AN ASSESSMENT OF THE QUALITY OF FORECAST TRAJECTORIES

Forecast and ''analysis'' (reference) trajectories were computed from six sites over North America at three altitudes (500, 1000, and 1500 m above ground) twice a day for a one-year period using Nested Grid Mod el wind fields. The reference meteorology was a series of short-term f orecasts. Absolute error (distance between reference and forecast traj ectory), relative error (absolute error divided by forecast trajectory travel distance), and the angle between the reference and forecast tr ajectory were also computed. The mean relative error for all the forec ast trajectories for a travel time of 36 h is about 35%; the 90th perc entile of the relative error is about 65%. The forecast is slightly bi ased to the left of the reference early in the forecast period. Absolu te error and travel distance both are larger in winter than summer, so that the relative error is generally constant throughout the year. Di fferences in mean error among the three starting altitudes, among the six origin sites, and between the two origin times are insignificant w hen compared to the variation in errors for a collection of trajectori es at a given origin. The forecast trajectories were objectively class ified through a cluster analysis, which groups trajectories by directi on and travel distance. For all clusters, by season, origin site, and altitude, differences between the minimum and maximum cluster-mean rel ative errors were about a factor of 2-3. Individual forecast trajector ies composing clusters with the minimum relative error (about 20%) ten ded to originate within stronger, steady flow either ahead of or behin d a cold front. Maximum relative error (about 45%) was associated with forecast trajectories originating in regions of generally slow wind f ields such as under a high pressure system or near stationary or slowl y moving fronts.