Jeremy M. Shannon
Michigan Technological University
Thesis for the Degree of M.S.
Abstract. Effective mitigation of volcanic hazards to aircraft, caused by the injection of ash and gas into the atmosphere, requires the timely detection, tracking and prediction of volcanic cloud movement as well as an understanding of the chemical and physical processes involved in the fate of volcanic constituents in the atmosphere.
Satellite detection and tracking of ash and SO2 with the Advanced Very High Resolution Radiometer (AVHRR) and the Total Ozone Mapping Spectrometer (TOMS), respectively, contribute to this effort because they effectively delineate the geographic position of large drifting plumes and are able to quantify amounts of material as they disperse int he atmosphere.
However, a significant limitation of AVHRR and TOMS satellite retrievals is that they cannot provide information on the vertical dimensions and altitude of the plume, especially important considerations for aviation safety.
In this thesis, we reconstruct the three-dimensional (3-D) transport of the volcanic ash and SO2 plumes from the June 1992 eruption of Crater Peak Vent, Mt. Spurr, Alaska by combining TOMS and AVHRR satellite data with an isentropic trajectory model. First, trajectory model simulations are coupled with 2-D satellite plume positions to determine the vertical distribution within the plume for the seven days of TOMS and AVHRR detection. Second, concentrations of SO2 and ash are computed using 2-D satellite mass retrievals normalized to the thickness of the plume. Third, meteorological effects on the fate of volcanic ash and gas are investigated.
Isentropic model wind trajectories indicate that the SO2 was distributed between 9.5 and 15.5 km during the first three days. Horizontal wind shear resulting from faster winds at lower altitudes caused: a) the SO2 plume to elongate nearly 3000 km in the first three days, producing a plume that was lowest in altitude at the leading edge and progressively higher toward the trailing edge; b) removal of the SO2 in the lower portions of the plume, resulting in a measurable SO2 distribution only from about 14 to 16 km by the seventh and final day of detection; c) the average vertical thickness of the SO2 plume to decrease from about 1.2 km on June 28 to about 0.7 km by June 30. AVHRR results show that the ash had roughly the same geographic distribution as the SO2, suggesting that the ash had the same vertical distribution and evolved with the same pattern of horizontal shear. A sharp decrease in ash cloud optical depth for the 12-hour period immediately following the eruption reflects the evolution of the cloud toward a smaller average thickness as it elongated. Computed concentrations of SO2 and ash indicate that both plumes remained poorly mixed within the first four days of detection. The distinctive vertical distribution of Mt. Spurr volcnaic clouds, as well as the influence of physical dispersion, illustrate the importance of understanding and modeling atmospheric transport and dispersion of volcanic clouds for the mitigation of volcanic hazards.