## Eddy diffusion in back trajectory?

Topics about the HYSPLIT trajectory model.
ryanxia
Posts: 19
Joined: July 1st, 2016, 1:05 pm
Registered HYSPLIT User: No

### Eddy diffusion in back trajectory?

There are two mechanisms in transporting water vapor in atmosphere, one is advection and one is eddy diffusion. I do not quite understand the numerical way HYSPLIT is modeling the back-trajectory, but it seems to me the most important are wind velocity and direction fields if I am right. If I modeled a back trajectory on a rainy hour, does the path only tell us the "advection" of air mass? Does the path also incorporate the eddy diffusion?

In addition, in tropical regions, precipitation is derived from convection. Is it appropriate to run back trajectory for convective rainfall hour to know where the water vapor is coming from?

Questions from non-expert...
barbara.stunder
Posts: 421
Joined: November 9th, 2012, 4:23 pm
Registered HYSPLIT User: Yes

### Re: Eddy diffusion in back trajectory?

The Basic Tutorial explains the trajectory calculation. https://www.ready.noaa.gov/HYSPLIT_Tutorials.php

Per the Tutorial, single trajectories have limitations, hence there are options for an ensemble, starting trajectories with time, etc. For a cloud, you would need to start back-trajectories at many heights within the cloud.
ryanxia
Posts: 19
Joined: July 1st, 2016, 1:05 pm
Registered HYSPLIT User: No

### Re: Eddy diffusion in back trajectory?

Thank you. I have a general technical question. I am interested in knowing the source of moisture for rainfall. For mid-latitude regions, rainfall has mainly caused by transient eddy rather than advection, but eddy is also embedded in advection. If I run back-trajectory for such region, does the single trajectory path tell us about the source of moisture?
barbara.stunder
Posts: 421
Joined: November 9th, 2012, 4:23 pm
Registered HYSPLIT User: Yes

### Re: Eddy diffusion in back trajectory?

The trajectory calculation uses the mean wind resolved in the meteorological model. For finer scale eddy motion you cannot use these trajectories.