Originally Posted by
peter we
No one has ever said that volcanic ash is imaginary; just that London VAAC/UKMO forecasts have included vast areas where the ash densities are so low and the particles so fine that neither engine damage nor abrasion is realistically going to happen.
Here's an extract from a
paper co-written by UKMO in 2007, which clearly shows that there are differences in approach, which make UKMO's figures significantly more pessimistic than other VAAC:
There is currently no consistency in the way that the participant VAACs define the ash-cloud boundary (Table II) with thresholds depending variously on the source release rate used, availability of satellite data for comparison and forecaster judgement. The London VAAC, for example uses a visual ash-cloud look-up table to determine hazardous ash concentration as a function of the plume height. This table was originally constructed by NOAA for the VAFTAD dispersion model and was based on correspondence between an ash cloud modelled with a 1 g source and that seen on satellite imagery for real eruption scenarios. As such, it is an empirical relation and may not be appropriate for all types of eruptions. The Washington VAAC has a variable concentration threshold for determining the ash boundary from the output of the model. The default threshold is the value in the NOAA table. The forecaster then previews output using thresholds 10, 100, and 1000 times the default and chooses the appropriate threshold based on forecaster judgement, in conjunction with satellite imagery.