As an airline pilot I always check the volcanic ash dispersion forecasts for Colima and Popoctptl whenever I fly Toronto-Mexico City, however I’m a little uncertain as to just how much gravity I should treat them with. If I was flying, say, from Miami to Mexico City at 18000ft can you tell me just what I might encounter if VAFTAD has a big red “blotch” along my route? Should I be planning to avoid it altogether or is the ash likely to be too dissipated to cause a problem?
First, you should see the current volcanic ash SIGMETs and Volcanic Ash Advisories. These products are based on various information sources, including the dispersion model output. The SIGMET is the official warning product. The dispersion model output is only for guidance purposes. That said, you raise a good point. We know that for some volcanic eruptions, the HYSPLIT dispersion model overpredicts the spatial extent of the ash “cloud”. This likely occurs because we do not know how much ash is in a given eruption and other uncertainties in describing the initial ash column. For instance, there may be unknown amounts of steam and/or sulfur dioxide (SO2) mixed with the ash. For a real volcanic eruption, based on satellite imagery, a “reduced ash” version is disseminated if the forecaster believes it more appropriate. The “hypothetical eruptions” web page ( http://www.ready.noaa.gov/ready-bin/ashhypo.pl) has HYSPLIT output from both the full-ash case and a “reduced ash” case. The differences in these two products indicate some uncertainty in the forecast based on the uncertainty in the eruption product mix (ash, steam, SO2, etc.) in the source. Typically for a large eruption (greater than about 30,000 ft) there are only small differences between the reduced and standard run, for smaller eruptions there is a greater difference.