Limitations of the Advection-Diffusion Equation for Modeling Tephra Fallout: 1992 Eruption of Cerro Negro Volcano, Nicaragua

• Main Author: Martin, Kristin Terese
• Format: Others
• Published: Scholar Commons 2004
• Subjects: volcanology; volcanic ash; hazard models; Marrabios Range; isomass; American Studies; Arts and Humanities
• Online Access: https://scholarcommons.usf.edu/etd/1150; https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=2149&context=etd

Detailed mapping and granulometric analyses of the 1992 Cerro Negro tephra blanket reveal remarkable departures from the expected distribution of tephra. Isomass maps show that the major axis of dispersion for the eruption was to the SW of the cone and that the coarser-grained particles, ranging from -4.0 -- 1.0 f, were deposited primarily along the major axis of dispersion with deposits thinning off of the axis. Comparable isomass maps for finer-grained particles, 1.5 - 3.5 f, show that these particles were primarily deposited along the edges of the deposit, off of the major axis of dispersion. Advection-diffusion models for tephra fallout currently widely used in volcanology do not account for this deposition pattern. Rather, it appears that interaction between the wind field, which developed a strong cross flow during the eruption, and the ascending tephra plume resulted in the formation of turbulent structure in the plume. Particles with a settling velocity greater than ~1-2m/s (diameter >0.5 mm) were able to overcome the turbulent structure and settled in a manner predicted by the advection-diffusion equation. Those with lower settling velocities were caught up in turbulent structure and deposited off of the major axis of dispersion, near the edges of the overall tephra blanket. Thus, this data set provides the first estimate of the strength of such turbulent structures in advecting plumes, and illustrates the limitations of the typical advection-diffusion models in describing some transport processes.