The HYSPLIT distribution models will often show significant areal
coverage. Is there a threshold particle density that defines the plume
boundaries and if so, is it one that corresponds to a known aircraft
danger/non-danger threshold ?
There is no known aircraft danger/non-danger threshold. From what we know, pilots
simply want to avoid the ash plume. HYSPLIT does not calculate actual ash
concentrations because when an eruption occurs, the quantity of ash in the eruption is
unknown. HYSPLIT assumes an initial quantity of one unit, and outputs concentration
with respect to that. A threshold ash concentration is chosen to define the visual ash
boundaries shown on the HYSPLIT maps. It is based on correlations between ash seen in
visible satellite imagery from eruptions of a few volcanoes, and further refined based on
the extent of the observed ash. More research is needed here, but little satellite or
other observational verification data exists.
The ash concentration threshold that is dangerous to aircraft is not
known. From what we know, pilots simply want to avoid the plume.