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.