Volcanic Transport
In our study of the transport of materials from large volcanic eruptions we focused initially on modern eruptions, where imagery documenting the time-resolved distribution, and wind measurements and related meteorological data are readily available.
Mt. St. Helens
The first case tackled was the 1980 eruption of Mt. St. Helens. The results confirmed the utility of using ash-tracking software in the study of modern eruptions, and suggested that the vertical distribution of ash transport is not controlled by the greatest height of penetration of the ascending plume. Wind fields from four reanalysis sets were used in PUFF simulations of the eruption. The ERA-40 set was prepared by the European Center for Medium-range Weather Forecasting. The fields available to us through the National Center for Atmospheric Research are at two spatial resolutions: 1.125 degree grid spacing, and 2.5 degree spacing. Both of these sets are archived at 6 hr temporal resolution, and have 23 vertical levels extending to ~1 hPa (48 km). We also used fields from the NCEP/NCAR 40 year Reanalysis set, which also has a spatial resolution of 2.5 degrees, but which has 17 vertical levels, extending to 10 hPa (~30 km). Finally, we made use of the North American Regional Reanalysis fields; these extend only to 100 hPa (~16 km), but have a spatial resolution of 32 km and are archived at intervals of 3 hr. The figure illustrates the development of the areal extent of the distribution from the plinian phase of the eruption.
Each color represents a different time step, at intervals of two hours. The solid outlines represent the cloud edge as interpreted from GOES satellite images. The modeled cloud utilizing the ERA-40 wind fields, regardless of resolution, best matches the observed outline of the MSH plume (panels a,b). The NCEP wind data failed to capture the sharp southward turn apparent in the outlines after 1515 PDT (panel c). The NARR wind fields show the southern component of the plume, but transport ash further to the north than the observed distribution (panel d). After times longer than 18 hours the results obtained using these different wind fields are indistinguishable by visual comparison.
The full story is in Julie's publication on this study: Fero, J., Carey, S. N., and Merrill, J. T., 2008. Simulation of the 1980 eruption of Mount St. Helens using the ash-tracking model PUFF. Journal of Volcanology and Geothermal Research, 175, 355-366, doi:10.1016/j.volgeores.2008.03.29.
Mt. Pinatubo
This cartoon illustrates some of the findings of our study of the 1991 eruption of Mt. Pinatubo.
PUFF and HAZMAP, two tephra dispersal models developed for volcanic hazard mitigation, are used to simulate the climactic 1991 eruption of Mt. Pinatubo. PUFF simulations indicate that the majority of ash was advected away from the source at the level of the tropopause (~ 17 km). Several eruptive pulses injected ash and SO2 gas to higher altitudes (~ 25 30 km), but these pulses represent only a small fraction (~ 1 %) of the total erupted material released during the simulation. Comparison with TOMS images of the SO2 cloud after 71 and 93 h indicate that the SO2 gas originated at an altitude of ~ 25 km near the source and descended to an altitude of ~ 22 km as the cloud moved across the Indian Ocean. HAZMAP simulations indicate that the Pinatubo tephra fall deposit in the South China Sea was formed by an eruption cloud with the majority of the ash concentrated at a height of ~ 17.5 km. Results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height (~ 40 km) and was thus controlled by atmospheric circulation patterns near the regional tropopause. In contrast, the movement of the SO2 cloud occurred at higher levels, along slightly different trajectories, and may have resulted from gas/particle segregations that took place during intrusion of the Pinatubo umbrella cloud as it moved away from source.
The above is excerpted from Julie's paper on this topic: Fero, J., Carey, S. N. and Merrill, J. T., 2009. Simulating the dispersal of tephra from the 1991 Pinatubo eruption: Implications for the formulation of widespread ash layers. Journal of Volcanology and Geothermal Research, DOI:10.1016/j.jvolgeores.2009.03.011