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How does HYSPLIT consider the vertical coordinate. If a trajectory meets the surface how does HISPLIT work? Does it reflect, or use another method to solve it?

In the trajectory model, trajectories only move up and down with the vertical velocity field. Trajectories that reach the bottom boundary continue at the surface, trajectories that reach the top boundary terminate.

In the dispersion model, particle trajectories reflect off both the top and bottom boundaries (to conserve pollutant mass).

Roland Draxler
NOAA ARL