OBSERVATIONS OF THE 1989-90 REDOUBT VOLCANO ERUPTION CLOUDS USING AVHRR SATELLITE IMAGERY

The 1989-90 eruptions of Redoubt Volcano, Alaska, generated ash clouds that reached altitudes of as much as 12 krn and provided an opportunity to test and refine the use of the advanced very high resolution radiometer (AVHRR) for satellite remote sensing of volcanic eruption clouds. Several aircraft encountered dispersed volcanic clouds, including a Boeing-747 that temporarily lost power from all four engines. Such near-tragic incidents highlighted the need for improved methods of discriminating and tracking volcanic clouds. Thirty-one AVHRR images from various stages of the Redoubt eruption were examined. In the early, more energetic phases of the eruption, the Redoubt volcanic clouds could be discriminated by a technique that employs an apparent temperature difference between bands 4 and 5 of AVHRR. This method was particularly successful in clouds that were older than 1 hour, demonstrating that slight aging of the cloud during dispersal enhances its discrimination by this method. In the later stages of Redoubt activity, this method fails, probably because of environmental variables associated with the changing character of the eruption. The ash clouds associated with dome collapses that marked the later Redoubt clouds were of small volume and may not have contained as high a proportion of fine ash particles. Laboratory biconical reflectance measurements were conducted to determine the spectral variability of ash that fell out of the Redoubt clouds. The results show that ash reflectance is controlled more by particle size than by particle composition, but these alone do not explain the variability we observed in the satellite data.