Why multiple trajectories are different than just one?

[arl.noaa]

I created two trajectory graphics. One has one trajectory starting at 500m AGL. The other has three trajectories starting at the same location but three different levels, including the 500m AGL. But the trajectory starting at 500m AGL on the two graphics is very different. Please give me suggestions.

[glenn.rolph]

We discovered that it is not an error with the data or HYSPLIT. The issue is that when you run one trajectory it works fine, but when you run more than one trajectory, the time step of the model may change based on where the other trajectories are. The time step is set by the fastest trajectory the last hour, when there are more than one trajectory, this could result in a different time step for any one trajectory than if it were run by itself. When a trajectory is sensitive to the time step, this means that the spatial gradients in the velocity field are too strong and there should be less confidence given to the accuracy of any one trajectory.

Until we decide how to handle this on the web site (fixing the time step of all the trajectories), we would recommend that you just run one trajectory.