> Why is the pressure not constant on HYSPLIT isobaric trajectories?
After calculating backward isobaric trajectories I see that the pressure is
not constant and looking at the vertical motion plot at the bottom of the plot,
I see vertical motion (about 500m difference from the starting point to some
other points ). Why is this?
The isobaric (and other options) are not truly "iso --" but quasi
because the motion is only approximated by computing the velocity
required to maintain the parcel on the iso surface. When the next time
period's meteorological data are read in, a parcel may find itself on
another surface. So the departure from "iso" is one measure of the
precision of that particular trajectory (precision not accuracy, that is
another issue). As to the vertical motion plot, you must be careful because
the vertical scale is height above model ground level, not pressure level.
So that as you are traveling along a pressure surface the distance between
the ground and the trajectory endpoint will change (decrease with
higher elevation and increase with lower elevation).
