HYSPLIT Forum: hysplitbbs.arl.noaa.gov

Hello,

I’d appreciate your input on a few questions related to trajectories initialized at or near the surface:

1. Reading through the HYSPLIT user guide PDF and tutorial sect. 4-6, I understand (mostly implicitly) that the trajectory model in its recent versions applies a PBL mixing scheme (I currently use the hyts_std program in v.5.3.2). This also seems to be the case based on runs in which the airmass trajectory intersects the surface but can then increase in altitude and exit the PBL. Several years ago, while I was still using version 4, a simulated airmass would have “stuck” to the 0 m AGL and didn’t climb up, somewhat consistent with a forum communication Ariel Stein had back in 2014. I think a response provided to a forum user in 2018 supports my current interpretation, but I’d be happy to hear whether my interpretation is correct.

2. If my interpretation is correct and a PBL mixing scheme takes place, I suppose that initialization at 0 m AGL (after verifying that kmsl=0 in SETUP.CFG) should still be valid given that the airmass is lifted up (as long as kmix0 is a few hundred meters). Do you think that is correct, or do you have other suggestions? (For PBL height, I use the input data from ERA5 using kmixd=0, if that matters)

Thank you,
Israel

If my interpretation is incorrect and the trajectory model does not account for PBL mixing, as suggested from Stein et al., 2015, then:
1. Should trajectories not be relied on once they hit the surface (unless using an ensemble approach)?
2. I presume that surface roughness is not considered either in the trajectory model, regardless of whether the roughness field is included in the meteorology files or not. Is that correct?
3. If roughness is considered, I understand that the ASCII file resolution must match the meteorology file resolution or otherwise the default 0.2 m value is used, is that correct?

Thanks.