HYSPLIT Forum: hysplitbbs.arl.noaa.gov

Hi,

The default Ztop is given as 10 km and in the tutorial, it is mention that Ztop defines the internal scaling height. I have two questions on this.
Firstly, does the internal scaling height means that at that height there is no effect of topographic features on the trajectory?

Secondly, if I change Ztop to let's say 20 km, then how is it affecting trajectories calculated at higher levels in PBL (for e.g; 8 km AGL)? What I understood from the Tutorial file is that above Ztop the model is not making any calculations. But will there be any difference between trajectories calculated for our e.g; case of the level 8 km AGL with i) Ztop = 10 km ii) Ztop = 20 km?

Thanks

1) No, Ztop does not impact simulated transport over topographical features.
2) 8 km AGL is well outside the PBL. A difference will only be found between a trajectory with Ztop set to 10km vs 20km if the trajectory encounters an extreme convective event that lofts particles above 10 km.

More details on Ztop:
Sets the vertical limit of the internal meteorological grid. If calculations are not required above a certain level, fewer meteorological data are processed thus speeding up the computation. Trajectories will terminate when they reach this level. A secondary use of this parameter is to set the model's internal scaling height - the height at which the internal sigma surfaces go flat relative to terrain. The default internal scaling height is set to 25 km but it is set to the top of the model domain if the entry exceeds 25 km. Further, when meteorological data are provided on terrain sigma surfaces it is assumed that the input data were scaled to a height of 20 km (RAMS) or 34.8 km (COAMPS). If a different height is required to decode the input data, it should be entered on this line as the negative of the height. HYSPLIT's internal scaling height remains at 25 km unless the absolute value of the domain top exceeds 25 km.