These parameters collectively should yield a baseline understanding of the distribution of ash layers, and we acknowledge that any one of these parameters is not necessarily preferable to the others.
Nevertheless, the overall similarity in results is comforting and suggests that for the purposes of rapid emergency management planning a simple advection diffusion model such as TEPHRA2 can capture the first-order distribution of ash fall hazard locally.
They modeled a monthlong eruption with wind conditions from January 2001, but the actual distribution of ash would ultimately depend on multiple factors, they note: not just the duration of the eruption and the direction of winds, but also the height of the ash cloud and the size of the ash particles.
Map A depicts the hazard using a fixed ash threshold – a distribution of probabilities of reaching and exceeding a fixed ash threshold, here, 1 mm, and map B depicts the hazard using a fixed probability threshold – a distribution of ash thicknesses expected at a fixed probability threshold, here, 25%.
Figure 8 Respondents' preference and perceived helpfulness of two different types of map content: maps depicting a distribution of probabilities of reaching and exceeding a fixed ash threshold, such as 10 mm (fixed ash threshold), and maps depicting a distribution of ash thicknesses expected at a fixed probability threshold, such as 25% (fixed probability threshold) (see Figure 3 ).
The distribution of ash layers and pockets were parameterized by (1) thickness with depth, (2) number of layers per 10 meters, (3) thickest layer (cm) over successive 10 meter depth increments, and (4) total thickness of ash (cm) in that same 10 meter window (Fig. 12).
During investigations as to how the wind information used by ASHFALL might be improved, we looked at the atmospheric dispersion model HYSPLIT (Stein et al., 2015), already in use by MetService for the movement of fine particles travelling in the atmosphere, such as volcanic ash, and found it has the capability for estimating the distribution of ash particles on the ground.
At longer time scales the footprint for a given threshold of ash loading might be expected to increase beyond the imposed 500 km limit (e.g. a Tambora 1815-style event; Sigurdsson and Carey 1989), and it is acknowledged that the distribution of ash at these longer timeframes (e.g. 10,000 years) in this PVAHA might be underestimated.
Firstly, the variable thickness of canopy cover, combined with ground layer obstacles such as fallen trees, rotting stems, and the location of living stems, precludes the even distribution of ash on the forest floor.3 Secondly, various authors have calculated the mean turnover rate of trees within rainforest to range between 40 and 180 years (Hartshorn 1978, 1980).
The overall grain size distribution of ash can vary greatly with different magma compositions.
