Forecast Trajectory Accuracy

Forecast trajectories are frequently used to provide an estimate of the forecast transport path of atmospheric species such as radioactivity and volcanic ash (precursor models to VAFTAD), manned balloons, and to position aircraft for tracer sampling in experiments such as ANATEX. Forecasts of ground-level evacuation regions, flight restriction zones, or aircraft sampling positions are dependent on the accuracy of forecast trajectories or dispersion model calculations. The purpose of the work by Stunder (1996) was to estimate the trajectory error associated with the quality of the meteorological forecast and to relate the error to synoptic patterns.

Research Summary

Forecast and reference HYSPLIT trajectories were computed from six sites at three altitudes twice-a-day for a one-year period using Nested Grid Model (NGM) wind fields. The reference meteorology was a series of short-term forecasts called the NGM Archive. Absolute error (distance between reference and forecast trajectory) and relative error (absolute error divided by forecast trajectory travel distance) were also computed. The mean relative error for all the forecast trajectories for a travel time of 36 h was about 35%; the 90th percentile of the relative error was about 65%; the boxplots shown in the figure summarize the absolute and relative error distributions. Absolute error and travel distance both were larger in winter than summer, so that the relative error was generally constant throughout the year. Differences in mean error among the three starting altitudes, among the six origin sites, and between the two origin times were insignificant when compared to the variation in errors for a collection of trajectories at a given origin. The forecast trajectories were objectively classified through a cluster analysis. For all clusters, by season, origin site, and altitude, differences between the minimum and maximum cluster-mean relative errors were about a factor of two to three. Individual forecast trajectories composing clusters with the minimum relative error (about 20%) tended to originate within stronger, steady flow either ahead of or behind a cold front. Maximum relative error (about 45%) was associated with forecast trajectories originating in regions of generally slow wind fields such as under a high pressure system or near stationary or slowly-moving fronts.

Publication

Stunder, B.J.B., 1996: An assessment of the quality of forecast trajectories. J. Appl. Meteor. 35(8) pp. 1319-1331.