### Previous Section Headers

## 2.6 Model Vertical Coordinate

The vertical coordinate is a hybrid sigma-pressure 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**_{s}) 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 2.1 Hybrid Vertical Coordinate.**

Pressure is defined as:

** p**_{(i,j,k) }= A_{k}P_{0}
+ B_{k}P_{s}(i,j)

where **p **is the pressure at a given level and latitude, longitude
grid point. The coefficients **A**, **B** and **P**_{0}
are constants. **P**_{s} is the model's current surface
pressure. **P**_{0} 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 mid-point
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 Hack *et al.* (1993).

Search page

Questions on these pages can be sent to...
erik@ucar.edu .

$Name: $ $Revision: 1.1 $ $Date: 2004/06/08 02:57:22 $ $Author: jmccaa $