Ground level (MAGL) refers to the terrain as it is defined in the
meteorological model we use (AVN, Eta, 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....
1. 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. 2. 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;