Orographic effects on the transport and deposition of volcanic ash: A case study of Mount Sakurajima, Japan. Issue 17 (6th September 2017)

• Record Type: Journal Article
• Authors: Poulidis, Alexandros P.; Takemi, Tetsuya; Iguchi, Masato; Renfrew, Ian A.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 17(2017)
• Page Start: 9332
• Page End: 9350
• Publication Date: 2017-09-06
• Subjects: orographic -- ashfall -- WRF -- Sakurajima -- tephra fall -- mountain wave; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2017JD026595
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Volcanic ash is a major atmospheric hazard that has a significant impact on local populations and international aviation. The topography surrounding a volcano affects the transport and deposition of volcanic ash, but these effects have not been studied in depth. Here we investigate orographic impacts on ash transport and deposition in the context of the Sakurajima volcano in Japan, using the chemistry‐resolving version of the Weather Research and Forecasting model. Sakurajima is an ideal location for such a study because of the surrounding mountainous topography, frequent eruptions, and comprehensive observing network. At Sakurajima, numerical experiments reveal that across the 2–8 ϕ grain size range, the deposition of "medium‐sized" ash (3–5 ϕ ) is most readily affected by orographic flows. The direct effects of resolving fine‐scale orographic phenomena are counteracting: mountain‐induced atmospheric gravity waves can keep ash afloat, while enhanced downslope winds in the lee of mountains (up to 50% stronger) can force the ash downward. Gravity waves and downslope winds were seen to have an effect along the dispersal path, in the vicinity of both the volcano and other mountains. Depending on the atmospheric conditions, resolving these orographic effects means that ash can be transported higher than the initial injection height (especially for ash finer than 2 ϕ ), shortly after the eruption (within 20 min) and close to the vent (within the first 10 km), effectively modifying the input plume height used in an ash dispersal model—an effect that should be taken into account when initializing simulations. Key Points: Gravity waves and downslope winds affect the transport and deposition of volcanic ash Meteorological‐ash‐dispersion modeling using WRF is able to resolve orographic process that leads to enhanced ash deposition The prescribed model plume height can be modified by orographic effects such as gravity waves near the volcano