Abstract
Geostationary Meteorological Satellite (GMS)-1 routinely returns 14 images a day at intervals of 30 minutes-4 hours with ground resolution of 1.25 km by visible sensor and of 5.0 km by infrared detector sub satellite point (Table 1 and Fig. 1). Images of eruption clouds from Alaid Volcano, the Kurile Islands (Fig. 2), and from Pagan Volcano, the Mariana Islands (Fig. 5), both of which erupted in April—May, 1981, are well detected by GMS observations (Plates 1-1, 2 and 2). Eruption clouds from Alaid continued for more than 4 days changing the moving directions by prevailing winds in the upper atmosphere in the latter part of April and intermittently continued through the end of May. Its eruption cloud is detected at a distance of more than 3,000 km SE of this volcano (Figs. 3 and 4). In case of Pagan, we can see an almost circular eruption cloud just after an occurrence of eruption and through the decay of the eruptive activity (Fig. 6), and active eruption clouds are detected for more than 10 hours.
By assuming that the surface temperature of eruption cloud that reached a very high altitude is cooled to the surrounding air temperature, the maximum altitudes of eruption clouds are estimated by isotherms in eruption clouds derived from GMS TBB values (Figs. 7 and 10) and also by using materials of radio sounding observations near these volcanoes (Figs. 8 and 11). The estimated maximum altitudes of eruption clouds at Alaid and Pagan are 11.7 km at 06 Z on April 30 and 16.5 km at 03 Z on May 15, respectively. According to radio sounding observations, it is considered that some of the eruption clouds from Alaid slightly penetrated the tropopause during the activity of April 27-30, but not in case of Pagan. Judging from scanning time around Pagan Volcano in the first image of the eruption cloud (Fig. 12), the eruption at Pagan is considered to have commenced its activity at around 2309 Z on May 14.
With time variations of the maximum altitudes and the maximum horizontal distances of eruption clouds from volcanoes, it is possible to see the time sequences of intensities of eruptive activities (Figs. 9 and 13). Estimated moving velocities of eruption clouds are 19-32 m/s at Alaid and 14-15 m/s at Pagan and both values are faster by 4-6 m/s than the surrounding wind speed at respective altitudes. Horizontal eddy diffusivity of eruption clouds by the method of Gifford (1959) shows large values of 109-1010 cm2/s for both volcanoes.
The method of Briggs (1969) is applied to estimating the thermal energy release of eruption cloud and the estimated values during the most active stages at both volcanoes are 1017 erg/s for Alaid and 1018 erg/s for Pagan. By using duration times of eruptive activities, the estimated total thermal energy releases by eruption clouds are 7×1022 erg at Alaid Volcano and 4×1022 erg at Pagan Volcano.
The eruption clouds spread over lower atmospheric clouds are clearly detected at Alaid, because the Alaid eruption clouds showed very low ALBEDO values, possibly owing to high concentration of ejected materials in clouds. However, there is no clear differences between eruption and atmospheric clouds in case of Pagan (Fig. 14).
It is considered that the detectability of eruption clouds by GMS images is severely hampered by existence of atmospheric clouds around erupting volcanoes. However, it will be possible to detect eruption clouds higher than several km and larger than about 20 km across under good conditions.
