When I ran a backward trajectory from a location on a previously computed forward trajectory, I did not return to the starting location of my forward trajectory. Why?

A backward trajectory from the forward end-point will rarely end up at the starting location due to the accumulation of various numerical integration errors. It depends upon the complexity of the wind field and the integration time step. The existing error tolerance is a compromise for speed and accuracy (concentration and trajectory models run with the same criterion) such that a point will not be advected more than 75% of the grid cell size in one time step. You can reduce the numerical error by reducing TRATIO from 0.75 to a smaller value, such as 0.25. TRATIO is defined in the SETUP.CFG namelist file that should be located in the root startup directory, either \working from the GUI or \trajmdl from the command line. This option will be available on the next Hysplit revision (coming within a few weeks).

Roland Draxler