Stunder, B.J.B., J.L. Heffter,and Roland R. Draxler

*Weather and Forecasting*,
2007,Volume 22, pages 1132-1139.

**Abstract** -
In support of aircraft flight safety operations, daily comparisons between modeled, hypothetical,
volcanic ash plumes calculated with meteorological forecasts and analyses were made over a
one-and-half year period. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)
model simulated the ash transport and dispersion. Ash forecasts and analyses from seven volcanoes
were studied. The volcanoes were chosen because of recent eruptions or because their airborne ash
could impinge on well-traveled commercial aircraft flight paths. For each forecast-analysis pair,
a statistic representing the degree of overlap, the threat score (TS), was calculated. A forecast
was classified as acceptable if the TS was greater than 0.25. Each forecast was also categorized
by two parameters: the forecast area quadrant with respect to the volcano and a factor related to
the complexity of the meteorology. The forecast complexity factor was based on the degree of spread
using NCEP ensemble output or using a HYSPLIT offset configuration. In general, the larger the
spread of the ensemble or offset forecasts, the greater the complexity. The forecasts were sorted
by complexity factor, and then classified by the quartile of the complexity. Volcanic ash forecast
area reliability (VAFAR) was calculated for each forecast area quadrant and for each quartile of
the complexity factor. VAFAR is the ratio of the number of acceptable forecasts to the total number
of forecasts. Most VAFAR values were above 70%. VAFAR values for two of the seven volcanoes
(Popocatepetl and Tungurahua) tended to be lower than the others. In general, VAFAR decreased with
increasing complexity of the meteorology. It should be noted that the VAFAR values reflect the
reliability of the meteorological forecasts when compared to the same calculation using analysis
data; the dispersion model itself was not evaluated.

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