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\subsection{Gridalt - Alternate Grid Package} |
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\label{sec:pkg:gridalt} |
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\begin{rawhtml} |
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<!-- CMIREDIR:package_gridalt: --> |
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\end{rawhtml} |
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\subsubsection {Introduction} |
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The gridalt package was developed to allow the high end atmospheric physics |
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(fizhi) physics to be run on a separate grid from the hydrodynamics. The package |
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could (with some user modification) be used in conjunction with other packages |
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or for other calculations within the GCM. For the case of the atmospheric |
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physics, a modified $p^*$ coordinate, which adds additional levels between |
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the lower levels of the existing $p^*$ grid (and perhaps between the levels near |
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the tropopause as well), is implemented. The vertical discretization is |
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different for each grid point, although it consist of the same number of |
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levels. This is illustrated as follows: |
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\begin{figure}[htbp] |
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\vspace*{-0.4in} |
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\begin{center} |
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\includegraphics[height=2.4in]{part6/vertical.eps} |
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\end{center} |
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\end{figure} |
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|
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\vspace*{-0.5in} |
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In addition to computing the physical forcing terms of the momentum, |
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thermodynamic and humidity equations on the modified (higher resolution) |
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grid, the higher resolution structure of the atmosphere (the boundary |
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layer) is retained between calculations. This neccessitates a second |
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set of evolution equations for the atmospheric state variables on the |
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modified grid. If the equations for the evolution of the state |
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on $p^*$ can be expressed as: |
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\[ |
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\left . {\partial U \over {\partial t}} \right |_{p^*}^{total} = |
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\left . {\partial U \over {\partial t}} \right |_{p^*}^{dynamics} + |
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\left . {\partial U \over {\partial t}} \right |_{p^*}^{physics} |
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\] |
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where the physics forcing terms on $p^*$ have been computed from a |
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mapping from the modified grid, then an additional set of equations |
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to govern the evolution of $U$ on the modified grid are written: |
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\[ |
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\left . {\partial U \over {\partial t}} \right |_{p^{*m}}^{total} = |
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\left . {\partial U \over {\partial t}} \right |_{p^{*m}}^{dynamics} + |
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\left . {\partial U \over {\partial t}} \right |_{p^{*m}}^{physics} + |
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\gamma ({\left . U \right |_{p^*}} - {\left . U \right |_{p^{*m}}}) |
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\] |
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where $p^{*m}$ refers to the modified higher resolution grid, and |
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the dynamics forcing terms have been mapped from the $p^*$ space. |
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The last term on the RHS is a relaxation term, meant to constrain |
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the state variables on the modified vertical grid to `track' the |
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state variables on the $p^*$ grid on some time scale, $\gamma$. |
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|
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\subsubsection {Key subroutines, parameters and files } |
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\subsubsection {Dos and donts} |
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In the context of a Held-Suarez type of model experiment (located |
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in the fizhi-gridalt-hs verification experiment) with |
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topography, the forcing terms which represent the physics are computed on |
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the modified grid. The forcing terms are computed as functions of the |
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state variables on the modified grid. The tendencies are then interpolated |
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to the standard grid |
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|
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\subsubsection {Gridalt Reference} |