Ensemble starting heights

[arl.noaa]

I am frequently calculating trajectory ensembles starting withing the PBL using the HYSPLIT ensemble mode. Usually I use 500 m agl as starting height and leave the default grid offset parameters (DXF, DYF, DZF) untouched. In the HYSPLIT manual it is said, that the default DZF of 0.01 sigma units corresponds to approx. 250 m.

I am not a meteorologist, but I now realized that in the standard atmosphere, 0.01 sigma units rather correspond to approx. 100 m close to the ground. Is the sigma in HYSPLIT defined differently?

Or, in other words, if I would like to start my trajectory ensembles closer to the ground (let's say at 100 m agl), what would be a good sigma to use for DZY in the HYSPLIT model?

Thanks for your help.

[glenn.rolph]

The standard atmosphere goes much higher. In the HYSPLIT sigma-height coordinate, the model top is 25 km, that is why 0.01 sigma = 250 m