo, K.C., Ropelewski, C.,
Wang, J., Leetmaa, A., Reynolds, R., Jenne, R. andJoseph, D. (+330) The NCEP/NCAR .*-year reanalysis
project. Bull. Am. Met. Soc., 11, .-1ῌ.1+.
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;
Prata, A. J. (+323)Observations of volcanic ash clouds inthe +*ῌ+, mm window using AVHRR/, data. Int. J.
Remote Sensing, +*, 1/+ῌ10+.
Rose, W.I. , Delene, D. J., Schneider, D. J., Bluth, G. J.S.,
Krueger, A. J., Sprod, I., McKee, C., Davies, H.L. and
Ernst, G.G. J. (+33/) Ice in the +33. Rabaul eruption
cloud : implications for volcano hazard and atmospherice#ects. Nature, -1/, .11ῌ.13.
Rose, W.I., Bluth, G. J.S. and Ernst, G.G. J. (,***)
Integrating retrievals of volcanic cloud characteristics
from satellite remote sensors : a summary. Phil. Trans.
Roy. Soc. London, A, -/2, +/2/ῌ +0*0.
M1N (+32-) ²³[\(´µ ¶·128  ,2 -/1ῌ-1-.
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Sawada, Y. (+330) Detection of explosive eruptions andregional tracking of volcanic ash clouds with Geostationary Meteorological Satellite (GMS). In Monitoringand Mitigation of Volcano Hazards. (Scarpa, R. andTilling, R.I. eds.), ,33ῌ-+., Springer-Verlag.
Watson, I.M., Realmuto, V. J., Rose, W.I., Prata, A. J.,  Bluth, G. J.S., Gu, Y., Bader, C.E. and Yu, T. (,**.)
Thermal infrared remote sensing of volcanic emissions
using the moderate resolution imaging spectroradiometer.
J. Volcanol. Geotherm. Res., +-/, 1/ῌ23.
Wen, S. and Rose, W.I. (+33.) Retrieval of size and total
masses of particles in volcanic clouds using AVHRR
bands . and /. J. Geophys. Res., 33, /.,+ῌ/.-+.
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