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The vertical coordinate is a hybrid sigmapressure system. In this system,
the upper regions of the atmosphere are discretized by pressure only. Lower
vertical levels use the sigma (i.e. p/p) vertical
coordinate smoothly merged in, with the lowest levels being pure sigma.
A schematic representation of the hybrid vertical coordinate and vertical
indexing is presented below. Both input and output datafiles follow
this format as well as internal model datastructures.
Figure 3.1:
Hybrid vertical coordinate

Pressure is defined as:
p
= AP+ BP(i,j)
where p is the pressure at a given level and latitude, longitude
grid point. The coefficients A, B and
Pare constants. P is the model's
current surface pressure. P is set in the model code. The
input model initial conditions dataset sets A and B through
the variables hyam, hyai, hybm, and hybi.
The subscript "i" refers to interface levels, and "m" refers to the midpoint
levels. "hyam" then refers to Hybrid level "A" coefficient
on the interfaces.
More details on the theoretical nature of the vertical coordinate system
can be found in Collins et al. [2004].
Next: 4. Simple Code Modifications
Up: 3. Model Output
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Jim McCaa
20041022