The trajectory starting height is defaulted to meters AGL (above ground
level), however height definitions can be changed to meters MSL (mean sea
level) from the Advanced / Configuration Setup menu tab as was done
in the previous exercise. Regardless of how the input heights are defined, internally
HYSPLIT treats all heights in a terrain following coordinate system based on
the chosen meteorological data. These heights may be quite
different from the actual terrain height at a point of interest.
As an example of how one might define a starting trajectory height, examine
the location for Broomfield (KBJC), Colorado, at 39.92N and 105.12W,
which has a surface height of 1724 m ASL. (If we did not know
the terrain height for Broomfield, we could estimate it using the profile
program for several different data sets (right) by taking the difference between the surface
pressure (PRSS) and mean sea-level pressure (MSLP) and multiplying the result by 10 m per hPa,
which is the approximate relation of height to pressure and is a function of the air density.)
|
Model |
Resolution |
Terrain |
MM5 |
15 km |
2000 m |
MM5 |
45 km |
2100 m |
NAM |
12 km |
1970 m |
NAM |
40 km |
1840 m |
RUC |
20 km |
1890 m |
GFS |
1 deg |
2020 m |
|
The terrain heights for the NAM 12 km (left) and GFS (right) are shown below (Bloomfield is
indicated by the orange star). The terrain in the vicinity of Bloomfield
is much smoother in the coarser GFS than the NAM and the terrain gradient is much steeper in the NAM and therefore
we would expect to see differences in the terrains. Also, when the model terrain is consistently above the
true terrain, one might suspect that the station location is in a valley, as in this case.
In this situation, all one can do is assume that true ground-level is at the model's terrain height and proceed
with the realization that the lower levels of the flow field may at times be constrained in ways that are
not evident in the coarser gridded meteorological data fields.
 |
 |
In the example below we compare the NAM 12 km trajectory (blue) to the MM5 45 km (green),
and the 1 degree GFS (red) originating from 10 m AGL. Even though all the trajectories
start out at the same height AGL, they start at different pressure levels
due to differences in elevation between the data.

|