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\section{Example: Four layer Baroclinic Ocean Gyre In Spherical Coordinates} |
\section{Example: Four layer Baroclinic Ocean Gyre In Spherical Coordinates} |
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\label{sec:eg-fourlayer} |
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%{\large May 2001} |
%{\large May 2001} |
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%\end{center} |
%\end{center} |
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\subsection{Introduction} |
This document describes an example experiment using MITgcm |
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to simulate a baroclinic ocean gyre in spherical |
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This document describes the second example MITgcm experiment. The first |
polar coordinates. The barotropic |
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example experiment ilustrated how to configure the code for a single layer |
example experiment in section \ref{sec:eg-baro} |
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ilustrated how to configure the code for a single layer |
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simulation in a cartesian grid. In this example a similar physical problem |
simulation in a cartesian grid. In this example a similar physical problem |
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is simulated, but the code is now configured |
is simulated, but the code is now configured |
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for four layers and in a spherical polar coordinate system. |
for four layers and in a spherical polar coordinate system. |
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In this experiment the model is configured to represent a mid-latitude |
In this experiment the model is configured to represent a mid-latitude |
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enclosed sector of fluid on a sphere, $60^{\circ} \times 60^{\circ}$ in |
enclosed sector of fluid on a sphere, $60^{\circ} \times 60^{\circ}$ in |
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lateral extent. The fluid is $2$~km deep and is forced |
lateral extent. The fluid is $2$~km deep and is forced |
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by a constant in time zonal wind stress, $\tau_x$, that varies sinusoidally |
by a constant in time zonal wind stress, $\tau_{\lambda}$, that varies |
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in the north-south direction. Topologically the simulated |
sinusoidally in the north-south direction. Topologically the simulated |
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domain is a sector on a sphere and the coriolis parameter, $f$, is defined |
domain is a sector on a sphere and the coriolis parameter, $f$, is defined |
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according to latitude, $\phi$ |
according to latitude, $\varphi$ |
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|
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\begin{equation} |
\begin{equation} |
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\label{EQ:fcori} |
\label{EQ:fcori} |
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f(\phi) = 2 \Omega \sin( \phi ) |
f(\varphi) = 2 \Omega \sin( \varphi ) |
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\end{equation} |
\end{equation} |
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\noindent with the rotation rate, $\Omega$ set to $\frac{2 \pi}{86400s}$. |
\noindent with the rotation rate, $\Omega$ set to $\frac{2 \pi}{86400s}$. |
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\begin{equation} |
\begin{equation} |
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\label{EQ:taux} |
\label{EQ:taux} |
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\tau_x(\phi) = \tau_{0}\sin(\pi \frac{\phi}{L_{\phi}}) |
\tau_{\lambda}(\varphi) = \tau_{0}\sin(\pi \frac{\varphi}{L_{\varphi}}) |
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\end{equation} |
\end{equation} |
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\noindent where $L_{\phi}$ is the lateral domain extent ($60^{\circ}$) and |
\noindent where $L_{\varphi}$ is the lateral domain extent ($60^{\circ}$) and |
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$\tau_0$ is set to $0.1N m^{-2}$. |
$\tau_0$ is set to $0.1N m^{-2}$. |
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\\ |
\\ |
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Figure \ref{FIG:simulation_config} |
Figure \ref{FIG:simulation_config} |
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summarises the configuration simulated. |
summarises the configuration simulated. |
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In contrast to example (1) \cite{baro_gyre_case_study}, the current |
In contrast to the example in section \ref{sec:eg-baro}, the |
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experiment simulates a spherical polar domain. However, as indicated |
current experiment simulates a spherical polar domain. As indicated |
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by the axes in the lower left of the figure the model code works internally |
by the axes in the lower left of the figure the model code works internally |
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in a locally orthoganal coordinate $(x,y,z)$. In the remainder of this |
in a locally orthoganal coordinate $(x,y,z)$. For this experiment description |
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document the local coordinate $(x,y,z)$ will be adopted. |
of this document the local orthogonal model coordinate $(x,y,z)$ is synonomous |
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with the spherical polar coordinate shown in figure |
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\ref{fig:spherical-polar-coord} |
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\\ |
\\ |
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The experiment has four levels in the vertical, each of equal thickness, |
The experiment has four levels in the vertical, each of equal thickness, |
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\noindent with $\rho_{0}=999.8\,{\rm kg\,m}^{-3}$ and |
\noindent with $\rho_{0}=999.8\,{\rm kg\,m}^{-3}$ and |
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$\alpha_{\theta}=2\times10^{-4}\,{\rm degrees}^{-1} $. Integrated forward in |
$\alpha_{\theta}=2\times10^{-4}\,{\rm degrees}^{-1} $. Integrated forward in |
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this configuration the model state variable {\bf theta} is synonomous with |
this configuration the model state variable {\bf theta} is equivalent to |
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either in-situ temperature, $T$, or potential temperature, $\theta$. For |
either in-situ temperature, $T$, or potential temperature, $\theta$. For |
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consistency with later examples, in which the equation of state is |
consistency with later examples, in which the equation of state is |
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non-linear, we use $\theta$ to represent temperature here. This is |
non-linear, we use $\theta$ to represent temperature here. This is |
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the quantity that is carried in the model core equations. |
the quantity that is carried in the model core equations. |
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\begin{figure} |
\begin{figure} |
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\centerline{ |
\begin{center} |
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\resizebox{7.5in}{5.5in}{ |
\resizebox{7.5in}{5.5in}{ |
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\includegraphics*[0.2in,0.7in][10.5in,10.5in] |
\includegraphics*[0.2in,0.7in][10.5in,10.5in] |
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{part3/case_studies/fourlayer_gyre/simulation_config.eps} } |
{part3/case_studies/fourlayer_gyre/simulation_config.eps} } |
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} |
\end{center} |
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\caption{Schematic of simulation domain and wind-stress forcing function |
\caption{Schematic of simulation domain and wind-stress forcing function |
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for the four-layer gyre numerical experiment. The domain is enclosed by solid |
for the four-layer gyre numerical experiment. The domain is enclosed by solid |
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walls at $0^{\circ}$~E, $60^{\circ}$~E, $0^{\circ}$~N and $60^{\circ}$~N. |
walls at $0^{\circ}$~E, $60^{\circ}$~E, $0^{\circ}$~N and $60^{\circ}$~N. |
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In the four-layer case an initial temperature stratification is |
An initial stratification is |
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imposed by setting the potential temperature, $\theta$, in each layer. |
imposed by setting the potential temperature, $\theta$, in each layer. |
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The vertical spacing, $\Delta z$, is constant and equal to $500$m. |
The vertical spacing, $\Delta z$, is constant and equal to $500$m. |
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} |
} |
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\label{FIG:simulation_config} |
\label{FIG:simulation_config} |
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\end{figure} |
\end{figure} |
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\subsection{Discrete Numerical Configuration} |
\subsection{Equations solved} |
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The model is configured in hydrostatic form. The domain is discretised with |
The implicit free surface {\bf HPE} form of the |
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a uniform grid spacing in latitude and longitude |
equations described in Marshall et. al \cite{Marshall97a} is |
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$\Delta x=\Delta y=1^{\circ}$, so |
employed. The flow is three-dimensional with just temperature, $\theta$, as |
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that there are sixty grid cells in the $x$ and $y$ directions. Vertically the |
an active tracer. The equation of state is linear. |
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model is configured with a four layers with constant depth, |
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$\Delta z$, of $500$~m. |
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The implicit free surface form of the |
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pressure equation described in Marshall et. al \cite{Marshall97a} is |
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employed. |
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A horizontal laplacian operator $\nabla_{h}^2$ provides viscous |
A horizontal laplacian operator $\nabla_{h}^2$ provides viscous |
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dissipation. The wind-stress momentum input is added to the momentum equation |
dissipation and provides a diffusive sub-grid scale closure for the |
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for the ``zonal flow'', $u$. Other terms in the model |
temperature equation. A wind-stress momentum forcing is added to the momentum |
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equation for the zonal flow, $u$. Other terms in the model |
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are explicitly switched off for this experiement configuration (see section |
are explicitly switched off for this experiement configuration (see section |
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\ref{SEC:code_config} ), yielding an active set of equations solved in this |
\ref{SEC:eg_fourl_code_config} ). This yields an active set of equations |
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configuration as follows |
solved in this configuration, written in spherical polar coordinates as |
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follows |
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\begin{eqnarray} |
\begin{eqnarray} |
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\label{EQ:model_equations} |
\label{EQ:model_equations} |
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\frac{Du}{Dt} - fv + |
\frac{Du}{Dt} - fv + |
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\frac{1}{\rho}\frac{\partial p^{'}}{\partial x} - |
\frac{1}{\rho}\frac{\partial p^{\prime}}{\partial \lambda} - |
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A_{h}\nabla_{h}^2u - A_{z}\frac{\partial^{2}u}{\partial z^{2}} |
A_{h}\nabla_{h}^2u - A_{z}\frac{\partial^{2}u}{\partial z^{2}} |
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& = & |
& = & |
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\cal{F} |
\cal{F}_{\lambda} |
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\\ |
\\ |
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\frac{Dv}{Dt} + fu + |
\frac{Dv}{Dt} + fu + |
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\frac{1}{\rho}\frac{\partial p^{'}}{\partial y} - |
\frac{1}{\rho}\frac{\partial p^{\prime}}{\partial \varphi} - |
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A_{h}\nabla_{h}^2v - A_{z}\frac{\partial^{2}v}{\partial z^{2}} |
A_{h}\nabla_{h}^2v - A_{z}\frac{\partial^{2}v}{\partial z^{2}} |
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& = & |
& = & |
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0 |
0 |
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\\ |
\\ |
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\frac{\partial \eta}{\partial t} + \nabla_{h}\cdot \vec{u} |
\frac{\partial \eta}{\partial t} + \frac{\partial H \widehat{u}}{\partial \lambda} + |
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\frac{\partial H \widehat{v}}{\partial \varphi} |
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&=& |
&=& |
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0 |
0 |
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\label{eq:fourl_example_continuity} |
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\\ |
\\ |
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\frac{D\theta}{Dt} - |
\frac{D\theta}{Dt} - |
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K_{h}\nabla_{h}^2\theta - K_{z}\frac{\partial^{2}\theta}{\partial z^{2}} |
K_{h}\nabla_{h}^2\theta - K_{z}\frac{\partial^{2}\theta}{\partial z^{2}} |
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& = & |
& = & |
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0 |
0 |
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\label{eq:eg_fourl_theta} |
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\\ |
\\ |
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g\rho_{0} \eta + \int^{0}_{-z}\rho^{'} dz & = & p^{'} |
p^{\prime} & = & |
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g\rho_{0} \eta + \int^{0}_{-z}\rho^{\prime} dz |
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\\ |
\\ |
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{\cal F} |_{s} & = & \frac{\tau_{x}}{\rho_{0}\Delta z_{s}} |
\rho^{\prime} & = &- \alpha_{\theta}\rho_{0}\theta^{\prime} |
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\\ |
\\ |
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{\cal F} |_{i} & = & 0 |
{\cal F}_{\lambda} |_{s} & = & \frac{\tau_{\lambda}}{\rho_{0}\Delta z_{s}} |
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\\ |
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{\cal F}_{\lambda} |_{i} & = & 0 |
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\end{eqnarray} |
\end{eqnarray} |
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\noindent where $u$ and $v$ are the $x$ and $y$ components of the |
\noindent where $u$ and $v$ are the components of the horizontal |
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flow vector $\vec{u}$. The suffices ${s},{i}$ indicate surface and |
flow vector $\vec{u}$ on the sphere ($u=\dot{\lambda},v=\dot{\varphi}$). |
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interior model levels respectively. As described in |
The terms $H\widehat{u}$ and $H\widehat{v}$ are the components of the vertical |
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MITgcm Numerical Solution Procedure \cite{MITgcm_Numerical_Scheme}, the time |
integral term given in equation \ref{eq:free-surface} and |
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evolution of potential temperature, $\theta$, equation is solved prognostically. |
explained in more detail in section \ref{sect:pressure-method-linear-backward}. |
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The total pressure, $p$, is diagnosed by summing pressure due to surface |
However, for the problem presented here, the continuity relation (equation |
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elevation $\eta$ and the hydrostatic pressure. |
\ref{eq:fourl_example_continuity}) differs from the general form given |
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\\ |
in section \ref{sect:pressure-method-linear-backward}, |
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equation \ref{eq:linear-free-surface=P-E+R}, because the source terms |
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${\cal P}-{\cal E}+{\cal R}$ |
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are all $0$. |
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The pressure field, $p^{\prime}$, is separated into a barotropic part |
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due to variations in sea-surface height, $\eta$, and a hydrostatic |
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part due to variations in density, $\rho^{\prime}$, integrated |
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through the water column. |
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The suffices ${s},{i}$ indicate surface and interior of the domain. |
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The windstress forcing, ${\cal F}_{\lambda}$, is applied in the surface layer |
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by a source term in the zonal momentum equation. In the ocean interior |
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this term is zero. |
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In the momentum equations |
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lateral and vertical boundary conditions for the $\nabla_{h}^{2}$ |
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and $\frac{\partial^{2}}{\partial z^{2}}$ operators are specified |
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when the numerical simulation is run - see section |
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\ref{SEC:eg_fourl_code_config}. For temperature |
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the boundary condition is ``zero-flux'' |
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e.g. $\frac{\partial \theta}{\partial \varphi}= |
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\frac{\partial \theta}{\partial \lambda}=\frac{\partial \theta}{\partial z}=0$. |
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\subsection{Discrete Numerical Configuration} |
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The domain is discretised with |
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a uniform grid spacing in latitude and longitude |
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$\Delta \lambda=\Delta \varphi=1^{\circ}$, so |
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that there are sixty grid cells in the zonal and meridional directions. |
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Vertically the |
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model is configured with four layers with constant depth, |
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$\Delta z$, of $500$~m. The internal, locally orthogonal, model coordinate |
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variables $x$ and $y$ are initialised from the values of |
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$\lambda$, $\varphi$, $\Delta \lambda$ and $\Delta \varphi$ in |
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radians according to |
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\begin{eqnarray} |
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x=r\cos(\varphi)\lambda,~\Delta x & = &r\cos(\varphi)\Delta \lambda \\ |
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y=r\varphi,~\Delta y &= &r\Delta \varphi |
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\end{eqnarray} |
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The procedure for generating a set of internal grid variables from a |
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spherical polar grid specification is discussed in section |
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\ref{sec:spatial_discrete_horizontal_grid}. |
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\noindent\fbox{ \begin{minipage}{5.5in} |
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{\em S/R INI\_SPHERICAL\_POLAR\_GRID} ({\em |
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model/src/ini\_spherical\_polar\_grid.F}) |
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$A_c$, $A_\zeta$, $A_w$, $A_s$: {\bf rAc}, {\bf rAz}, {\bf rAw}, {\bf rAs} |
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({\em GRID.h}) |
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$\Delta x_g$, $\Delta y_g$: {\bf DXg}, {\bf DYg} ({\em GRID.h}) |
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$\Delta x_c$, $\Delta y_c$: {\bf DXc}, {\bf DYc} ({\em GRID.h}) |
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$\Delta x_f$, $\Delta y_f$: {\bf DXf}, {\bf DYf} ({\em GRID.h}) |
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$\Delta x_v$, $\Delta y_u$: {\bf DXv}, {\bf DYu} ({\em GRID.h}) |
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\end{minipage} }\\ |
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As described in \ref{sec:tracer_equations}, the time evolution of potential |
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temperature, |
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$\theta$, (equation \ref{eq:eg_fourl_theta}) |
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is evaluated prognostically. The centered second-order scheme with |
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Adams-Bashforth time stepping described in section |
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\ref{sec:tracer_equations_abII} is used to step forward the temperature |
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equation. The pressure forces that drive the fluid motions, ( |
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$\frac{\partial p^{'}}{\partial \lambda}$ and $\frac{\partial p^{'}}{\partial \varphi}$), are found by summing pressure due to surface |
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elevation $\eta$ and the hydrostatic pressure. The hydrostatic part of the |
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pressure is evaluated explicitly by integrating density. The sea-surface |
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height, $\eta$, is solved for implicitly as described in section |
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\ref{sect:pressure-method-linear-backward}. |
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\subsubsection{Numerical Stability Criteria} |
\subsubsection{Numerical Stability Criteria} |
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\noindent of $\approx 100$km. This is greater than the model |
\noindent of $\approx 100$km. This is greater than the model |
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resolution in mid-latitudes $\Delta x$, ensuring that the frictional |
resolution in mid-latitudes $\Delta x$, ensuring that the frictional |
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boundary layer is well resolved. |
boundary layer is well resolved. |
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\\ |
\\ |
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\noindent The model is stepped forward with a |
\noindent The model is stepped forward with a |
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stability limit of 0.25. |
stability limit of 0.25. |
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\subsection{Code Configuration} |
\subsection{Code Configuration} |
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\label{SEC:code_config} |
\label{SEC:eg_fourl_code_config} |
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The model configuration for this experiment resides under the |
The model configuration for this experiment resides under the |
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directory {\it verification/exp1/}. The experiment files |
directory {\it verification/exp1/}. The experiment files |
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