The 1992 Crater Peak vent eruptions of Mount Spurr provided a robust test of the new High Resolution Picture Transmission (HRPT) Information Processing System (HIPS), which was installed in Anchorage for real-time tracking of volcanic clouds. One principle objective of the tracking system is to avoid aviation hazards. The system receives Advanced Very High Resolution Radiometer (AVHRR) images from polar-orbiting satellites and processes them using a two-channel technique. The resulting processed imagery reflects the extensive real-time capability now available. The frequency of data collection (about once every few hours) allows for an understanding of the patterns of change in spectral response that occur during the evolution of a volcanic cloud. Volcanic clouds imaged during and shortly after eruption are optically thick and can contain abundant water droplets and (or) ice, and these characteristics cause their spectral signal to closely resemble a meteorological cloud. As the volcanic cloud disperses, the spectral properties of the cloud change, first at the edge and then throughout. These changes produce a volcanic cloud signal that can be distinguished by using a brightness temperature difference determined from thermal bands 4 and 5 of the AVHRR. The usefulness of this technique of tracking clouds for long distances was investigated using archived data. Two clouds were tracked for more than 80 hours and thousands of kilometers, during which the band 4 minus 5 brightness temperature signal decayed. These clouds traveled southward over the continental United States and posed a potential hazard to aircraft. To alleviate this potential hazard, the measurable brightness temperature difference signal needs to be correlated with ash concentration and particle size to determine when drifting volcanic clouds are a threat to aircraft.
Available at: http://works.bepress.com/william-rose/149/