Terrain Height Issues |
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| 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 AMSL. 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 terrain heights between the 2 models. Also, when the all model terrain heights (see table to the right) are consistently higher than the true terrain height, one might suspect that the station is located 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 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. |
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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.
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