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% $Name$ |
% $Name$ |
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\section{Flux-form momentum equations} |
\section{Flux-form momentum equations} |
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\label{sec:flux-form_momentum_eqautions} |
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The original finite volume model was based on the Eulerian flux form |
The original finite volume model was based on the Eulerian flux form |
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momentum equations. This is the default though the vector invariant |
momentum equations. This is the default though the vector invariant |
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\end{eqnarray} |
\end{eqnarray} |
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where the Coriolis parameters $f$ and $f'$ are defined: |
where the Coriolis parameters $f$ and $f'$ are defined: |
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\begin{eqnarray} |
\begin{eqnarray} |
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f & = & 2 \Omega \sin{\phi} \\ |
f & = & 2 \Omega \sin{\varphi} \\ |
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f' & = & 2 \Omega \cos{\phi} |
f' & = & 2 \Omega \cos{\varphi} |
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\end{eqnarray} |
\end{eqnarray} |
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where $\phi$ is geographic latitude when using spherical geometry, |
where $\varphi$ is geographic latitude when using spherical geometry, |
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otherwise the $\beta$-plane definition is used: |
otherwise the $\beta$-plane definition is used: |
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\begin{eqnarray} |
\begin{eqnarray} |
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f & = & f_o + \beta y \\ |
f & = & f_o + \beta y \\ |
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\marginpar{Need to change the default in code to match this} |
\marginpar{Need to change the default in code to match this} |
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where the subscripts on $f$ and $f'$ indicate evaluation of the |
where the subscripts on $f$ and $f'$ indicate evaluation of the |
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Coriolis parameters at the appropriate points in space. The above |
Coriolis parameters at the appropriate points in space. The above |
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discretization does {\em not} conserve anything, especially energy. An |
discretization does {\em not} conserve anything, especially energy and |
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option to recover this discretization has been retained for backward |
for historical reasons is the default for the code. A |
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compatibility testing (set run-time logical {\bf |
flag controls this discretization: set run-time logical {\bf |
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useNonconservingCoriolis} to {\em true} which otherwise defaults to |
useEnergyConservingCoriolis} to {\em true} which otherwise defaults to |
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{\em false}). |
{\em false}. |
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\fbox{ \begin{minipage}{4.75in} |
\fbox{ \begin{minipage}{4.75in} |
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{\em S/R MOM\_CDSCHEME} ({\em mom\_cdscheme.F}) |
{\em S/R MOM\_CDSCHEME} ({\em mom\_cdscheme.F}) |
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\subsection{Curvature metric terms} |
\subsection{Curvature metric terms} |
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The most commonly used coordinate system on the sphere is the |
The most commonly used coordinate system on the sphere is the |
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geographic system $(\lambda,\phi)$. The curvilinear nature of these |
geographic system $(\lambda,\varphi)$. The curvilinear nature of these |
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coordinates on the sphere lead to some ``metric'' terms in the |
coordinates on the sphere lead to some ``metric'' terms in the |
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component momentum equations. Under the thin-atmosphere and |
component momentum equations. Under the thin-atmosphere and |
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hydrostatic approximations these terms are discretized: |
hydrostatic approximations these terms are discretized: |
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\begin{eqnarray} |
\begin{eqnarray} |
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{\cal A}_w \Delta r_f h_w G_u^{metric} & = & |
{\cal A}_w \Delta r_f h_w G_u^{metric} & = & |
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\overline{ \frac{ \overline{u}^i }{a} \tan{\phi} {\cal A}_c \Delta r_f h_c \overline{ v }^j }^i \\ |
\overline{ \frac{ \overline{u}^i }{a} \tan{\varphi} {\cal A}_c \Delta r_f h_c \overline{ v }^j }^i \\ |
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{\cal A}_s \Delta r_f h_s G_v^{metric} & = & |
{\cal A}_s \Delta r_f h_s G_v^{metric} & = & |
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- \overline{ \frac{ \overline{u}^i }{a} \tan{\phi} {\cal A}_c \Delta r_f h_c \overline{ u }^i }^j \\ |
- \overline{ \frac{ \overline{u}^i }{a} \tan{\varphi} {\cal A}_c \Delta r_f h_c \overline{ u }^i }^j \\ |
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G_w^{metric} & = & 0 |
G_w^{metric} & = & 0 |
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\end{eqnarray} |
\end{eqnarray} |
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where $a$ is the radius of the planet (sphericity is assumed) or the |
where $a$ is the radius of the planet (sphericity is assumed) or the |
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radial distance of the particle (i.e. a function of height). It is |
radial distance of the particle (i.e. a function of height). It is |
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easy to see that this discretization satisfies all the properties of |
easy to see that this discretization satisfies all the properties of |
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the discrete Coriolis terms since the metric factor $\frac{u}{a} |
the discrete Coriolis terms since the metric factor $\frac{u}{a} |
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\tan{\phi}$ can be viewed as a modification of the vertical Coriolis |
\tan{\varphi}$ can be viewed as a modification of the vertical Coriolis |
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parameter: $f \rightarrow f+\frac{u}{a} \tan{\phi}$. |
parameter: $f \rightarrow f+\frac{u}{a} \tan{\varphi}$. |
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However, as for the Coriolis terms, a non-energy conserving form has |
However, as for the Coriolis terms, a non-energy conserving form has |
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exclusively been used to date: |
exclusively been used to date: |
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\begin{eqnarray} |
\begin{eqnarray} |
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G_u^{metric} & = & \frac{u \overline{v}^{ij} }{a} \tan{\phi} \\ |
G_u^{metric} & = & \frac{u \overline{v}^{ij} }{a} \tan{\varphi} \\ |
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G_v^{metric} & = & \frac{ \overline{u}^{ij} \overline{u}^{ij}}{a} \tan{\phi} |
G_v^{metric} & = & \frac{ \overline{u}^{ij} \overline{u}^{ij}}{a} \tan{\varphi} |
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\end{eqnarray} |
\end{eqnarray} |
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where $\tan{\phi}$ is evaluated at the $u$ and $v$ points |
where $\tan{\varphi}$ is evaluated at the $u$ and $v$ points |
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respectively. |
respectively. |
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\fbox{ \begin{minipage}{4.75in} |
\fbox{ \begin{minipage}{4.75in} |
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The lateral viscous stresses are discretized: |
The lateral viscous stresses are discretized: |
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\begin{eqnarray} |
\begin{eqnarray} |
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\tau_{11} & = & A_h c_{11\Delta}(\phi) \frac{1}{\Delta x_f} \delta_i u |
\tau_{11} & = & A_h c_{11\Delta}(\varphi) \frac{1}{\Delta x_f} \delta_i u |
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-A_4 c_{11\Delta^2}(\phi) \frac{1}{\Delta x_f} \delta_i \nabla^2 u \\ |
-A_4 c_{11\Delta^2}(\varphi) \frac{1}{\Delta x_f} \delta_i \nabla^2 u \\ |
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\tau_{12} & = & A_h c_{12\Delta}(\phi) \frac{1}{\Delta y_u} \delta_j u |
\tau_{12} & = & A_h c_{12\Delta}(\varphi) \frac{1}{\Delta y_u} \delta_j u |
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-A_4 c_{12\Delta^2}(\phi)\frac{1}{\Delta y_u} \delta_j \nabla^2 u \\ |
-A_4 c_{12\Delta^2}(\varphi)\frac{1}{\Delta y_u} \delta_j \nabla^2 u \\ |
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\tau_{21} & = & A_h c_{21\Delta}(\phi) \frac{1}{\Delta x_v} \delta_i v |
\tau_{21} & = & A_h c_{21\Delta}(\varphi) \frac{1}{\Delta x_v} \delta_i v |
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-A_4 c_{21\Delta^2}(\phi) \frac{1}{\Delta x_v} \delta_i \nabla^2 v \\ |
-A_4 c_{21\Delta^2}(\varphi) \frac{1}{\Delta x_v} \delta_i \nabla^2 v \\ |
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\tau_{22} & = & A_h c_{22\Delta}(\phi) \frac{1}{\Delta y_f} \delta_j v |
\tau_{22} & = & A_h c_{22\Delta}(\varphi) \frac{1}{\Delta y_f} \delta_j v |
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-A_4 c_{22\Delta^2}(\phi) \frac{1}{\Delta y_f} \delta_j \nabla^2 v |
-A_4 c_{22\Delta^2}(\varphi) \frac{1}{\Delta y_f} \delta_j \nabla^2 v |
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\end{eqnarray} |
\end{eqnarray} |
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where the non-dimensional factors $c_{lm\Delta^n}(\phi), \{l,m,n\} \in |
where the non-dimensional factors $c_{lm\Delta^n}(\varphi), \{l,m,n\} \in |
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\{1,2\}$ define the ``cosine'' scaling with latitude which can be |
\{1,2\}$ define the ``cosine'' scaling with latitude which can be |
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applied in various ad-hoc ways. For instance, $c_{11\Delta} = |
applied in various ad-hoc ways. For instance, $c_{11\Delta} = |
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c_{21\Delta} = (\cos{\phi})^{3/2}$, $c_{12\Delta}=c_{22\Delta}=0$ would |
c_{21\Delta} = (\cos{\varphi})^{3/2}$, $c_{12\Delta}=c_{22\Delta}=0$ would |
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represent the an-isotropic cosine scaling typically used on the |
represent the an-isotropic cosine scaling typically used on the |
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``lat-lon'' grid for Laplacian viscosity. |
``lat-lon'' grid for Laplacian viscosity. |
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\marginpar{Need to tidy up method for controlling this in code} |
\marginpar{Need to tidy up method for controlling this in code} |
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\begin{eqnarray} |
\begin{eqnarray} |
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G_u^{side-drag} & = & |
G_u^{side-drag} & = & |
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\frac{4}{\Delta z_f} \overline{ (1-h_\zeta) \frac{\Delta x_v}{\Delta y_u} }^j |
\frac{4}{\Delta z_f} \overline{ (1-h_\zeta) \frac{\Delta x_v}{\Delta y_u} }^j |
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\left( A_h c_{12\Delta}(\phi) u - A_4 c_{12\Delta^2}(\phi) \nabla^2 u \right) |
\left( A_h c_{12\Delta}(\varphi) u - A_4 c_{12\Delta^2}(\varphi) \nabla^2 u \right) |
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\\ |
\\ |
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G_v^{side-drag} & = & |
G_v^{side-drag} & = & |
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\frac{4}{\Delta z_f} \overline{ (1-h_\zeta) \frac{\Delta y_u}{\Delta x_v} }^i |
\frac{4}{\Delta z_f} \overline{ (1-h_\zeta) \frac{\Delta y_u}{\Delta x_v} }^i |
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\left( A_h c_{21\Delta}(\phi) v - A_4 c_{21\Delta^2}(\phi) \nabla^2 v \right) |
\left( A_h c_{21\Delta}(\varphi) v - A_4 c_{21\Delta^2}(\varphi) \nabla^2 v \right) |
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\end{eqnarray} |
\end{eqnarray} |
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In fact, the above discretization is not quite complete because it |
In fact, the above discretization is not quite complete because it |
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