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1 \subsection{Gridalt - Alternate Grid Package}
2 \label{sec:pkg:gridalt}
3 \begin{rawhtml}
4 <!-- CMIREDIR:package_gridalt: -->
5 \end{rawhtml}
6
7 \subsubsection {Introduction}
8
9 The gridalt package was developed to allow the high end atmospheric physics
10 (fizhi) physics to be run on a separate grid from the hydrodynamics. The package
11 could (with some user modification) be used in conjunction with other packages
12 or for other calculations within the GCM. For the case of the atmospheric
13 physics, a modified $p^*$ coordinate, which adds additional levels between
14 the lower levels of the existing $p^*$ grid (and perhaps between the levels near
15 the tropopause as well), is implemented. The vertical discretization is
16 different for each grid point, although it consist of the same number of
17 levels. This is illustrated as follows:
18 \begin{figure}[htbp]
19 \vspace*{-0.4in}
20 \begin{center}
21 \includegraphics[height=2.4in]{part6/vertical.eps}
22 \end{center}
23 \end{figure}
24
25 \vspace*{-0.5in}
26 In addition to computing the physical forcing terms of the momentum,
27 thermodynamic and humidity equations on the modified (higher resolution)
28 grid, the higher resolution structure of the atmosphere (the boundary
29 layer) is retained between calculations. This neccessitates a second
30 set of evolution equations for the atmospheric state variables on the
31 modified grid. If the equations for the evolution of the state
32 on $p^*$ can be expressed as:
33 \[
34 \left . {\partial U \over {\partial t}} \right |_{p^*}^{total} =
35 \left . {\partial U \over {\partial t}} \right |_{p^*}^{dynamics} +
36 \left . {\partial U \over {\partial t}} \right |_{p^*}^{physics}
37 \]
38 where the physics forcing terms on $p^*$ have been computed from a
39 mapping from the modified grid, then an additional set of equations
40 to govern the evolution of $U$ on the modified grid are written:
41 \[
42 \left . {\partial U \over {\partial t}} \right |_{p^{*m}}^{total} =
43 \left . {\partial U \over {\partial t}} \right |_{p^{*m}}^{dynamics} +
44 \left . {\partial U \over {\partial t}} \right |_{p^{*m}}^{physics} +
45 \gamma ({\left . U \right |_{p^*}} - {\left . U \right |_{p^{*m}}})
46 \]
47 where $p^{*m}$ refers to the modified higher resolution grid, and
48 the dynamics forcing terms have been mapped from the $p^*$ space.
49 The last term on the RHS is a relaxation term, meant to constrain
50 the state variables on the modified vertical grid to `track' the
51 state variables on the $p^*$ grid on some time scale, $\gamma$.
52
53 \subsubsection {Key subroutines, parameters and files }
54
55 \subsubsection {Dos and donts}
56
57 In the context of a Held-Suarez type of model experiment (located
58 in the fizhi-gridalt-hs verification experiment) with
59 topography, the forcing terms which represent the physics are computed on
60 the modified grid. The forcing terms are computed as functions of the
61 state variables on the modified grid. The tendencies are then interpolated
62 to the standard grid
63
64 \subsubsection {Gridalt Reference}

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