Ending trajectories after reaching a 0 mts height.

Topics about the HYSPLIT trajectory model.
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Christian813
Posts: 16
Joined: January 25th, 2018, 1:28 pm
Registered HYSPLIT User: Yes

Ending trajectories after reaching a 0 mts height.

Post by Christian813 »

Hi, I would like to know if there is a way to end particles trajectories as soon as they touch the surface, I've seen that there are particles that keep their direction after touching the surface.

Thank you very much,
Christian
alicec
Posts: 251
Joined: February 8th, 2016, 12:56 pm
Registered HYSPLIT User: Yes

Re: Ending trajectories after reaching a 0 mts height.

Post by alicec »

There is not a way currently to automatically do this.
If you need to, you may simply disregard the points in the trajectory after it reaches the ground.
The tractory output files are ascii files.
https://ready.arl.noaa.gov/hysplitusersguide/S263.htm

There are good reasons NOT to end a trajectory once it has intersected the ground.
The trajectory is generally seen as representing the movement of an air mass and the air mass does
not stop when it "intersects" the ground and not all the material in the air mass would be deposited at that point either.
I suggest reading sections 4-6 of the tutorial.
https://www.ready.noaa.gov/documents/Tu ... basic.html

Also some points from a FAQ

Trajectories
● A trajectory represents the “centerline” of a dispersion
● Can be run forward or backward
○ Forward trajectories tell you where the air will go
○ Backward trajectories tell you where the air came from
● Back-trajectories are commonly used to help interpret air pollution measurements
○ Where did the air come from corresponding to any given sample time?
○ Where did the air come from when the concentrations of the pollutant were high?
○ Where did the air come from when the concentrations were low?


What height(s) should you start a trajectory at?
● Should you start it at the height of your sampler, e.g., 2 meters above the ground?
● Not recommended, as the trajectory can then easily hit the ground and lose accuracy
● If only starting at one height, then a good choice might be “1⁄2 of the height of the planetary
boundary layer (PBL)” (aka mixed layer).
● This choice reflects the idea that what you are seeing at your sampler is the average
concentration of pollutant in the well-mixed boundary layer.
● To use this option, you must use the Advanced menu in the GUI (or a SETUP.CFG file in
command line or script execution)

If you think that your application really needs this capability, we would be interested in hearing more about it.
Christian813
Posts: 16
Joined: January 25th, 2018, 1:28 pm
Registered HYSPLIT User: Yes

Re: Ending trajectories after reaching a 0 mts height.

Post by Christian813 »

I understand. I'm thinking on making some kind of deposition rate above water, that's why I thought on this question. I have some ideas like just eliminating the next position after a particle touch 0 m (which would be surface water). But I had to ask first here if there were other way, thanks!
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