[707] | 1 | |
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| 2 | % ================================================================ |
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[4147] | 3 | % Chapter � Lateral Ocean Physics (LDF) |
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[707] | 4 | % ================================================================ |
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| 5 | \chapter{Lateral Ocean Physics (LDF)} |
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| 6 | \label{LDF} |
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| 7 | \minitoc |
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| 8 | |
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[2282] | 9 | |
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| 10 | \newpage |
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[707] | 11 | $\ $\newline % force a new ligne |
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| 12 | |
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[2282] | 13 | |
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[999] | 14 | The lateral physics terms in the momentum and tracer equations have been |
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| 15 | described in \S\ref{PE_zdf} and their discrete formulation in \S\ref{TRA_ldf} |
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| 16 | and \S\ref{DYN_ldf}). In this section we further discuss each lateral physics option. |
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| 17 | Choosing one lateral physics scheme means for the user defining, (1) the space |
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| 18 | and time variations of the eddy coefficients ; (2) the direction along which the |
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| 19 | lateral diffusive fluxes are evaluated (model level, geopotential or isopycnal |
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| 20 | surfaces); and (3) the type of operator used (harmonic, or biharmonic operators, |
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| 21 | and for tracers only, eddy induced advection on tracers). These three aspects |
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| 22 | of the lateral diffusion are set through namelist parameters and CPP keys |
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[4147] | 23 | (see the \textit{\ngn{nam\_traldf}} and \textit{\ngn{nam\_dynldf}} below). Note |
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[3294] | 24 | that this chapter describes the default implementation of iso-neutral |
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| 25 | tracer mixing, and Griffies's implementation, which is used if |
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| 26 | \np{traldf\_grif}=true, is described in Appdx\ref{sec:triad} |
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[999] | 27 | |
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[707] | 28 | %-----------------------------------nam_traldf - nam_dynldf-------------------------------------------- |
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[2282] | 29 | \namdisplay{namtra_ldf} |
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| 30 | \namdisplay{namdyn_ldf} |
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[707] | 31 | %-------------------------------------------------------------------------------------------------------------- |
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| 32 | |
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| 33 | |
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| 34 | % ================================================================ |
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| 35 | % Lateral Mixing Coefficients |
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| 36 | % ================================================================ |
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[2282] | 37 | \section [Lateral Mixing Coefficient (\textit{ldftra}, \textit{ldfdyn})] |
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| 38 | {Lateral Mixing Coefficient (\mdl{ldftra}, \mdl{ldfdyn}) } |
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[707] | 39 | \label{LDF_coef} |
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| 40 | |
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| 41 | |
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| 42 | Introducing a space variation in the lateral eddy mixing coefficients changes |
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[999] | 43 | the model core memory requirement, adding up to four extra three-dimensional |
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| 44 | arrays for the geopotential or isopycnal second order operator applied to |
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| 45 | momentum. Six CPP keys control the space variation of eddy coefficients: |
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| 46 | three for momentum and three for tracer. The three choices allow: |
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[2282] | 47 | a space variation in the three space directions (\key{traldf\_c3d}, \key{dynldf\_c3d}), |
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| 48 | in the horizontal plane (\key{traldf\_c2d}, \key{dynldf\_c2d}), |
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| 49 | or in the vertical only (\key{traldf\_c1d}, \key{dynldf\_c1d}). |
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| 50 | The default option is a constant value over the whole ocean on both momentum and tracers. |
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| 51 | |
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[707] | 52 | The number of additional arrays that have to be defined and the gridpoint |
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| 53 | position at which they are defined depend on both the space variation chosen |
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[999] | 54 | and the type of operator used. The resulting eddy viscosity and diffusivity |
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| 55 | coefficients can be a function of more than one variable. Changes in the |
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| 56 | computer code when switching from one option to another have been |
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| 57 | minimized by introducing the eddy coefficients as statement functions |
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| 58 | (include file \hf{ldftra\_substitute} and \hf{ldfdyn\_substitute}). The functions |
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| 59 | are replaced by their actual meaning during the preprocessing step (CPP). |
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| 60 | The specification of the space variation of the coefficient is made in |
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| 61 | \mdl{ldftra} and \mdl{ldfdyn}, or more precisely in include files |
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[2349] | 62 | \textit{traldf\_cNd.h90} and \textit{dynldf\_cNd.h90}, with N=1, 2 or 3. |
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[999] | 63 | The user can modify these include files as he/she wishes. The way the |
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| 64 | mixing coefficient are set in the reference version can be briefly described |
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| 65 | as follows: |
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[707] | 66 | |
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| 67 | \subsubsection{Constant Mixing Coefficients (default option)} |
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[2349] | 68 | When none of the \textbf{key\_dynldf\_...} and \textbf{key\_traldf\_...} keys are |
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[999] | 69 | defined, a constant value is used over the whole ocean for momentum and |
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[2282] | 70 | tracers, which is specified through the \np{rn\_ahm0} and \np{rn\_aht0} namelist |
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[999] | 71 | parameters. |
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[707] | 72 | |
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[2349] | 73 | \subsubsection{Vertically varying Mixing Coefficients (\key{traldf\_c1d} and \key{dynldf\_c1d})} |
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[999] | 74 | The 1D option is only available when using the $z$-coordinate with full step. |
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| 75 | Indeed in all the other types of vertical coordinate, the depth is a 3D function |
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| 76 | of (\textbf{i},\textbf{j},\textbf{k}) and therefore, introducing depth-dependent |
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[2282] | 77 | mixing coefficients will require 3D arrays. In the 1D option, a hyperbolic variation |
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| 78 | of the lateral mixing coefficient is introduced in which the surface value is |
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| 79 | \np{rn\_aht0} (\np{rn\_ahm0}), the bottom value is 1/4 of the surface value, |
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| 80 | and the transition takes place around z=300~m with a width of 300~m |
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| 81 | ($i.e.$ both the depth and the width of the inflection point are set to 300~m). |
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[2349] | 82 | This profile is hard coded in file \hf{traldf\_c1d}, but can be easily modified by users. |
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[707] | 83 | |
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[2349] | 84 | \subsubsection{Horizontally Varying Mixing Coefficients (\key{traldf\_c2d} and \key{dynldf\_c2d})} |
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[1224] | 85 | By default the horizontal variation of the eddy coefficient depends on the local mesh |
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| 86 | size and the type of operator used: |
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[707] | 87 | \begin{equation} \label{Eq_title} |
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| 88 | A_l = \left\{ |
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| 89 | \begin{aligned} |
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| 90 | & \frac{\max(e_1,e_2)}{e_{max}} A_o^l & \text{for laplacian operator } \\ |
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[2282] | 91 | & \frac{\max(e_1,e_2)^{3}}{e_{max}^{3}} A_o^l & \text{for bilaplacian operator } |
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[707] | 92 | \end{aligned} \right. |
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| 93 | \end{equation} |
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[1224] | 94 | where $e_{max}$ is the maximum of $e_1$ and $e_2$ taken over the whole masked |
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[2282] | 95 | ocean domain, and $A_o^l$ is the \np{rn\_ahm0} (momentum) or \np{rn\_aht0} (tracer) |
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[1224] | 96 | namelist parameter. This variation is intended to reflect the lesser need for subgrid |
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| 97 | scale eddy mixing where the grid size is smaller in the domain. It was introduced in |
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[2282] | 98 | the context of the DYNAMO modelling project \citep{Willebrand_al_PO01}. |
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| 99 | Note that such a grid scale dependance of mixing coefficients significantly increase |
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| 100 | the range of stability of model configurations presenting large changes in grid pacing |
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| 101 | such as global ocean models. Indeed, in such a case, a constant mixing coefficient |
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| 102 | can lead to a blow up of the model due to large coefficient compare to the smallest |
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| 103 | grid size (see \S\ref{STP_forward_imp}), especially when using a bilaplacian operator. |
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[707] | 104 | |
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[999] | 105 | Other formulations can be introduced by the user for a given configuration. |
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[4147] | 106 | For example, in the ORCA2 global ocean model (see Configurations), the laplacian |
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[2282] | 107 | viscosity operator uses \np{rn\_ahm0}~= 4.10$^4$ m$^2$/s poleward of 20$^{\circ}$ |
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| 108 | north and south and decreases linearly to \np{rn\_aht0}~= 2.10$^3$ m$^2$/s |
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| 109 | at the equator \citep{Madec_al_JPO96, Delecluse_Madec_Bk00}. This modification |
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[999] | 110 | can be found in routine \rou{ldf\_dyn\_c2d\_orca} defined in \mdl{ldfdyn\_c2d}. |
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| 111 | Similar modified horizontal variations can be found with the Antarctic or Arctic |
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[4147] | 112 | sub-domain options of ORCA2 and ORCA05 (see \&namcfg namelist). |
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[707] | 113 | |
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[2349] | 114 | \subsubsection{Space Varying Mixing Coefficients (\key{traldf\_c3d} and \key{dynldf\_c3d})} |
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[707] | 115 | |
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[999] | 116 | The 3D space variation of the mixing coefficient is simply the combination of the |
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| 117 | 1D and 2D cases, $i.e.$ a hyperbolic tangent variation with depth associated with |
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| 118 | a grid size dependence of the magnitude of the coefficient. |
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[707] | 119 | |
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[999] | 120 | \subsubsection{Space and Time Varying Mixing Coefficients} |
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[707] | 121 | |
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[999] | 122 | There is no default specification of space and time varying mixing coefficient. |
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| 123 | The only case available is specific to the ORCA2 and ORCA05 global ocean |
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[4147] | 124 | configurations. It provides only a tracer |
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[999] | 125 | mixing coefficient for eddy induced velocity (ORCA2) or both iso-neutral and |
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| 126 | eddy induced velocity (ORCA05) that depends on the local growth rate of |
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| 127 | baroclinic instability. This specification is actually used when an ORCA key |
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| 128 | and both \key{traldf\_eiv} and \key{traldf\_c2d} are defined. |
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[707] | 129 | |
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[2282] | 130 | $\ $\newline % force a new ligne |
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| 131 | |
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[3294] | 132 | The following points are relevant when the eddy coefficient varies spatially: |
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[707] | 133 | |
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[999] | 134 | (1) the momentum diffusion operator acting along model level surfaces is |
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[707] | 135 | written in terms of curl and divergent components of the horizontal current |
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[3294] | 136 | (see \S\ref{PE_ldf}). Although the eddy coefficient could be set to different values |
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| 137 | in these two terms, this option is not currently available. |
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[707] | 138 | |
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[999] | 139 | (2) with an horizontally varying viscosity, the quadratic integral constraints |
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[707] | 140 | on enstrophy and on the square of the horizontal divergence for operators |
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[999] | 141 | acting along model-surfaces are no longer satisfied |
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| 142 | (Appendix~\ref{Apdx_dynldf_properties}). |
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[707] | 143 | |
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| 144 | (3) for isopycnal diffusion on momentum or tracers, an additional purely |
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| 145 | horizontal background diffusion with uniform coefficient can be added by |
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[2282] | 146 | setting a non zero value of \np{rn\_ahmb0} or \np{rn\_ahtb0}, a background horizontal |
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[999] | 147 | eddy viscosity or diffusivity coefficient (namelist parameters whose default |
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| 148 | values are $0$). However, the technique used to compute the isopycnal |
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| 149 | slopes is intended to get rid of such a background diffusion, since it introduces |
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| 150 | spurious diapycnal diffusion (see {\S\ref{LDF_slp}). |
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[707] | 151 | |
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[2349] | 152 | (4) when an eddy induced advection term is used (\key{traldf\_eiv}), $A^{eiv}$, |
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[999] | 153 | the eddy induced coefficient has to be defined. Its space variations are controlled |
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| 154 | by the same CPP variable as for the eddy diffusivity coefficient ($i.e.$ |
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| 155 | \textbf{key\_traldf\_cNd}). |
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[707] | 156 | |
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[999] | 157 | (5) the eddy coefficient associated with a biharmonic operator must be set to a \emph{negative} value. |
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[707] | 158 | |
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[1954] | 159 | (6) it is possible to use both the laplacian and biharmonic operators concurrently. |
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[707] | 160 | |
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[2282] | 161 | (7) it is possible to run without explicit lateral diffusion on momentum (\np{ln\_dynldf\_lap} = |
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| 162 | \np{ln\_dynldf\_bilap} = false). This is recommended when using the UBS advection |
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| 163 | scheme on momentum (\np{ln\_dynadv\_ubs} = true, see \ref{DYN_adv_ubs}) |
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| 164 | and can be useful for testing purposes. |
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[1954] | 165 | |
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[707] | 166 | % ================================================================ |
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| 167 | % Direction of lateral Mixing |
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| 168 | % ================================================================ |
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[817] | 169 | \section [Direction of Lateral Mixing (\textit{ldfslp})] |
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| 170 | {Direction of Lateral Mixing (\mdl{ldfslp})} |
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[707] | 171 | \label{LDF_slp} |
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| 172 | |
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[817] | 173 | %%% |
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[1224] | 174 | \gmcomment{ we should emphasize here that the implementation is a rather old one. |
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[2282] | 175 | Better work can be achieved by using \citet{Griffies_al_JPO98, Griffies_Bk04} iso-neutral scheme. } |
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[707] | 176 | |
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[999] | 177 | A direction for lateral mixing has to be defined when the desired operator does |
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| 178 | not act along the model levels. This occurs when $(a)$ horizontal mixing is |
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| 179 | required on tracer or momentum (\np{ln\_traldf\_hor} or \np{ln\_dynldf\_hor}) |
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| 180 | in $s$- or mixed $s$-$z$- coordinates, and $(b)$ isoneutral mixing is required |
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| 181 | whatever the vertical coordinate is. This direction of mixing is defined by its |
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| 182 | slopes in the \textbf{i}- and \textbf{j}-directions at the face of the cell of the |
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| 183 | quantity to be diffused. For a tracer, this leads to the following four slopes : |
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| 184 | $r_{1u}$, $r_{1w}$, $r_{2v}$, $r_{2w}$ (see \eqref{Eq_tra_ldf_iso}), while |
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[2282] | 185 | for momentum the slopes are $r_{1t}$, $r_{1uw}$, $r_{2f}$, $r_{2uw}$ for |
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| 186 | $u$ and $r_{1f}$, $r_{1vw}$, $r_{2t}$, $r_{2vw}$ for $v$. |
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[707] | 187 | |
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| 188 | %gm% add here afigure of the slope in i-direction |
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| 189 | |
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[999] | 190 | \subsection{slopes for tracer geopotential mixing in the $s$-coordinate} |
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[707] | 191 | |
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[999] | 192 | In $s$-coordinates, geopotential mixing ($i.e.$ horizontal mixing) $r_1$ and |
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| 193 | $r_2$ are the slopes between the geopotential and computational surfaces. |
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| 194 | Their discrete formulation is found by locally solving \eqref{Eq_tra_ldf_iso} |
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| 195 | when the diffusive fluxes in the three directions are set to zero and $T$ is |
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| 196 | assumed to be horizontally uniform, $i.e.$ a linear function of $z_T$, the |
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| 197 | depth of a $T$-point. |
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| 198 | %gm { Steven : My version is obviously wrong since I'm left with an arbitrary constant which is the local vertical temperature gradient} |
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[707] | 199 | |
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| 200 | \begin{equation} \label{Eq_ldfslp_geo} |
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| 201 | \begin{aligned} |
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| 202 | r_{1u} &= \frac{e_{3u}}{ \left( e_{1u}\;\overline{\overline{e_{3w}}}^{\,i+1/2,\,k} \right)} |
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[2282] | 203 | \;\delta_{i+1/2}[z_t] |
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| 204 | &\approx \frac{1}{e_{1u}}\; \delta_{i+1/2}[z_t] |
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[707] | 205 | \\ |
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| 206 | r_{2v} &= \frac{e_{3v}}{\left( e_{2v}\;\overline{\overline{e_{3w}}}^{\,j+1/2,\,k} \right)} |
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[2282] | 207 | \;\delta_{j+1/2} [z_t] |
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| 208 | &\approx \frac{1}{e_{2v}}\; \delta_{j+1/2}[z_t] |
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[707] | 209 | \\ |
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[2282] | 210 | r_{1w} &= \frac{1}{e_{1w}}\;\overline{\overline{\delta_{i+1/2}[z_t]}}^{\,i,\,k+1/2} |
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[707] | 211 | &\approx \frac{1}{e_{1w}}\; \delta_{i+1/2}[z_{uw}] |
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| 212 | \\ |
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[2282] | 213 | r_{2w} &= \frac{1}{e_{2w}}\;\overline{\overline{\delta_{j+1/2}[z_t]}}^{\,j,\,k+1/2} |
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[707] | 214 | &\approx \frac{1}{e_{2w}}\; \delta_{j+1/2}[z_{vw}] |
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| 215 | \\ |
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| 216 | \end{aligned} |
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| 217 | \end{equation} |
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| 218 | |
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| 219 | %gm% caution I'm not sure the simplification was a good idea! |
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| 220 | |
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[999] | 221 | These slopes are computed once in \rou{ldfslp\_init} when \np{ln\_sco}=True, |
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| 222 | and either \np{ln\_traldf\_hor}=True or \np{ln\_dynldf\_hor}=True. |
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| 223 | |
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[3294] | 224 | \subsection{Slopes for tracer iso-neutral mixing}\label{LDF_slp_iso} |
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[999] | 225 | In iso-neutral mixing $r_1$ and $r_2$ are the slopes between the iso-neutral |
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| 226 | and computational surfaces. Their formulation does not depend on the vertical |
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| 227 | coordinate used. Their discrete formulation is found using the fact that the |
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| 228 | diffusive fluxes of locally referenced potential density ($i.e.$ $in situ$ density) |
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| 229 | vanish. So, substituting $T$ by $\rho$ in \eqref{Eq_tra_ldf_iso} and setting the |
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| 230 | diffusive fluxes in the three directions to zero leads to the following definition for |
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| 231 | the neutral slopes: |
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[707] | 232 | |
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| 233 | \begin{equation} \label{Eq_ldfslp_iso} |
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| 234 | \begin{split} |
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| 235 | r_{1u} &= \frac{e_{3u}}{e_{1u}}\; \frac{\delta_{i+1/2}[\rho]} |
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| 236 | {\overline{\overline{\delta_{k+1/2}[\rho]}}^{\,i+1/2,\,k}} |
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| 237 | \\ |
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| 238 | r_{2v} &= \frac{e_{3v}}{e_{2v}}\; \frac{\delta_{j+1/2}\left[\rho \right]} |
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| 239 | {\overline{\overline{\delta_{k+1/2}[\rho]}}^{\,j+1/2,\,k}} |
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| 240 | \\ |
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| 241 | r_{1w} &= \frac{e_{3w}}{e_{1w}}\; |
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| 242 | \frac{\overline{\overline{\delta_{i+1/2}[\rho]}}^{\,i,\,k+1/2}} |
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| 243 | {\delta_{k+1/2}[\rho]} |
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| 244 | \\ |
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| 245 | r_{2w} &= \frac{e_{3w}}{e_{2w}}\; |
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| 246 | \frac{\overline{\overline{\delta_{j+1/2}[\rho]}}^{\,j,\,k+1/2}} |
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| 247 | {\delta_{k+1/2}[\rho]} |
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| 248 | \\ |
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| 249 | \end{split} |
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| 250 | \end{equation} |
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| 251 | |
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[1224] | 252 | %gm% rewrite this as the explanation is not very clear !!! |
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[707] | 253 | %In practice, \eqref{Eq_ldfslp_iso} is of little help in evaluating the neutral surface slopes. Indeed, for an unsimplified equation of state, the density has a strong dependancy on pressure (here approximated as the depth), therefore applying \eqref{Eq_ldfslp_iso} using the $in situ$ density, $\rho$, computed at T-points leads to a flattening of slopes as the depth increases. This is due to the strong increase of the $in situ$ density with depth. |
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| 254 | |
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| 255 | %By definition, neutral surfaces are tangent to the local $in situ$ density \citep{McDougall1987}, therefore in \eqref{Eq_ldfslp_iso}, all the derivatives have to be evaluated at the same local pressure (which in decibars is approximated by the depth in meters). |
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| 256 | |
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[999] | 257 | %In the $z$-coordinate, the derivative of the \eqref{Eq_ldfslp_iso} numerator is evaluated at the same depth \nocite{as what?} ($T$-level, which is the same as the $u$- and $v$-levels), so the $in situ$ density can be used for its evaluation. |
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[707] | 258 | |
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[999] | 259 | As the mixing is performed along neutral surfaces, the gradient of $\rho$ in |
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| 260 | \eqref{Eq_ldfslp_iso} has to be evaluated at the same local pressure (which, |
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| 261 | in decibars, is approximated by the depth in meters in the model). Therefore |
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| 262 | \eqref{Eq_ldfslp_iso} cannot be used as such, but further transformation is |
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| 263 | needed depending on the vertical coordinate used: |
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[707] | 264 | |
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| 265 | \begin{description} |
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| 266 | |
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[999] | 267 | \item[$z$-coordinate with full step : ] in \eqref{Eq_ldfslp_iso} the densities |
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| 268 | appearing in the $i$ and $j$ derivatives are taken at the same depth, thus |
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| 269 | the $in situ$ density can be used. This is not the case for the vertical |
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| 270 | derivatives: $\delta_{k+1/2}[\rho]$ is replaced by $-\rho N^2/g$, where $N^2$ |
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| 271 | is the local Brunt-Vais\"{a}l\"{a} frequency evaluated following |
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| 272 | \citet{McDougall1987} (see \S\ref{TRA_bn2}). |
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[707] | 273 | |
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[999] | 274 | \item[$z$-coordinate with partial step : ] this case is identical to the full step |
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| 275 | case except that at partial step level, the \emph{horizontal} density gradient |
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| 276 | is evaluated as described in \S\ref{TRA_zpshde}. |
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[707] | 277 | |
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[999] | 278 | \item[$s$- or hybrid $s$-$z$- coordinate : ] in the current release of \NEMO, |
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[3294] | 279 | iso-neutral mixing is only employed for $s$-coordinates if the |
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| 280 | Griffies scheme is used (\np{traldf\_grif}=true; see Appdx \ref{sec:triad}). |
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[999] | 281 | In other words, iso-neutral mixing will only be accurately represented with a |
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[2282] | 282 | linear equation of state (\np{nn\_eos}=1 or 2). In the case of a "true" equation |
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[999] | 283 | of state, the evaluation of $i$ and $j$ derivatives in \eqref{Eq_ldfslp_iso} |
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| 284 | will include a pressure dependent part, leading to the wrong evaluation of |
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| 285 | the neutral slopes. |
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[707] | 286 | |
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| 287 | %gm% |
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[1224] | 288 | Note: The solution for $s$-coordinate passes trough the use of different |
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| 289 | (and better) expression for the constraint on iso-neutral fluxes. Following |
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[2282] | 290 | \citet{Griffies_Bk04}, instead of specifying directly that there is a zero neutral |
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[1224] | 291 | diffusive flux of locally referenced potential density, we stay in the $T$-$S$ |
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| 292 | plane and consider the balance between the neutral direction diffusive fluxes |
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| 293 | of potential temperature and salinity: |
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[707] | 294 | \begin{equation} |
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| 295 | \alpha \ \textbf{F}(T) = \beta \ \textbf{F}(S) |
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| 296 | \end{equation} |
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[999] | 297 | %gm{ where vector F is ....} |
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[707] | 298 | |
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| 299 | This constraint leads to the following definition for the slopes: |
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| 300 | |
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| 301 | \begin{equation} \label{Eq_ldfslp_iso2} |
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| 302 | \begin{split} |
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| 303 | r_{1u} &= \frac{e_{3u}}{e_{1u}}\; \frac |
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| 304 | {\alpha_u \;\delta_{i+1/2}[T] - \beta_u \;\delta_{i+1/2}[S]} |
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| 305 | {\alpha_u \;\overline{\overline{\delta_{k+1/2}[T]}}^{\,i+1/2,\,k} |
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| 306 | -\beta_u \;\overline{\overline{\delta_{k+1/2}[S]}}^{\,i+1/2,\,k} } |
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| 307 | \\ |
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| 308 | r_{2v} &= \frac{e_{3v}}{e_{2v}}\; \frac |
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| 309 | {\alpha_v \;\delta_{j+1/2}[T] - \beta_v \;\delta_{j+1/2}[S]} |
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| 310 | {\alpha_v \;\overline{\overline{\delta_{k+1/2}[T]}}^{\,j+1/2,\,k} |
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| 311 | -\beta_v \;\overline{\overline{\delta_{k+1/2}[S]}}^{\,j+1/2,\,k} } |
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| 312 | \\ |
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| 313 | r_{1w} &= \frac{e_{3w}}{e_{1w}}\; \frac |
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| 314 | {\alpha_w \;\overline{\overline{\delta_{i+1/2}[T]}}^{\,i,\,k+1/2} |
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| 315 | -\beta_w \;\overline{\overline{\delta_{i+1/2}[S]}}^{\,i,\,k+1/2} } |
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| 316 | {\alpha_w \;\delta_{k+1/2}[T] - \beta_w \;\delta_{k+1/2}[S]} |
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| 317 | \\ |
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| 318 | r_{2w} &= \frac{e_{3w}}{e_{2w}}\; \frac |
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| 319 | {\alpha_w \;\overline{\overline{\delta_{j+1/2}[T]}}^{\,j,\,k+1/2} |
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| 320 | -\beta_w \;\overline{\overline{\delta_{j+1/2}[S]}}^{\,j,\,k+1/2} } |
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| 321 | {\alpha_w \;\delta_{k+1/2}[T] - \beta_w \;\delta_{k+1/2}[S]} |
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| 322 | \\ |
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| 323 | \end{split} |
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| 324 | \end{equation} |
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[999] | 325 | where $\alpha$ and $\beta$, the thermal expansion and saline contraction |
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| 326 | coefficients introduced in \S\ref{TRA_bn2}, have to be evaluated at the three |
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| 327 | velocity points. In order to save computation time, they should be approximated |
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| 328 | by the mean of their values at $T$-points (for example in the case of $\alpha$: |
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| 329 | $\alpha_u=\overline{\alpha_T}^{i+1/2}$, $\alpha_v=\overline{\alpha_T}^{j+1/2}$ |
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| 330 | and $\alpha_w=\overline{\alpha_T}^{k+1/2}$). |
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[707] | 331 | |
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[999] | 332 | Note that such a formulation could be also used in the $z$-coordinate and |
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| 333 | $z$-coordinate with partial steps cases. |
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[707] | 334 | |
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| 335 | \end{description} |
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| 336 | |
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[3294] | 337 | This implementation is a rather old one. It is similar to the one |
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| 338 | proposed by Cox [1987], except for the background horizontal |
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| 339 | diffusion. Indeed, the Cox implementation of isopycnal diffusion in |
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| 340 | GFDL-type models requires a minimum background horizontal diffusion |
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| 341 | for numerical stability reasons. To overcome this problem, several |
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| 342 | techniques have been proposed in which the numerical schemes of the |
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| 343 | ocean model are modified \citep{Weaver_Eby_JPO97, |
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| 344 | Griffies_al_JPO98}. Griffies's scheme is now available in \NEMO if |
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| 345 | \np{traldf\_grif\_iso} is set true; see Appdx \ref{sec:triad}. Here, |
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| 346 | another strategy is presented \citep{Lazar_PhD97}: a local |
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| 347 | filtering of the iso-neutral slopes (made on 9 grid-points) prevents |
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| 348 | the development of grid point noise generated by the iso-neutral |
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| 349 | diffusion operator (Fig.~\ref{Fig_LDF_ZDF1}). This allows an |
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| 350 | iso-neutral diffusion scheme without additional background horizontal |
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| 351 | mixing. This technique can be viewed as a diffusion operator that acts |
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| 352 | along large-scale (2~$\Delta$x) \gmcomment{2deltax doesnt seem very |
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| 353 | large scale} iso-neutral surfaces. The diapycnal diffusion required |
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| 354 | for numerical stability is thus minimized and its net effect on the |
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| 355 | flow is quite small when compared to the effect of an horizontal |
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| 356 | background mixing. |
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[707] | 357 | |
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[1224] | 358 | Nevertheless, this iso-neutral operator does not ensure that variance cannot increase, |
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[2282] | 359 | contrary to the \citet{Griffies_al_JPO98} operator which has that property. |
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[707] | 360 | |
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| 361 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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[2376] | 362 | \begin{figure}[!ht] \begin{center} |
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[998] | 363 | \includegraphics[width=0.70\textwidth]{./TexFiles/Figures/Fig_LDF_ZDF1.pdf} |
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[2376] | 364 | \caption { \label{Fig_LDF_ZDF1} |
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| 365 | averaging procedure for isopycnal slope computation.} |
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| 366 | \end{center} \end{figure} |
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[707] | 367 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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| 368 | |
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[1224] | 369 | %There are three additional questions about the slope calculation. |
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| 370 | %First the expression for the rotation tensor has been obtain assuming the "small slope" approximation, so a bound has to be imposed on slopes. |
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| 371 | %Second, numerical stability issues also require a bound on slopes. |
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| 372 | %Third, the question of boundary condition specified on slopes... |
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[707] | 373 | |
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| 374 | %from griffies: chapter 13.1.... |
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| 375 | |
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| 376 | |
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| 377 | |
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[3294] | 378 | % In addition and also for numerical stability reasons \citep{Cox1987, Griffies_Bk04}, |
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| 379 | % the slopes are bounded by $1/100$ everywhere. This limit is decreasing linearly |
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| 380 | % to zero fom $70$ meters depth and the surface (the fact that the eddies "feel" the |
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| 381 | % surface motivates this flattening of isopycnals near the surface). |
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[707] | 382 | |
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[2282] | 383 | For numerical stability reasons \citep{Cox1987, Griffies_Bk04}, the slopes must also |
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[999] | 384 | be bounded by $1/100$ everywhere. This constraint is applied in a piecewise linear |
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| 385 | fashion, increasing from zero at the surface to $1/100$ at $70$ metres and thereafter |
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| 386 | decreasing to zero at the bottom of the ocean. (the fact that the eddies "feel" the |
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| 387 | surface motivates this flattening of isopycnals near the surface). |
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| 388 | |
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[707] | 389 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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[2376] | 390 | \begin{figure}[!ht] \begin{center} |
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[998] | 391 | \includegraphics[width=0.70\textwidth]{./TexFiles/Figures/Fig_eiv_slp.pdf} |
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[2376] | 392 | \caption { \label{Fig_eiv_slp} |
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| 393 | Vertical profile of the slope used for lateral mixing in the mixed layer : |
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[999] | 394 | \textit{(a)} in the real ocean the slope is the iso-neutral slope in the ocean interior, |
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| 395 | which has to be adjusted at the surface boundary (i.e. it must tend to zero at the |
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| 396 | surface since there is no mixing across the air-sea interface: wall boundary |
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| 397 | condition). Nevertheless, the profile between the surface zero value and the interior |
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| 398 | iso-neutral one is unknown, and especially the value at the base of the mixed layer ; |
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| 399 | \textit{(b)} profile of slope using a linear tapering of the slope near the surface and |
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| 400 | imposing a maximum slope of 1/100 ; \textit{(c)} profile of slope actually used in |
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| 401 | \NEMO: a linear decrease of the slope from zero at the surface to its ocean interior |
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| 402 | value computed just below the mixed layer. Note the huge change in the slope at the |
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| 403 | base of the mixed layer between \textit{(b)} and \textit{(c)}.} |
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[707] | 404 | \end{center} \end{figure} |
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| 405 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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| 406 | |
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| 407 | \colorbox{yellow}{add here a discussion about the flattening of the slopes, vs tapering the coefficient.} |
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| 408 | |
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| 409 | \subsection{slopes for momentum iso-neutral mixing} |
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| 410 | |
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[999] | 411 | The iso-neutral diffusion operator on momentum is the same as the one used on |
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| 412 | tracers but applied to each component of the velocity separately (see |
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| 413 | \eqref{Eq_dyn_ldf_iso} in section~\ref{DYN_ldf_iso}). The slopes between the |
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| 414 | surface along which the diffusion operator acts and the surface of computation |
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| 415 | ($z$- or $s$-surfaces) are defined at $T$-, $f$-, and \textit{uw}- points for the |
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| 416 | $u$-component, and $T$-, $f$- and \textit{vw}- points for the $v$-component. |
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| 417 | They are computed from the slopes used for tracer diffusion, $i.e.$ |
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| 418 | \eqref{Eq_ldfslp_geo} and \eqref{Eq_ldfslp_iso} : |
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[707] | 419 | |
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| 420 | \begin{equation} \label{Eq_ldfslp_dyn} |
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| 421 | \begin{aligned} |
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[2282] | 422 | &r_{1t}\ \ = \overline{r_{1u}}^{\,i} &&& r_{1f}\ \ &= \overline{r_{1u}}^{\,i+1/2} \\ |
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| 423 | &r_{2f} \ \ = \overline{r_{2v}}^{\,j+1/2} &&& r_{2t}\ &= \overline{r_{2v}}^{\,j} \\ |
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[707] | 424 | &r_{1uw} = \overline{r_{1w}}^{\,i+1/2} &&\ \ \text{and} \ \ & r_{1vw}&= \overline{r_{1w}}^{\,j+1/2} \\ |
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| 425 | &r_{2uw}= \overline{r_{2w}}^{\,j+1/2} &&& r_{2vw}&= \overline{r_{2w}}^{\,j+1/2}\\ |
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| 426 | \end{aligned} |
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| 427 | \end{equation} |
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| 428 | |
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[999] | 429 | The major issue remaining is in the specification of the boundary conditions. |
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| 430 | The same boundary conditions are chosen as those used for lateral |
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[707] | 431 | diffusion along model level surfaces, i.e. using the shear computed along |
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| 432 | the model levels and with no additional friction at the ocean bottom (see |
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| 433 | {\S\ref{LBC_coast}). |
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| 434 | |
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| 435 | |
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| 436 | % ================================================================ |
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| 437 | % Eddy Induced Mixing |
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| 438 | % ================================================================ |
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[817] | 439 | \section [Eddy Induced Velocity (\textit{traadv\_eiv}, \textit{ldfeiv})] |
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| 440 | {Eddy Induced Velocity (\mdl{traadv\_eiv}, \mdl{ldfeiv})} |
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[707] | 441 | \label{LDF_eiv} |
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| 442 | |
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[999] | 443 | When Gent and McWilliams [1990] diffusion is used (\key{traldf\_eiv} defined), |
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| 444 | an eddy induced tracer advection term is added, the formulation of which |
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| 445 | depends on the slopes of iso-neutral surfaces. Contrary to the case of iso-neutral |
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[1224] | 446 | mixing, the slopes used here are referenced to the geopotential surfaces, $i.e.$ |
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| 447 | \eqref{Eq_ldfslp_geo} is used in $z$-coordinates, and the sum \eqref{Eq_ldfslp_geo} |
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[999] | 448 | + \eqref{Eq_ldfslp_iso} in $s$-coordinates. The eddy induced velocity is given by: |
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[707] | 449 | \begin{equation} \label{Eq_ldfeiv} |
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| 450 | \begin{split} |
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| 451 | u^* & = \frac{1}{e_{2u}e_{3u}}\; \delta_k \left[e_{2u} \, A_{uw}^{eiv} \; \overline{r_{1w}}^{\,i+1/2} \right]\\ |
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| 452 | v^* & = \frac{1}{e_{1u}e_{3v}}\; \delta_k \left[e_{1v} \, A_{vw}^{eiv} \; \overline{r_{2w}}^{\,j+1/2} \right]\\ |
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| 453 | w^* & = \frac{1}{e_{1w}e_{2w}}\; \left\{ \delta_i \left[e_{2u} \, A_{uw}^{eiv} \; \overline{r_{1w}}^{\,i+1/2} \right] + \delta_j \left[e_{1v} \, A_{vw}^{eiv} \; \overline{r_{2w}}^{\,j+1/2} \right] \right\} \\ |
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| 454 | \end{split} |
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| 455 | \end{equation} |
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[999] | 456 | where $A^{eiv}$ is the eddy induced velocity coefficient whose value is set |
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[2282] | 457 | through \np{rn\_aeiv}, a \textit{nam\_traldf} namelist parameter. |
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[999] | 458 | The three components of the eddy induced velocity are computed and add |
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| 459 | to the eulerian velocity in \mdl{traadv\_eiv}. This has been preferred to a |
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| 460 | separate computation of the advective trends associated with the eiv velocity, |
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| 461 | since it allows us to take advantage of all the advection schemes offered for |
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| 462 | the tracers (see \S\ref{TRA_adv}) and not just the $2^{nd}$ order advection |
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| 463 | scheme as in previous releases of OPA \citep{Madec1998}. This is particularly |
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| 464 | useful for passive tracers where \emph{positivity} of the advection scheme is |
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| 465 | of paramount importance. |
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[707] | 466 | |
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[999] | 467 | At the surface, lateral and bottom boundaries, the eddy induced velocity, |
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| 468 | and thus the advective eddy fluxes of heat and salt, are set to zero. |
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[707] | 469 | |
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| 470 | |
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| 471 | |
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| 472 | |
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| 473 | |
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