Distal Enhanced Sedimentation From Volcanic Plumes: Insights From the Secondary Mass Maxima in the 1992 Mount Spurr Fallout Deposits

Abstract

Some tephra fallout deposits show an increase of mass and thickness at distances from the source >100 km (areas of secondary mass maximum, ASMM) which demonstrates distal enhanced sedimentation from volcanic plumes. We explore development of the ASMMs during the 1992 August and September Mount Spurr eruptions, USA, by combining field data on the spatial distribution of mass and grain size with (1) simulations of individual particle settling through a homogeneous and horizontally stratified atmosphere and (2) mesoscale models of the three‐dimensional wind field that include the effect of the underlying topography. The crosswind and downwind variations of deposit characteristics indicate that the increase of sedimentation at the ASMMs is not formed solely because of preferential settling of small ash particles (<125 μm), as commonly assumed in aggregation models. Instead, ASMM grain sizes correspond to the fine modes of the bimodal total grain size distributions. There also appears to be a link between the ASMM and the topography: the mass local minima occur across the windward flank of 2 km high mountain ranges, while the ASMMs spread on the leeward flank. Mesoscale models of the three‐dimensional wind field show vertical oscillations in the wind over mountainous regions which may enhance mechanisms of en masse sedimentation (aggregation, hydrometeor formation, and particle boundary layers), as well as strong spatial variations of the horizontal wind field in the lower troposphere. Our study demonstrates the importance of using grain size, as well as mass, data to constrain the complex processes responsible for particle sedimentation from volcanic plumes.

Meteorological Controls on Local and Regional Volcanic Ash Dispersal

“…Although this leads to some improvements (especially on the southern coast) it underestimates ashfall inland and overestimates deposition along the western coast. The impact of topographic effects on ash transport deposition has been noted in several studies 11 , 20 , 40 , 41 and can also be seen in the simulations here: flattening the topography of St Vincent dramatically changes the low-level winds and reduces the amount of turbulent mixing over the island. The Control simulation has strong descent associated with down-slope winds on the leeside of the island, which drives ash towards the ground (see Supp.…”

Experimental High-Resolution Forecasting of Volcanic Ash Hazard at Sakurajima, Japan

“…These secondary maxima are ubiquitous in ash deposition studies and have also been noticed using observational data around Sakurajima [3,26]. A number of mechanisms have been proposed to offer explanations, including particle aggregation [63], localized orographic effects [77] leading to force deposition of volcanic ash [27,78,79], plume and atmosphere interaction [80], as well as due to the effect of different shear layers in the atmosphere [30]. Overall, the control simulation offers a notable improvement over the benchmark forecast (Fig.…”

Statistical analysis of dispersal and deposition patterns of volcanic emissions from Mt. Sakurajima, Japan

“…At greater distances from the vent the impact of volcanological effects is decreased due to lower concentrations of ash (Gilbert and Lane, 1994;Scollo et al, 2017) and deposition patterns can be assumed to be more readily affected by preferential dispersal and the effects of the topography. Topographic heights located along the dispersal path have been implicated in enhancing deposition of ash in their lee side (Watt et al, 2015;Eychenne et al, 2017). While this effect was also confirmed for Sakurajima A.P.…”

“…points of flow splitting) could introduce localised maxima in the concentration of airborne ash, and enhancing both aggregation (Gilbert and Lane, 1994) and deposition via gravitational instabilities (Manzella et al, 2015;Scollo et al, 2017). Despite an increase of research in the field in recent years (Watt et al, 2015;Eychenne et al, 2017), the interactions between volcanic ash and local orographic phenomena have currently not been comprehensively studied and analysed, mainly due to the lack of concrete observational data.…”