Silicate dusts, volcanic ash clouds and Asian dust, are well detected by the 'split-window' method, which calculates the brightness temperature difference of the 11 and 12 μm bands. Volcanic plumes containing less ash are enhanced by the difference of the visible and near infrared bands of NOAA/AVHRR data. In order to supplement these difference images and improve the discrimination of volcanic ash- rich/poor plumes and of Asian dust from various meteorological clouds, various combinations of AVHRR imagery were investigated. It was found that the differences of the 3.7 and 11 μm bands and of the 1.6 μm and visible bands are useful to distinguish between thick clouds and dusts. Colour composite images containing the 1.6 or 3.7 μm bands are useful for distinguishing the objects from any meteorological clouds, because these bands are sensitive to droplet size.
The advection and dispersion of Asian dust events during 2000-2002 from China to the Pacific Ocean around Japan were investigated using meteorological satellite data, NOAA/AVHRR and GMS-5/VISSR. Aerosol Vapor Index (AVI) images, taking the brightness temperature difference between 11 micron and 12 micron, are very effective to
monitor the Asian dust phenomenon in the east Asia region, because of their capacity for night-time detection. The remarkable dust events during 2000-2002 were classified into three types based on the weather patterns: 'Dry Slot' type, 'Wedge of High Pressure' type, and
'Travelling High' type.
The eruption of Miyakejima volcano started on 8 July 2000. As of September 2002, SO2 emissions measured by COSPEC average 11000 t/day. As the volcanic gas tends to behave together with the plume, we may infer the advection of the gas from the observation of the plume. Ground observations have been performed from Mikurajima using CCD cameras since September 2000. These pictures clarify the vertical structure of volcanic clouds. The horizontal dispersion of the clouds is well shown in NOAA/AVHRR, Terra/MODIS and other satellite images. Ash-rich clouds at the first stage are well detected by the difference image of NOAA/AVHRR 5 and 4, while vapor-rich clouds are insensitive to the difference. Instead, the latter are detected by the difference of AVHRR 1 and 2, and discriminated well from meteorological clouds in many cases. MODIS data is sensitive to the ash and sulfates of Miyakejima plumes in the 8.6 mm channel, and can detect the fine structure of the plumes with its high-resolution visible channels. Miyakejima plumes contain large quantities of volcanic gases, and their daily monitoring using satellites and ground observations in conjunction with upper wind data helps to understand high concentration events of sulfur dioxide downstream from the volcano.