> Which value should be used as the start height (MAGL) in HYSPLIT? What does "above model
ground level" mean?
Which value should I input as the start height (MAGL)? What does "above model
ground level" mean?
Ground level (MAGL) refers to the terrain as it is defined in the
meteorological model we use (GFS, NAM, EDAS, etc). The higher the
resolution of the data, the better the terrain will be modeled.
Since the meteorological data is sampled from
selected points on a grid, valleys and mountains tend to get smoothed out.
Therefore, when you want to start a trajectory at the top of a mountain you
have to realize that the meteorological model might have that mountain
only half the height that it really is! The best we can do is to look at
the surface pressure and the mean sea-level pressure at the starting location
and estimate the height above sea level of the model terrain at your
location.
This can be done using the vertical sounding program available on the web
in the READY system. The text listing will show the mean sea-level pressure
(MSLP) and the surface pressure (PRSS) at your location. Here is an example
for two sites....
35.66N, -82.38W; 1030mb MSLP and 939 mb PRSS
taking the difference gives 91mb. A quick estimate is to assume a
hydrostatic atmosphere, or approximately 10 meters per mb. This
gives 910 MASL, which is what the model sees as the terrain height.
Since you know that the terrain is at 951 MASL, you should
start a trajectory at 41 MAGL (951-910m) if you are interested in the
free atmospheric flow (where did the air originate at some time in the
past). If you are interested in surface frictional effects this will
not work, you would want to start near the model surface, say at 10 MAGL.
Your second location, which is very close (in terms of the model
resolution) to the first point is very interesting because the
model cannot see the tall mountain....
35.78N, -82.29W; 1030mb MSLP and 940 mb PRSS
gives 90mb difference or 900m MASL. Since you know the real height
is at 2006 MASL you can see that the model only sees the mountain
as less than half that height, so you will need to start your trajectory
at 1100 MAGL to get the long range transport!
Here is an example of the opposite case;
Given a point in Colorado, the sounding
program gives the MSLP as 1018 mb and the surface pressure as 733 mb. The difference is 285 mb.
By assuming 10 mb/m, one gets a model elevation of 2850 m. The actual
elevation of this site is 2501 m. So in this case the model is saying that
the elevation is ~350 m "above" the actual elevation instead of below.
If I am wanting to run trajectories at 200 m, 500 m, and 1000 m from this site how
do I adjust? If I adjust downward from 200 m, I would be below actual ground elevation.
If the actual elevation of the location from which you are starting trajectories is
below the average terrain of the corresponding grid cell of the meteorological model
you are using (perhaps the location is in a valley), then yes some
heights above ground will still be below the average elevation of the surrounding area.
In that situation all you can do is assume that true ground-level is at the model's
terrrain height and proceed ... with the realization that the real lower
levels of the flow field may at times be constrained in ways that are not evident in the
coarser gridded meteorological data fields.
