AGL to sigma coordinates conversion

[lagom]

Hi,

I have two questions about the conversion from AGL to sigma terrain-following internal coordinates.

1- this conversion is done several times in the code but in the PAROUT subroutine it seems to be done differently than before, i.e. the scaling factor is the top of the model height instead of the difference between the top of the model and the local ground level height. Is it actually so? What is the reason of this choice?
2- Forcing particles to stay above a fixed location it seems that the variable ZSFC (height of the ground) changes as time is going on, is it right?

Thank you in advance any help

bests

D.

[lagom]

I forgot to specify that, as meteorological dataset, I'm using the NARR reanalysis in the ARL Hysplit format, downloaded from your site.

Thank you in advance,

bests

D.