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