s_examples/advection_in_gyre/adv_gyre.tex

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\section[Gyre Advection Example]{Ocean Gyre Advection Schemes}
\label{www:tutorials}
\label{sect:eg-adv-gyre}
\begin{rawhtml}
<!-- CMIREDIR:eg-adv-gyre: -->
\end{rawhtml}
\begin{center} 
(in directory: {\it verification/tutorial\_advection\_in\_gyre/})
\end{center}

Author: Oliver Jahn and Chris Hill


This set of examples is based on the barotropic and baroclinic gyre MITgcm configurations,
that are described in the tutorial sections \ref{sect:eg-baro} and \ref{sect:eg-fourlayer}. 
The examples in this section explain how to introduce a passive tracer into the flow 
field of the barotropic and baroclinic gyre setups and looks at how the time evolution
of the passive tracer depends on the advection or transport scheme that is selected 
for the tracer. 

Passive tracers are useful in many numerical experiments. In some cases tracers are
used to track flow pathways, for example in \cite{Dutay02} a passive tracer is used
to track pathways of CFC-11 in 13 global ocean models, using a numerical
configuration similar to the example described in section \ref{sect:eg-offline-cfc}).
In other cases tracers are used as a way
to infer bulk mixing coefficients for a turbulent flow field, for example in 
\cite{marsh06} a tracer is used to infer eddy mixing coefficients in the
Antarctic Circumpolar Current region. In biogeochemical and ecological simulations large numbers 
of tracers are used that carry the concentrations of biological nutrients and concentrations of 
biological species, for example in ....
When using tracers for these and other purposes it is useful to have a feel for the role
that the advection scheme employed plays in determining properties of the tracer distribution.
In particular, in a discrete numerical model tracer advection only approximates the 
continuum behavior in space and time and different advection schemes introduce diferent 
approximations so that the resulting tracer distributions vary. In the following 
text we illustrate how
to use the different advection schemes available in MITgcm here, and discuss which properties 
are well represented by each one. The advection schemes selections also apply to active
tracers (e.g. $T$ and $S$) and the character of the schemes also affect their distributions
and behavior.

\subsection{Advection and tracer transport}

In general, the tracer problem we want to solve can be written

\begin{equation}
\label{EQ:eg-adv-gyre-generic-tracer}
\frac{\partial C}{partial t} = -U \cdot \nabla C + S
\end{equation}

where $C$ is the tracer concentration in a model cell, $U$ is the model three-dimensional
flow field ( $U=(u,v,w)$ ). In (\ref{EQ:eg-adv-gyre-generic-tracer}) $S$ represents source, sink 
and tendency terms not associated with advective transport. Example of terms in $S$ include
(i) air-sea fluxes for a dissolved gas, (ii) biological grazing and growth terms (for a 
biogeochemical problem) or (iii) convective mixing and other sub-grid parameterizations of 
mixing. In this section we are primarily concerned with 
\begin{enumerate}
\item how to introduce the tracer term, $C$, into an integration
\item the different discretized forms of 
the $-U \cdot \nabla C$ term that are available
\end{enumerate}


\subsection{Introducing a tracer into the flow}

 The MITgcm ptracers package (see section \ref{sec:pkg:ptracers} for a more complete discussion
of the ptracers package and section \ref{sec:pkg:using} for a general introduction to MITgcm 
packages) provides pre-coded support for a simple passive tracer with an initial
distribution at simulation time $t=0$ of $C_0(x,y,z)$. The steps required to use this capability
are 
\begin{enumerate}
\item{\bf Activating the ptracers package.} This simply requires adding the line {\tt ptracers} to
the {\tt packages.conf} file in the {\it code/} directory for the experiment. 
\item{\bf Setting an initial tracer distribution.} 
\end{enumerate}

Once the two steps above are complete we can proceed to examine how the tracer we have created is
carried by the flow field and what properties of the tracer distribution are preserved under
different advection schemes.

\subsection{Selecting an advection scheme}

- flags in data and data.ptracers

- overlap width

- CPP GAD\_ALLOW\_SOM\_ADVECT required for SOM case

\subsection{Comparison of different advection schemes}

\begin{enumerate}
\item{Conservation}
\item{Dispersion}
\item{Diffusion}
\item{Positive definite}
\end{enumerate}

\input{s_examples/advection_in_gyre/adv_gyre_figure.tex}

\begin{figure}
\begin{center}
 \includegraphics*[width=\textwidth]{s_examples/advection_in_gyre/stats.eps}
\end{center}
\caption{Maxima, minima and standard deviation (from left) as a function of time (in months)
for the gyre circulation experiment from figure~\ref{fig:adv-gyre-all}.}
\label{fig:adv-gyre-stats}
\end{figure}

\subsection{Code and Parameters files for this tutorial}

The code and parameters for the experiments can be found in the MITgcm example experiments 
directory {\it verification/tutorial\_advection\_in\_gyre/}.


1	% $Header: /u/gcmpack/manual/s_examples/advection_in_gyre/adv_gyre.tex,v 1.14 2009/05/01 17:18:14 jmc Exp $
2	% $Name: $
3
4	\bodytext{bgcolor="#FFFFFFFF"}
5
6
7	\section[Gyre Advection Example]{Ocean Gyre Advection Schemes}
8	\label{www:tutorials}
9	\label{sect:eg-adv-gyre}
10	\begin{rawhtml}
11	<!-- CMIREDIR:eg-adv-gyre: -->
12	\end{rawhtml}
13	\begin{center}
14	(in directory: {\it verification/tutorial\_advection\_in\_gyre/})
15	\end{center}
16
17	Author: Oliver Jahn and Chris Hill
18
19
20
21	This set of examples is based on the barotropic and baroclinic gyre MITgcm configurations,
22	that are described in the tutorial sections \ref{sect:eg-baro} and \ref{sect:eg-fourlayer}.
23	The examples in this section explain how to introduce a passive tracer into the flow
24	field of the barotropic and baroclinic gyre setups and looks at how the time evolution
25	of the passive tracer depends on the advection or transport scheme that is selected
26	for the tracer.
27
28	Passive tracers are useful in many numerical experiments. In some cases tracers are
29	used to track flow pathways, for example in \cite{Dutay02} a passive tracer is used
30	to track pathways of CFC-11 in 13 global ocean models, using a numerical
31	configuration similar to the example described in section \ref{sect:eg-offline-cfc}).
32	In other cases tracers are used as a way
33	to infer bulk mixing coefficients for a turbulent flow field, for example in
34	\cite{marsh06} a tracer is used to infer eddy mixing coefficients in the
35	Antarctic Circumpolar Current region. In biogeochemical and ecological simulations large numbers
36	of tracers are used that carry the concentrations of biological nutrients and concentrations of
37	biological species, for example in ....
38	When using tracers for these and other purposes it is useful to have a feel for the role
39	that the advection scheme employed plays in determining properties of the tracer distribution.
40	In particular, in a discrete numerical model tracer advection only approximates the
41	continuum behavior in space and time and different advection schemes introduce diferent
42	approximations so that the resulting tracer distributions vary. In the following
43	text we illustrate how
44	to use the different advection schemes available in MITgcm here, and discuss which properties
45	are well represented by each one. The advection schemes selections also apply to active
46	tracers (e.g. $T$ and $S$) and the character of the schemes also affect their distributions
47	and behavior.
48
49	\subsection{Advection and tracer transport}
50
51	In general, the tracer problem we want to solve can be written
52
53	\begin{equation}
54	\label{EQ:eg-adv-gyre-generic-tracer}
55	\frac{\partial C}{partial t} = -U \cdot \nabla C + S
56	\end{equation}
57
58	where $C$ is the tracer concentration in a model cell, $U$ is the model three-dimensional
59	flow field ( $U=(u,v,w)$ ). In (\ref{EQ:eg-adv-gyre-generic-tracer}) $S$ represents source, sink
60	and tendency terms not associated with advective transport. Example of terms in $S$ include
61	(i) air-sea fluxes for a dissolved gas, (ii) biological grazing and growth terms (for a
62	biogeochemical problem) or (iii) convective mixing and other sub-grid parameterizations of
63	mixing. In this section we are primarily concerned with
64	\begin{enumerate}
65	\item how to introduce the tracer term, $C$, into an integration
66	\item the different discretized forms of
67	the $-U \cdot \nabla C$ term that are available
68	\end{enumerate}
69
70
71	\subsection{Introducing a tracer into the flow}
72
73	The MITgcm ptracers package (see section \ref{sec:pkg:ptracers} for a more complete discussion
74	of the ptracers package and section \ref{sec:pkg:using} for a general introduction to MITgcm
75	packages) provides pre-coded support for a simple passive tracer with an initial
76	distribution at simulation time $t=0$ of $C_0(x,y,z)$. The steps required to use this capability
77	are
78	\begin{enumerate}
79	\item{\bf Activating the ptracers package.} This simply requires adding the line {\tt ptracers} to
80	the {\tt packages.conf} file in the {\it code/} directory for the experiment.
81	\item{\bf Setting an initial tracer distribution.}
82	\end{enumerate}
83
84	Once the two steps above are complete we can proceed to examine how the tracer we have created is
85	carried by the flow field and what properties of the tracer distribution are preserved under
86	different advection schemes.
87
88	\subsection{Selecting an advection scheme}
89
90	- flags in data and data.ptracers
91
92	- overlap width
93
94	- CPP GAD\_ALLOW\_SOM\_ADVECT required for SOM case
95
96	\subsection{Comparison of different advection schemes}
97
98	\begin{enumerate}
99	\item{Conservation}
100	\item{Dispersion}
101	\item{Diffusion}
102	\item{Positive definite}
103	\end{enumerate}
104
105	\input{s_examples/advection_in_gyre/adv_gyre_figure.tex}
106
107	\begin{figure}
108	\begin{center}
109	\includegraphics*[width=\textwidth]{s_examples/advection_in_gyre/stats.eps}
110	\end{center}
111	\caption{Maxima, minima and standard deviation (from left) as a function of time (in months)
112	for the gyre circulation experiment from figure~\ref{fig:adv-gyre-all}.}
113	\label{fig:adv-gyre-stats}
114	\end{figure}
115
116	\subsection{Code and Parameters files for this tutorial}
117
118	The code and parameters for the experiments can be found in the MITgcm example experiments
119	directory {\it verification/tutorial\_advection\_in\_gyre/}.
120
121
122
123
124
125
126