I am trying to do a backward trajectory for parcels involved in historical weather events, and I know that even minor changes in the point or time can yield much different results for the parcel origin. Is there a way to do an “ensemble” run, slightly perturbing the starting point?

On the web, there is an ensemble method available to registered users that will shift the meteorology grid in X, Y, and Z by 1 grid point in X and Y and 0.01 sigma units in the vertical (the PC version is more flexible in choosing these parameters). Just click on the Ensemble option in the trajectory model options page.

Glenn Rolph