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NEMO Workspace
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2d52af2e
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4 months ago
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Sibylle Techene
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@@ -392,163 +392,6 @@ the change of KE due to the gradient of KE (see \autoref{apdx:INVARIANTS}).
\right
.
\]
%% =================================================================================================
\section
{
Coriolis and advection: flux form
}
\label
{
sec:DYN
_
adv
_
cor
_
flux
}
Options are defined through the
\nam
{
dyn
_
adv
}{
dyn
\_
adv
}
namelist variables.
In the flux form (as in the vector invariant form),
the Coriolis and momentum advection terms are evaluated using either a leapfrog scheme or a RK3 scheme.
In the leapfrog case the velocity appearing in these expressions is centred in time (
\textit
{
now
}
velocity).
In the RK3 case the velocity appearing in these expressions is forward in time (
\textit
{
before
}
velocity) at stage 1,
it is is centred in time (
\textit
{
now
}
velocity) at stage 2 and 3.
At the lateral boundaries either free slip,
no slip or partial slip boundary conditions are applied following
\autoref
{
chap:LBC
}
.
%% =================================================================================================
\subsection
[Coriolis plus curvature metric terms (\textit{dynvor.F90})]
{
Coriolis plus curvature metric terms (
\protect\mdl
{
dynvor
}
)
}
\label
{
subsec:DYN
_
cor
_
flux
}
In flux form, the vorticity term reduces to a Coriolis term in which the Coriolis parameter has been modified to account for the "metric" term.
This altered Coriolis parameter is thus discretised at
$
f
$
-points.
It is given by:
\begin{aligned*}
% \label{eq:DYN_cor_metric}
f+
\frac
{
1
}{
e
_
1 e
_
2
}
\left
(
{
v
\frac
{
\partial
e
_
2
}{
\partial
i
}
- u
\frac
{
\partial
e
_
1
}{
\partial
j
}}
\right
)
\equiv
f +
\frac
{
1
}{
e
_{
1f
}
e
_{
2f
}
}
\left
(
\overline
v
^{
i+1/2
}
\delta
_{
i+1/2
}
\left
[ {e_{2u} } \right]
-
\overline
u
^{
j+1/2
}
\delta
_{
j+1/2
}
\left
[ {e_{1u} } \right]
\right
)
\end{aligned*}
% energy conserving scheme at T-point
%% =================================================================================================
\subsubsection
[Energy conserving scheme (\forcode{ln_dynvor_enT})]
{
Energy conserving scheme (
\protect\np
{
ln
_
dynvor
_
enT
}{
ln
\_
dynvor
\_
enT
}
)
}
\label
{
subsec:DYN
_
vor
_
enT
}
The kinetic energy conserving scheme at T-point (ENT scheme) conserves the global kinetic energy but not the global enstrophy.
It is given by:
\begin{equation}
\label
{
eq:DYN
_
vor
_
enT
}
\left\{
\begin{aligned}
&
+
\frac
{
1
}{
e
_{
1u
}
\,
e
_{
2u
}
\,
e
_{
3u
}}
\,
\overline
{
\left
( f
^
T +
\overline
{
\overline
{
\zeta
}}^{
\,
i,j
}
\right
)
e
_{
1t
}
\,
e
_{
2t
}
\,
e
_{
3t
}
\,
\overline
{
v
}^{
\,
j
}}^{
\,
i+1/2
}
\\
&
-
\frac
{
1
}{
e
_{
1v
}
\,
e
_{
2v
}
\,
e
_{
3v
}}
\,
\overline
{
\left
( f
^
T +
\overline
{
\overline
{
\zeta
}}^{
\,
i,j
}
\right
)
e
_{
1t
}
\,
e
_{
2t
}
\,
e
_{
3t
}
\,
\overline
{
u
}^{
\,
i
}}^{
\,
j+1/2
}
\end{aligned}
\right
.
\end{equation}
\noindent
Any of the (
\autoref
{
eq:DYN
_
vor
_
ens
}
), (
\autoref
{
eq:DYN
_
vor
_
ene
}
), (
\autoref
{
eq:DYN
_
vor
_
enT
}
) and (
\autoref
{
eq:DYN
_
vor
_
een
}
)
schemes can be used to
compute the product of the Coriolis parameter and the vorticity.
However, the energy-conserving schemes (
\autoref
{
eq:DYN
_
vor
_
een
}
and
\autoref
{
eq:DYN
_
vor
_
enT
}
)
have exclusively been used to date.
\vskip
0.5cm
\noindent
This term is evaluated using either a leapfrog scheme or a RK3 scheme.
In the leapfrog case it is centred in time (
\textit
{
now
}
velocity).
In the RK3 case it is forward in time (
\textit
{
before
}
velocity) at stage 1,
it is is centred in time (
\textit
{
now
}
velocity) at stage 2 and 3.
%% =================================================================================================
\subsection
[Flux form advection term (\textit{dynadv.F90})]
{
Flux form advection term (
\protect\mdl
{
dynadv
}
)
}
\label
{
subsec:DYN
_
adv
_
flux
}
The discrete expression of the advection term is given by:
\[
% \label{eq:DYN_adv}
\left\{
\begin
{
aligned
}
\frac
{
1
}{
e
_{
1
u
}
\,
e
_{
2
u
}
\,
e
_{
3
u
}}
\left
(
\delta
_{
i
+
1
/
2
}
\left
[
\overline
{
e
_{
2
u
}
\,
e
_{
3
u
}
\;
u
}^{
i
}
\
u
_
t
\right
]
+
&
\delta
_{
j
}
\left
[
\overline
{
e
_{
1
u
}
\,
e
_{
3
u
}
\;
v
}^{
i
+
1
/
2
}
\
u
_
f
\right
]
\right
.
\\
\left
.
+
&
\delta
_{
k
}
\left
[
\overline
{
e
_{
1
w
}
\,
e
_{
2
w
}
\;
w
}^{
i
+
1
/
2
}
\
u
_{
uw
}
\right
]
\right
)
\\
[
10
pt
]
\frac
{
1
}{
e
_{
1
v
}
\,
e
_{
2
v
}
\,
e
_{
3
v
}}
\left
(
\delta
_{
i
}
\left
[
\overline
{
e
_{
2
u
}
\,
e
_{
3
u
}
\;
u
}^{
j
+
1
/
2
}
\
v
_
f
\right
]
+
&
\delta
_{
j
+
1
/
2
}
\left
[
\overline
{
e
_{
1
u
}
\,
e
_{
3
u
}
\;
v
}^{
i
}
\
v
_
t
\right
]
\right
.
\\
\left
.
+
&
\delta
_{
k
}
\left
[
\overline
{
e
_{
1
w
}
\,
e
_{
2
w
}
\;
w
}^{
j
+
1
/
2
}
\
v
_{
vw
}
\right
]
\right
)
\\
\end
{
aligned
}
\right
.
\]
Two advection schemes are available:
a
$
2
^{
nd
}$
order centered finite difference scheme, CEN2,
or a
$
3
^{
rd
}$
order upstream biased scheme, UP3.
The latter is described in
\citet
{
shchepetkin.mcwilliams
_
OM05
}
.
The schemes are selected using the namelist logicals
\np
{
ln
_
dynadv
_
cen2
}{
ln
\_
dynadv
\_
cen2
}
and
\np
{
ln
_
dynadv
_
up3
}{
ln
\_
dynadv
\_
up3
}
.
In flux form, the schemes differ by the choice of a space and time interpolation to define the value of
$
u
$
and
$
v
$
at the centre of each face of
$
u
$
- and
$
v
$
-cells,
\ie\
at the
$
T
$
-,
$
f
$
-,
and
$
uw
$
-points for
$
u
$
and at the
$
f
$
-,
$
T
$
- and
$
vw
$
-points for
$
v
$
.
% 2nd order centred scheme
%% =================================================================================================
\subsubsection
[CEN2: $2^{nd}$ order centred scheme (\forcode{ln_dynadv_cen2})]
{
CEN2:
$
2
^{
nd
}$
order centred scheme (
\protect\np
{
ln
_
dynadv
_
cen2
}{
ln
\_
dynadv
\_
cen2
}
)
}
\label
{
subsec:DYN
_
adv
_
cen2
}
In the centered
$
2
^{
nd
}$
order formulation, the velocity is evaluated as the mean of the two neighbouring points:
\begin{equation}
\label
{
eq:DYN
_
adv
_
cen2
}
\left\{
\begin{aligned}
u
_
T
^{
cen2
}
&
=
\overline
u
^{
i
}
\quad
&
u
_
F
^{
cen2
}
&
=
\overline
u
^{
j+1/2
}
\quad
&
u
_{
uw
}^{
cen2
}
&
=
\overline
u
^{
k+1/2
}
\\
v
_
F
^{
cen2
}
&
=
\overline
v
^{
i+1/2
}
\quad
&
v
_
F
^{
cen2
}
&
=
\overline
v
^
j
\quad
&
v
_{
vw
}^{
cen2
}
&
=
\overline
v
^{
k+1/2
}
\\
\end{aligned}
\right
.
\end{equation}
The scheme is non diffusive (
\ie\
conserves the kinetic energy) but dispersive (
\ie\
it may create false extrema).
It is therefore notoriously noisy and must be used in conjunction with an explicit diffusion operator to
produce a sensible solution.
% UP3 scheme
%% =================================================================================================
\subsubsection
[UP3: Upstream Biased Scheme (\forcode{ln_dynadv_up3})]
{
UP3: Upstream Biased Scheme (
\protect\np
{
ln
_
dynadv
_
up3
}{
ln
\_
dynadv
\_
up3
}
)
}
\label
{
subsec:DYN
_
adv
_
up3
}
The UP3 advection scheme is an upstream biased third order scheme based on
an upstream-biased parabolic interpolation.
For example, the evaluation of
$
u
_
T
^{
up
3
}
$
is done as follows:
\begin{equation}
\label
{
eq:DYN
_
adv
_
up3
}
u
_
T
^{
up3
}
=
\overline
u
^
i-
\;\frac
{
1
}{
6
}
\begin{cases}
u"
_{
i-1/2
}&
\text
{
if
$
\ \overline
{
e
_{
2
u
}
\,
e
_{
3
u
}
\
u
}^
i
\geqslant
0
$
}
\\
u"
_{
i+1/2
}&
\text
{
if
$
\ \overline
{
e
_{
2
u
}
\,
e
_{
3
u
}
\
u
}^
i <
0
$
}
\end{cases}
\end{equation}
where
$
u"
_{
i
+
1
/
2
}
=
\delta
_{
i
+
1
/
2
}
\left
[
{
\delta
_
i
\left
[
u
\right
]
}
\right
]
$
.
This results in a dissipatively dominant (
\ie\
hyper-diffusive) truncation error
\citep
{
shchepetkin.mcwilliams
_
OM05
}
.
The overall performance of the advection scheme is similar to that reported in
\citet
{
farrow.stevens
_
JPO95
}
.
It is a relatively good compromise between accuracy and smoothness.
It is not a
\emph
{
positive
}
scheme, meaning that false extrema are permitted.
But the amplitudes of the false extrema are significantly reduced over those in the centred second order method.
As the scheme already includes a diffusion component, it can be used without explicit lateral diffusion on momentum
(
\ie\ \np
[=.true.]
{
ln
_
dynldf
_
OFF
}{
ln
\_
dynldf
\_
OFF
}
),
and it is recommended to do so.
The UP3 scheme is used in all directions.
UP3 is diffusive and is associated with vertical mixing of momentum.
\cmtgm
{
gm pursue the
sentence:Since vertical mixing of momentum is a source term of the TKE equation...
}
In a leapfrog environment, for stability reasons, the first term in (
\autoref
{
eq:DYN
_
adv
_
up3
}
),
which corresponds to a second order centred scheme, is evaluated using the
\textit
{
now
}
velocity (centred in time),
while the second term, which is the diffusion part of the scheme,
is evaluated using the
\textit
{
before
}
velocity (forward in time).
In an RK3 environment, the first term in (
\autoref
{
eq:DYN
_
adv
_
up3
}
),
which corresponds to a second order centred scheme, is evaluated using the
\textit
{
before
}
velocity at stage 1
and using the
\textit
{
before
}
velocity (centred in time) at stage 2 and 3,
while the second term, which is the diffusion part of the scheme,
is evaluated using the
\textit
{
before
}
velocity (forward in time).
This is discussed by
\citet
{
webb.de-cuevas.ea
_
JAOT98
}
in the context of the Quick advection scheme.
Note that the UP3 and QUICK (Quadratic Upstream Interpolation for Convective Kinematics) schemes only differ by
one coefficient.
Replacing
$
1
/
6
$
by
$
1
/
8
$
in (
\autoref
{
eq:DYN
_
adv
_
up3
}
) leads to the QUICK advection scheme
\citep
{
webb.de-cuevas.ea
_
JAOT98
}
.
This option is not available through a namelist parameter, since the
$
1
/
6
$
coefficient is hard coded.
Nevertheless it is quite easy to make the substitution in the
\mdl
{
dynadv
\_
up3
}
module and obtain a QUICK scheme.
%% =================================================================================================
\section
[Hydrostatic pressure gradient (\textit{dynhpg.F90})]
{
Hydrostatic pressure gradient (
\protect\mdl
{
dynhpg
}
)
}
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Andrew Coward
@acoward
mentioned in commit
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·
4 months ago
mentioned in commit
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mentioned in commit 76725a189ff9b1f22062d6441d548afc22849e3b
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