Volcanic Ash Transport from Mount Asama to the Tokyo Metropolitan Area Influenced by Large-Scale Local Wind Circulation

The eruption of the Mount Asama volcano on 16 September 2004 produced an ash cloud and led to ashfall in the Tokyo metropolitan area that lies on the Kanto Plain. Satellite images showed the ash cloud drifting toward the south in the morning but to the southeast in the afternoon. An aerosol lidar and a ceilometer, installed in the metropolitan area, continuously observed the southeastward-transported ash particles passing at altitudes of 2.6–4.5 km above mean sea level (MSL) in the nighttime. Results of meteorological analyses and numerical experiments showed that the south-to-southeasterly sea breezes and valley winds prevailed at altitudes below 1.5 km MSL over the Kanto Plain in the afternoon and the compensatory return flow (CRF) was formed aloft at altitudes of 1.5–4.5 km MSL as strong northwesterly winds, which were encouraged by a synoptic wind. The numerical experiments also showed that the direction of the ash transport turned from the south to the southeast following the formation of the northwesterly CRF. This demonstrates that the daytime ash transport was influenced by the CRF. The nocturnal ash transport, however, depended on the intensified synoptic wind. Thus, in addition to synoptic winds, the large-scale local wind circulation prevailing over the Kanto Plain can determine the direction of ash transport originating from the Mount Asama volcano and increase the possibility of ashfall in the Tokyo metropolitan area.