Assessment of depositional ash loadings from the 2009 eruption of Mt. Redoubt

摘要

Volcanic ash deposited onto ice and snow in the Arctic has the potential to perturb the regional radiation balance by altering the surface reflectivity. In order to determine changes in surface albedo, loading fields of deposits must be quantified. Loading fields are defined as the total mass of ash deposited over an area. We assessed the areal extent and loading of ash deposits from the 2009 eruption of Mt. Redoubt using an Eulerian volcanic ash transport and dispersion model, Fall3D, combined with satellite and deposit observations. Because direct observations are often limited in remote Arctic regions, we devised a novel method for modeling ash deposit loading fields for the entire eruption based on best-fit parameters of a well-studied eruptive event. The major land-depositing eruptive events (events 2-6 on March 23-24) were chosen for analyses. Realistic ranges of model parameters were selected based on the well-described event 5 (March 23) and were estimated for other events. The model results were validated against NASA A-train satellite data and field measurements reported by the Alaska Volcano Observatory. Overall, good to moderate agreement was found. A total cumulative deposit area of 3.7 × 10~6 km~2 was produced, and loadings ranged from ～7000 ± 3000 g m~(-2) near the vent to ＜0.1 ± 0.002 g m~(-2) on the outer margins of the deposits. Spatial gradients of total ash loading along a cross-section extending from Mt. Redoubt to ～600 km revealed event 5 contributed most to the total loading from 0 to 200 km, followed by events 3, 4, 6, and 2. Ash loading histories for total deposits showed that fallout time along the cross-section for each of events 2-6 ranged from ～5-17 h. Our deposit loading results suggest that ash from short-duration events can produce regionally significant deposits hundreds of kilometers from the volcano, with the potential of significantly modifying albedo over wide regions of ice and snow covered terrain.