## Different trajectory outputs for the same inputs

Topics about the HYSPLIT trajectory model.
dimmamali
Posts: 1
Joined: March 21st, 2018, 10:20 am
Registered HYSPLIT User: No

### Different trajectory outputs for the same inputs

Hello,

I get different results when I run the online hysplit model for 2 backtrajectories at altitudes 1000 and 1500 m https://ready.arl.noaa.gov/hypubout/trajplot_142809.pdf and for 3 backtrajectories at 500, 1000 and 1500 m https://ready.arl.noaa.gov/hypubout/trajplot_142754.pdf (all other parameters being equal).

Why does this happen and how can I understand which situation is the one closer to reality?

Dimitra

barbara.stunder
Posts: 278
Joined: November 9th, 2012, 4:23 pm
Registered HYSPLIT User: Yes

### Re: Different trajectory outputs for the same inputs

Yes, this is a known issue that multiple trajectories in the same run can sometimes be different than when run separately. Please see the "More info" button to the right of "Level 1 height" at https://www.ready.noaa.gov/hypub-bin/traj1.pl.

For reference here it is:
Up to 3 simultaneous trajectories can be calculated at multiple levels. Enter the heights with the lowest height in input box 1. Then check the box for either meters above model ground level or meters above mean sea-level. If entered as meters above mean sea-level, the model will convert them to meters above model ground level during the trajectory calculation. If they are below the model ground level, they will be assigned a height of zero meters above model ground level.

NOTE: Please be aware of the following issue: when running more than one trajectory (multiple levels) in the same run, the time step of the model may change based on where the other trajectories are located. 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. If this is an issue for your work, run only one trajectory at a time.