[10414] | 1 | \documentclass[../main/NEMO_manual]{subfiles} |
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| 2 | |
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[14113] | 3 | \begin{document} |
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| 4 | |
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[10442] | 5 | \chapter{Iso-Neutral Diffusion and Eddy Advection using Triads} |
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[11543] | 6 | \label{apdx:TRIADS} |
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[10414] | 7 | |
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[11435] | 8 | \chaptertoc |
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[10414] | 9 | |
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[11598] | 10 | \paragraph{Changes record} ~\\ |
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| 11 | |
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| 12 | {\footnotesize |
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| 13 | \begin{tabularx}{\textwidth}{l||X|X} |
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| 14 | Release & Author(s) & Modifications \\ |
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| 15 | \hline |
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| 16 | {\em 4.0} & {\em ...} & {\em ...} \\ |
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| 17 | {\em 3.6} & {\em ...} & {\em ...} \\ |
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| 18 | {\em 3.4} & {\em ...} & {\em ...} \\ |
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| 19 | {\em <=3.4} & {\em ...} & {\em ...} |
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| 20 | \end{tabularx} |
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| 21 | } |
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| 22 | |
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| 23 | \clearpage |
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| 24 | |
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[11597] | 25 | %% ================================================================================================= |
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[14644] | 26 | \section[Choice of \forcode{namtra_ldf} namelist parameters]{Choice of \protect\nam{tra_ldf}{tra\_ldf} namelist parameters} |
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[10146] | 27 | |
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[10354] | 28 | Two scheme are available to perform the iso-neutral diffusion. |
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[11577] | 29 | If the namelist logical \np{ln_traldf_triad}{ln\_traldf\_triad} is set true, |
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[11435] | 30 | \NEMO\ updates both active and passive tracers using the Griffies triad representation of iso-neutral diffusion and |
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[10354] | 31 | the eddy-induced advective skew (GM) fluxes. |
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[11577] | 32 | If the namelist logical \np{ln_traldf_iso}{ln\_traldf\_iso} is set true, |
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[10354] | 33 | the filtered version of Cox's original scheme (the Standard scheme) is employed (\autoref{sec:LDF_slp}). |
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| 34 | In the present implementation of the Griffies scheme, |
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[11577] | 35 | the advective skew fluxes are implemented even if \np{ln_traldf_eiv}{ln\_traldf\_eiv} is false. |
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[6289] | 36 | |
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[10354] | 37 | Values of iso-neutral diffusivity and GM coefficient are set as described in \autoref{sec:LDF_coef}. |
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[11435] | 38 | Note that when GM fluxes are used, the eddy-advective (GM) velocities are output for diagnostic purposes using XIOS, |
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[6289] | 39 | even though the eddy advection is accomplished by means of the skew fluxes. |
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[2282] | 40 | |
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[3294] | 41 | The options specific to the Griffies scheme include: |
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[10442] | 42 | \begin{description} |
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[11598] | 43 | \item [{\np{ln_triad_iso}{ln\_triad\_iso}}] See \autoref{sec:TRIADS_taper}. |
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[10354] | 44 | If this is set false (the default), |
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| 45 | then `iso-neutral' mixing is accomplished within the surface mixed-layer along slopes linearly decreasing with |
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[11543] | 46 | depth from the value immediately below the mixed-layer to zero (flat) at the surface (\autoref{sec:TRIADS_lintaper}). |
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[10354] | 47 | This is the same treatment as used in the default implementation |
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[11543] | 48 | \autoref{subsec:LDF_slp_iso}; \autoref{fig:LDF_eiv_slp}. |
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[11577] | 49 | Where \np{ln_triad_iso}{ln\_triad\_iso} is set true, |
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[10354] | 50 | the vertical skew flux is further reduced to ensure no vertical buoyancy flux, |
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| 51 | giving an almost pure horizontal diffusive tracer flux within the mixed layer. |
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[11543] | 52 | This is similar to the tapering suggested by \citet{gerdes.koberle.ea_CD91}. See \autoref{subsec:TRIADS_Gerdes-taper} |
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[11598] | 53 | \item [{\np{ln_botmix_triad}{ln\_botmix\_triad}}] See \autoref{sec:TRIADS_iso_bdry}. |
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[6289] | 54 | If this is set false (the default) then the lateral diffusive fluxes |
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[11543] | 55 | associated with triads partly masked by topography are neglected. |
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| 56 | If it is set true, however, then these lateral diffusive fluxes are applied, |
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[6289] | 57 | giving smoother bottom tracer fields at the cost of introducing diapycnal mixing. |
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[11598] | 58 | \item [{\np{rn_sw_triad}{rn\_sw\_triad}}] blah blah to be added.... |
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[3294] | 59 | \end{description} |
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[6289] | 60 | The options shared with the Standard scheme include: |
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[10442] | 61 | \begin{description} |
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[11598] | 62 | \item [{\np{ln_traldf_msc}{ln\_traldf\_msc}}] blah blah to be added |
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| 63 | \item [{\np{rn_slpmax}{rn\_slpmax}}] blah blah to be added |
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[6289] | 64 | \end{description} |
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[9393] | 65 | |
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[11597] | 66 | %% ================================================================================================= |
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[3294] | 67 | \section{Triad formulation of iso-neutral diffusion} |
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[11543] | 68 | \label{sec:TRIADS_iso} |
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[10414] | 69 | |
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[11435] | 70 | We have implemented into \NEMO\ a scheme inspired by \citet{griffies.gnanadesikan.ea_JPO98}, |
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| 71 | but formulated within the \NEMO\ framework, using scale factors rather than grid-sizes. |
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[2282] | 72 | |
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[11597] | 73 | %% ================================================================================================= |
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[9393] | 74 | \subsection{Iso-neutral diffusion operator} |
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[10414] | 75 | |
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[10354] | 76 | The iso-neutral second order tracer diffusive operator for small angles between |
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[11543] | 77 | iso-neutral surfaces and geopotentials is given by \autoref{eq:TRIADS_iso_tensor_1}: |
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[10414] | 78 | \begin{subequations} |
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[11543] | 79 | \label{eq:TRIADS_iso_tensor_1} |
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[3294] | 80 | \begin{equation} |
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[10442] | 81 | D^{lT}=-\nabla \cdot\vect{f}^{lT}\equiv |
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[3294] | 82 | -\frac{1}{e_1e_2e_3}\left[\pd{i}\left (f_1^{lT}e_2e_3\right) + |
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| 83 | \pd{j}\left (f_2^{lT}e_2e_3\right) + \pd{k}\left (f_3^{lT}e_1e_2\right)\right], |
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| 84 | \end{equation} |
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| 85 | where the diffusive flux per unit area of physical space |
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| 86 | \begin{equation} |
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[10442] | 87 | \vect{f}^{lT}=-{A^{lT}}\Re\cdot\nabla T, |
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[3294] | 88 | \end{equation} |
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| 89 | \begin{equation} |
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[11543] | 90 | \label{eq:TRIADS_iso_tensor_2} |
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[3294] | 91 | \mbox{with}\quad \;\;\Re = |
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| 92 | \begin{pmatrix} |
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[10442] | 93 | 1 & 0 & -r_1 \rule[-.9 em]{0pt}{1.79 em} \\ |
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| 94 | 0 & 1 & -r_2 \rule[-.9 em]{0pt}{1.79 em} \\ |
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| 95 | -r_1 & -r_2 & r_1 ^2+r_2 ^2 \rule[-.9 em]{0pt}{1.79 em} |
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[3294] | 96 | \end{pmatrix} |
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[10442] | 97 | \quad \text{and} \quad\nabla T= |
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[3294] | 98 | \begin{pmatrix} |
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[10442] | 99 | \frac{1}{e_1} \pd[T]{i} \rule[-.9 em]{0pt}{1.79 em} \\ |
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| 100 | \frac{1}{e_2} \pd[T]{j} \rule[-.9 em]{0pt}{1.79 em} \\ |
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| 101 | \frac{1}{e_3} \pd[T]{k} \rule[-.9 em]{0pt}{1.79 em} |
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[10414] | 102 | \end{pmatrix} |
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| 103 | . |
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[3294] | 104 | \end{equation} |
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| 105 | \end{subequations} |
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| 106 | % \left( {{\begin{array}{*{20}c} |
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| 107 | % 1 \hfill & 0 \hfill & {-r_1 } \hfill \\ |
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| 108 | % 0 \hfill & 1 \hfill & {-r_2 } \hfill \\ |
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| 109 | % {-r_1 } \hfill & {-r_2 } \hfill & {r_1 ^2+r_2 ^2} \hfill \\ |
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| 110 | % \end{array} }} \right) |
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[11543] | 111 | Here \autoref{eq:MB_iso_slopes} |
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[2282] | 112 | \begin{align*} |
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| 113 | r_1 &=-\frac{e_3 }{e_1 } \left( \frac{\partial \rho }{\partial i} |
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[10414] | 114 | \right) |
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| 115 | \left( {\frac{\partial \rho }{\partial k}} \right)^{-1} \\ |
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| 116 | &=-\frac{e_3 }{e_1 } \left( -\alpha\frac{\partial T }{\partial i} + |
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| 117 | \beta\frac{\partial S }{\partial i} \right) \left( |
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| 118 | -\alpha\frac{\partial T }{\partial k} + \beta\frac{\partial S |
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| 119 | }{\partial k} \right)^{-1} |
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[2282] | 120 | \end{align*} |
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[10354] | 121 | is the $i$-component of the slope of the iso-neutral surface relative to the computational surface, |
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| 122 | and $r_2$ is the $j$-component. |
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[2282] | 123 | |
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[10354] | 124 | We will find it useful to consider the fluxes per unit area in $i,j,k$ space; we write |
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[10414] | 125 | \[ |
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[11543] | 126 | % \label{eq:TRIADS_Fijk} |
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[3294] | 127 | \vect{F}_{\mathrm{iso}}=\left(f_1^{lT}e_2e_3, f_2^{lT}e_1e_3, f_3^{lT}e_1e_2\right). |
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[10414] | 128 | \] |
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[10354] | 129 | Additionally, we will sometimes write the contributions towards the fluxes $\vect{f}$ and |
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| 130 | $\vect{F}_{\mathrm{iso}}$ from the component $R_{ij}$ of $\Re$ as $f_{ij}$, $F_{\mathrm{iso}\: ij}$, |
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| 131 | with $f_{ij}=R_{ij}e_i^{-1}\partial T/\partial x_i$ (no summation) etc. |
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[3294] | 132 | |
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| 133 | The off-diagonal terms of the small angle diffusion tensor |
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[11543] | 134 | \autoref{eq:TRIADS_iso_tensor_1}, \autoref{eq:TRIADS_iso_tensor_2} produce skew-fluxes along |
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[10354] | 135 | the $i$- and $j$-directions resulting from the vertical tracer gradient: |
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[3294] | 136 | \begin{align} |
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[11543] | 137 | \label{eq:TRIADS_i13c} |
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[10442] | 138 | f_{13}=&+{A^{lT}} r_1\frac{1}{e_3}\frac{\partial T}{\partial k},\qquad f_{23}=+{A^{lT}} r_2\frac{1}{e_3}\frac{\partial T}{\partial k}\\ |
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[10414] | 139 | \intertext{and in the k-direction resulting from the lateral tracer gradients} |
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[11543] | 140 | \label{eq:TRIADS_i31c} |
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[10442] | 141 | f_{31}+f_{32}=& {A^{lT}} r_1\frac{1}{e_1}\frac{\partial T}{\partial i}+{A^{lT}} r_2\frac{1}{e_1}\frac{\partial T}{\partial i} |
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[3294] | 142 | \end{align} |
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| 143 | |
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[10354] | 144 | The vertical diffusive flux associated with the $_{33}$ component of the small angle diffusion tensor is |
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[2282] | 145 | \begin{equation} |
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[11543] | 146 | \label{eq:TRIADS_i33c} |
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[10442] | 147 | f_{33}=-{A^{lT}}(r_1^2 +r_2^2) \frac{1}{e_3}\frac{\partial T}{\partial k}. |
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[2282] | 148 | \end{equation} |
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| 149 | |
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[10354] | 150 | Since there are no cross terms involving $r_1$ and $r_2$ in the above, |
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| 151 | we can consider the iso-neutral diffusive fluxes separately in the $i$-$k$ and $j$-$k$ planes, |
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| 152 | just adding together the vertical components from each plane. |
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| 153 | The following description will describe the fluxes on the $i$-$k$ plane. |
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[2282] | 154 | |
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[11543] | 155 | There is no natural discretization for the $i$-component of the skew-flux, \autoref{eq:TRIADS_i13c}, |
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[10354] | 156 | as although it must be evaluated at $u$-points, |
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| 157 | it involves vertical gradients (both for the tracer and the slope $r_1$), defined at $w$-points. |
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[11543] | 158 | Similarly, the vertical skew flux, \autoref{eq:TRIADS_i31c}, |
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[10354] | 159 | is evaluated at $w$-points but involves horizontal gradients defined at $u$-points. |
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[2282] | 160 | |
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[11597] | 161 | %% ================================================================================================= |
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[9393] | 162 | \subsection{Standard discretization} |
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[10414] | 163 | |
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[2282] | 164 | The straightforward approach to discretize the lateral skew flux |
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[11543] | 165 | \autoref{eq:TRIADS_i13c} from tracer cell $i,k$ to $i+1,k$, introduced in 1995 into OPA, |
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| 166 | \autoref{eq:TRA_ldf_iso}, is to calculate a mean vertical gradient at the $u$-point from |
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[10354] | 167 | the average of the four surrounding vertical tracer gradients, and multiply this by a mean slope at the $u$-point, |
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| 168 | calculated from the averaged surrounding vertical density gradients. |
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| 169 | The total area-integrated skew-flux (flux per unit area in $ijk$ space) from tracer cell $i,k$ to $i+1,k$, |
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| 170 | noting that the $e_{{3}_{i+1/2}^k}$ in the area $e{_{3}}_{i+1/2}^k{e_{2}}_{i+1/2}i^k$ at the $u$-point cancels out with |
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[11543] | 171 | the $1/{e_{3}}_{i+1/2}^k$ associated with the vertical tracer gradient, is then \autoref{eq:TRA_ldf_iso} |
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[10406] | 172 | \[ |
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[10442] | 173 | \left(F_u^{13} \right)_{i+\frac{1}{2}}^k = {A}_{i+\frac{1}{2}}^k |
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[3294] | 174 | {e_{2}}_{i+1/2}^k \overline{\overline |
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[2282] | 175 | r_1} ^{\,i,k}\,\overline{\overline{\delta_k T}}^{\,i,k}, |
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[10406] | 176 | \] |
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[2282] | 177 | where |
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[10406] | 178 | \[ |
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[2282] | 179 | \overline{\overline |
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[10414] | 180 | r_1} ^{\,i,k} = -\frac{{e_{3u}}_{i+1/2}^k}{{e_{1u}}_{i+1/2}^k} |
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[3294] | 181 | \frac{\delta_{i+1/2} [\rho]}{\overline{\overline{\delta_k \rho}}^{\,i,k}}, |
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[10406] | 182 | \] |
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[10442] | 183 | and here and in the following we drop the $^{lT}$ superscript from ${A^{lT}}$ for simplicity. |
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[10354] | 184 | Unfortunately the resulting combination $\overline{\overline{\delta_k\bullet}}^{\,i,k}$ of a $k$ average and |
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| 185 | a $k$ difference of the tracer reduces to $\bullet_{k+1}-\bullet_{k-1}$, |
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| 186 | so two-grid-point oscillations are invisible to this discretization of the iso-neutral operator. |
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| 187 | These \emph{computational modes} will not be damped by this operator, and may even possibly be amplified by it. |
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| 188 | Consequently, applying this operator to a tracer does not guarantee the decrease of its global-average variance. |
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| 189 | To correct this, we introduced a smoothing of the slopes of the iso-neutral surfaces (see \autoref{chap:LDF}). |
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| 190 | This technique works for $T$ and $S$ in so far as they are active tracers |
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[11435] | 191 | (\ie\ they enter the computation of density), but it does not work for a passive tracer. |
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[9393] | 192 | |
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[11597] | 193 | %% ================================================================================================= |
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[2282] | 194 | \subsection{Expression of the skew-flux in terms of triad slopes} |
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[10414] | 195 | |
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[11123] | 196 | \citep{griffies.gnanadesikan.ea_JPO98} introduce a different discretization of the off-diagonal terms that |
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[10354] | 197 | nicely solves the problem. |
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[3294] | 198 | % Instead of multiplying the mean slope calculated at the $u$-point by |
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| 199 | % the mean vertical gradient at the $u$-point, |
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[10414] | 200 | \begin{figure}[tb] |
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[11558] | 201 | \centering |
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[11690] | 202 | \includegraphics[width=0.66\textwidth]{TRIADS_GRIFF_triad_fluxes} |
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[11558] | 203 | \caption[Triads arrangement and tracer gradients to give lateral and vertical tracer fluxes]{ |
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| 204 | (a) Arrangement of triads $S_i$ and tracer gradients to |
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| 205 | give lateral tracer flux from box $i,k$ to $i+1,k$ |
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| 206 | (b) Triads $S'_i$ and tracer gradients to give vertical tracer flux from |
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| 207 | box $i,k$ to $i,k+1$.} |
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| 208 | \label{fig:TRIADS_ISO_triad} |
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[10414] | 209 | \end{figure} |
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[10354] | 210 | They get the skew flux from the products of the vertical gradients at each $w$-point surrounding the $u$-point with |
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| 211 | the corresponding `triad' slope calculated from the lateral density gradient across the $u$-point divided by |
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| 212 | the vertical density gradient at the same $w$-point as the tracer gradient. |
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[11543] | 213 | See \autoref{fig:TRIADS_ISO_triad}a, where the thick lines denote the tracer gradients, |
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[10354] | 214 | and the thin lines the corresponding triads, with slopes $s_1, \dotsc s_4$. |
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| 215 | The total area-integrated skew-flux from tracer cell $i,k$ to $i+1,k$ |
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[2282] | 216 | \begin{multline} |
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[11543] | 217 | \label{eq:TRIADS_i13} |
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[10442] | 218 | \left( F_u^{13} \right)_{i+\frac{1}{2}}^k = {A}_{i+1}^k a_1 s_1 |
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[10406] | 219 | \delta_{k+\frac{1}{2}} \left[ T^{i+1} |
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[10442] | 220 | \right]/e_{{3w}_{i+1}}^{k+\frac{1}{2}} + {A} _i^k a_2 s_2 \delta |
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[2282] | 221 | _{k+\frac{1}{2}} \left[ T^i |
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| 222 | \right]/e_{{3w}_{i+1}}^{k+\frac{1}{2}} \\ |
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[10442] | 223 | +{A} _{i+1}^k a_3 s_3 \delta_{k-\frac{1}{2}} \left[ T^{i+1} |
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| 224 | \right]/e_{{3w}_{i+1}}^{k+\frac{1}{2}} +{A} _i^k a_4 s_4 \delta |
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[2282] | 225 | _{k-\frac{1}{2}} \left[ T^i \right]/e_{{3w}_{i+1}}^{k+\frac{1}{2}}, |
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| 226 | \end{multline} |
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[10354] | 227 | where the contributions of the triad fluxes are weighted by areas $a_1, \dotsc a_4$, |
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[10442] | 228 | and ${A}$ is now defined at the tracer points rather than the $u$-points. |
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[10354] | 229 | This discretization gives a much closer stencil, and disallows the two-point computational modes. |
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[2282] | 230 | |
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[11543] | 231 | The vertical skew flux \autoref{eq:TRIADS_i31c} from tracer cell $i,k$ to $i,k+1$ at |
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| 232 | the $w$-point $i,k+\frac{1}{2}$ is constructed similarly (\autoref{fig:TRIADS_ISO_triad}b) by |
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[10354] | 233 | multiplying lateral tracer gradients from each of the four surrounding $u$-points by the appropriate triad slope: |
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[2282] | 234 | \begin{multline} |
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[11543] | 235 | \label{eq:TRIADS_i31} |
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[10442] | 236 | \left( F_w^{31} \right) _i ^{k+\frac{1}{2}} = {A}_i^{k+1} a_{1}' |
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[10406] | 237 | s_{1}' \delta_{i-\frac{1}{2}} \left[ T^{k+1} \right]/{e_{3u}}_{i-\frac{1}{2}}^{k+1} |
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[10442] | 238 | +{A}_i^{k+1} a_{2}' s_{2}' \delta_{i+\frac{1}{2}} \left[ T^{k+1} \right]/{e_{3u}}_{i+\frac{1}{2}}^{k+1} \\ |
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| 239 | + {A}_i^k a_{3}' s_{3}' \delta_{i-\frac{1}{2}} \left[ T^k\right]/{e_{3u}}_{i-\frac{1}{2}}^k |
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| 240 | +{A}_i^k a_{4}' s_{4}' \delta_{i+\frac{1}{2}} \left[ T^k \right]/{e_{3u}}_{i+\frac{1}{2}}^k. |
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[2282] | 241 | \end{multline} |
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[3294] | 242 | |
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| 243 | We notate the triad slopes $s_i$ and $s'_i$ in terms of the `anchor point' $i,k$ |
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[10354] | 244 | (appearing in both the vertical and lateral gradient), |
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| 245 | and the $u$- and $w$-points $(i+i_p,k)$, $(i,k+k_p)$ at the centres of the `arms' of the triad as follows |
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[11543] | 246 | (see also \autoref{fig:TRIADS_ISO_triad}): |
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[2282] | 247 | \begin{equation} |
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[11543] | 248 | \label{eq:TRIADS_R} |
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[3294] | 249 | _i^k \mathbb{R}_{i_p}^{k_p} |
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| 250 | =-\frac{ {e_{3w}}_{\,i}^{\,k+k_p}} { {e_{1u}}_{\,i+i_p}^{\,k}} |
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[2282] | 251 | \ |
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[3294] | 252 | \frac |
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[6289] | 253 | { \alpha_i^k \ \delta_{i+i_p}[T^k] - \beta_i^k \ \delta_{i+i_p}[S^k] } |
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| 254 | { \alpha_i^k \ \delta_{k+k_p}[T^i] - \beta_i^k \ \delta_{k+k_p}[S^i] }. |
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[2282] | 255 | \end{equation} |
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[10354] | 256 | In calculating the slopes of the local neutral surfaces, |
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| 257 | the expansion coefficients $\alpha$ and $\beta$ are evaluated at the anchor points of the triad, |
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[6289] | 258 | while the metrics are calculated at the $u$- and $w$-points on the arms. |
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[2282] | 259 | |
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[10414] | 260 | \begin{figure}[tb] |
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[11558] | 261 | \centering |
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[11690] | 262 | \includegraphics[width=0.66\textwidth]{TRIADS_GRIFF_qcells} |
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[11558] | 263 | \caption[Triad notation for quarter cells]{ |
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| 264 | Triad notation for quarter cells. |
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| 265 | $T$-cells are inside boxes, |
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| 266 | while the $i+\fractext{1}{2},k$ $u$-cell is shaded in green and |
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| 267 | the $i,k+\fractext{1}{2}$ $w$-cell is shaded in pink.} |
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| 268 | \label{fig:TRIADS_qcells} |
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[10414] | 269 | \end{figure} |
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[2282] | 270 | |
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[11543] | 271 | Each triad $\{_i^{k}\:_{i_p}^{k_p}\}$ is associated (\autoref{fig:TRIADS_qcells}) with the quarter cell that is |
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[10354] | 272 | the intersection of the $i,k$ $T$-cell, the $i+i_p,k$ $u$-cell and the $i,k+k_p$ $w$-cell. |
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[11543] | 273 | Expressing the slopes $s_i$ and $s'_i$ in \autoref{eq:TRIADS_i13} and \autoref{eq:TRIADS_i31} in this notation, |
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[11435] | 274 | we have \eg\ \ $s_1=s'_1={\:}_i^k \mathbb{R}_{1/2}^{1/2}$. |
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[10354] | 275 | Each triad slope $_i^k\mathbb{R}_{i_p}^{k_p}$ is used once (as an $s$) to |
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| 276 | calculate the lateral flux along its $u$-arm, at $(i+i_p,k)$, |
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| 277 | and then again as an $s'$ to calculate the vertical flux along its $w$-arm at $(i,k+k_p)$. |
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| 278 | Each vertical area $a_i$ used to calculate the lateral flux and horizontal area $a'_i$ used to |
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| 279 | calculate the vertical flux can also be identified as the area across the $u$- and $w$-arms of a unique triad, |
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| 280 | and we notate these areas, similarly to the triad slopes, |
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| 281 | as $_i^k{\mathbb{A}_u}_{i_p}^{k_p}$, $_i^k{\mathbb{A}_w}_{i_p}^{k_p}$, |
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[11543] | 282 | where \eg\ in \autoref{eq:TRIADS_i13} $a_{1}={\:}_i^k{\mathbb{A}_u}_{1/2}^{1/2}$, |
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| 283 | and in \autoref{eq:TRIADS_i31} $a'_{1}={\:}_i^k{\mathbb{A}_w}_{1/2}^{1/2}$. |
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[2282] | 284 | |
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[11597] | 285 | %% ================================================================================================= |
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[9393] | 286 | \subsection{Full triad fluxes} |
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[10414] | 287 | |
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[10354] | 288 | A key property of iso-neutral diffusion is that it should not affect the (locally referenced) density. |
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| 289 | In particular there should be no lateral or vertical density flux. |
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| 290 | The lateral density flux disappears so long as the area-integrated lateral diffusive flux from |
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| 291 | tracer cell $i,k$ to $i+1,k$ coming from the $_{11}$ term of the diffusion tensor takes the form |
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[2282] | 292 | \begin{equation} |
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[11543] | 293 | \label{eq:TRIADS_i11} |
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[2282] | 294 | \left( F_u^{11} \right) _{i+\frac{1}{2}} ^{k} = |
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[10442] | 295 | - \left( {A}_i^{k+1} a_{1} + {A}_i^{k+1} a_{2} + {A}_i^k |
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| 296 | a_{3} + {A}_i^k a_{4} \right) |
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[10406] | 297 | \frac{\delta_{i+1/2} \left[ T^k\right]}{{e_{1u}}_{\,i+1/2}^{\,k}}, |
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[2282] | 298 | \end{equation} |
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[11543] | 299 | where the areas $a_i$ are as in \autoref{eq:TRIADS_i13}. |
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| 300 | In this case, separating the total lateral flux, the sum of \autoref{eq:TRIADS_i13} and \autoref{eq:TRIADS_i11}, |
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[10354] | 301 | into triad components, a lateral tracer flux |
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[2282] | 302 | \begin{equation} |
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[11543] | 303 | \label{eq:TRIADS_latflux-triad} |
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[10442] | 304 | _i^k {\mathbb{F}_u}_{i_p}^{k_p} (T) = - {A}_i^k{ \:}_i^k{\mathbb{A}_u}_{i_p}^{k_p} |
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[2282] | 305 | \left( |
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| 306 | \frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 307 | -\ {_i^k\mathbb{R}_{i_p}^{k_p}} \ |
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| 308 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
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| 309 | \right) |
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| 310 | \end{equation} |
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[10354] | 311 | can be identified with each triad. |
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| 312 | Then, because the same metric factors ${e_{3w}}_{\,i}^{\,k+k_p}$ and ${e_{1u}}_{\,i+i_p}^{\,k}$ are employed for both |
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| 313 | the density gradients in $ _i^k \mathbb{R}_{i_p}^{k_p}$ and the tracer gradients, |
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| 314 | the lateral density flux associated with each triad separately disappears. |
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[2282] | 315 | \begin{equation} |
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[11543] | 316 | \label{eq:TRIADS_latflux-rho} |
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[2282] | 317 | {\mathbb{F}_u}_{i_p}^{k_p} (\rho)=-\alpha _i^k {\:}_i^k {\mathbb{F}_u}_{i_p}^{k_p} (T) + \beta_i^k {\:}_i^k {\mathbb{F}_u}_{i_p}^{k_p} (S)=0 |
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| 318 | \end{equation} |
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[10354] | 319 | Thus the total flux $\left( F_u^{31} \right) ^i _{i,k+\frac{1}{2}} + \left( F_u^{11} \right) ^i _{i,k+\frac{1}{2}}$ from |
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| 320 | tracer cell $i,k$ to $i+1,k$ must also vanish since it is a sum of four such triad fluxes. |
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[2282] | 321 | |
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[11543] | 322 | The squared slope $r_1^2$ in the expression \autoref{eq:TRIADS_i33c} for the $_{33}$ component is also expressed in |
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[10354] | 323 | terms of area-weighted squared triad slopes, |
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| 324 | so the area-integrated vertical flux from tracer cell $i,k$ to $i,k+1$ resulting from the $r_1^2$ term is |
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[2282] | 325 | \begin{equation} |
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[11543] | 326 | \label{eq:TRIADS_i33} |
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[2282] | 327 | \left( F_w^{33} \right) _i^{k+\frac{1}{2}} = |
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[10442] | 328 | - \left( {A}_i^{k+1} a_{1}' s_{1}'^2 |
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| 329 | + {A}_i^{k+1} a_{2}' s_{2}'^2 |
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| 330 | + {A}_i^k a_{3}' s_{3}'^2 |
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| 331 | + {A}_i^k a_{4}' s_{4}'^2 \right)\delta_{k+\frac{1}{2}} \left[ T^{i+1} \right], |
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[2282] | 332 | \end{equation} |
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[11543] | 333 | where the areas $a'$ and slopes $s'$ are the same as in \autoref{eq:TRIADS_i31}. |
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| 334 | Then, separating the total vertical flux, the sum of \autoref{eq:TRIADS_i31} and \autoref{eq:TRIADS_i33}, |
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[10354] | 335 | into triad components, a vertical flux |
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[2282] | 336 | \begin{align} |
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[11543] | 337 | \label{eq:TRIADS_vertflux-triad} |
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[2282] | 338 | _i^k {\mathbb{F}_w}_{i_p}^{k_p} (T) |
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[10442] | 339 | &= {A}_i^k{\: }_i^k{\mathbb{A}_w}_{i_p}^{k_p} |
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[10414] | 340 | \left( |
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[2282] | 341 | {_i^k\mathbb{R}_{i_p}^{k_p}}\frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 342 | -\ \left({_i^k\mathbb{R}_{i_p}^{k_p}}\right)^2 \ |
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| 343 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
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[10414] | 344 | \right) \\ |
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[2282] | 345 | &= - \left(\left.{ }_i^k{\mathbb{A}_w}_{i_p}^{k_p}\right/{ }_i^k{\mathbb{A}_u}_{i_p}^{k_p}\right) |
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[11543] | 346 | {_i^k\mathbb{R}_{i_p}^{k_p}}{\: }_i^k{\mathbb{F}_u}_{i_p}^{k_p} (T) \label{eq:TRIADS_vertflux-triad2} |
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[2282] | 347 | \end{align} |
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[10354] | 348 | may be associated with each triad. |
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| 349 | Each vertical density flux $_i^k {\mathbb{F}_w}_{i_p}^{k_p} (\rho)$ associated with a triad then |
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| 350 | separately disappears (because the lateral flux $_i^k{\mathbb{F}_u}_{i_p}^{k_p} (\rho)$ disappears). |
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| 351 | Consequently the total vertical density flux |
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| 352 | $\left( F_w^{31} \right)_i ^{k+\frac{1}{2}} + \left( F_w^{33} \right)_i^{k+\frac{1}{2}}$ from |
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| 353 | tracer cell $i,k$ to $i,k+1$ must also vanish since it is a sum of four such triad fluxes. |
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[2282] | 354 | |
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[11543] | 355 | We can explicitly identify (\autoref{fig:TRIADS_qcells}) the triads associated with the $s_i$, $a_i$, |
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| 356 | and $s'_i$, $a'_i$ used in the definition of the $u$-fluxes and $w$-fluxes in \autoref{eq:TRIADS_i31}, |
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| 357 | \autoref{eq:TRIADS_i13}, \autoref{eq:TRIADS_i11} \autoref{eq:TRIADS_i33} and \autoref{fig:TRIADS_ISO_triad} to write out |
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[10354] | 358 | the iso-neutral fluxes at $u$- and $w$-points as sums of the triad fluxes that cross the $u$- and $w$-faces: |
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[11543] | 359 | %(\autoref{fig:TRIADS_ISO_triad}): |
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[10414] | 360 | \begin{flalign} |
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[11543] | 361 | \label{eq:TRIADS_iso_flux} \vect{F}_{\mathrm{iso}}(T) &\equiv |
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[2282] | 362 | \sum_{\substack{i_p,\,k_p}} |
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| 363 | \begin{pmatrix} |
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[10414] | 364 | {_{i+1/2-i_p}^k {\mathbb{F}_u}_{i_p}^{k_p} } (T) \\ \\ |
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| 365 | {_i^{k+1/2-k_p} {\mathbb{F}_w}_{i_p}^{k_p} } (T) \\ |
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[2282] | 366 | \end{pmatrix}. |
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| 367 | \end{flalign} |
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[9393] | 368 | |
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[11597] | 369 | %% ================================================================================================= |
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[3294] | 370 | \subsection{Ensuring the scheme does not increase tracer variance} |
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[11543] | 371 | \label{subsec:TRIADS_variance} |
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[2282] | 372 | |
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[10354] | 373 | We now require that this operator should not increase the globally-integrated tracer variance. |
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[2282] | 374 | %This changes according to |
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| 375 | % \begin{align*} |
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| 376 | % &\int_D D_l^T \; T \;dv \equiv \sum_{i,k} \left\{ T \ D_l^T \ b_T \right\} \\ |
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[3294] | 377 | % &\equiv + \sum_{i,k} \sum_{\substack{i_p,\,k_p}} \left\{ |
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| 378 | % \delta_{i} \left[{_{i+1/2-i_p}^k {\mathbb{F}_u }_{i_p}^{k_p}} \right] |
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[2282] | 379 | % + \delta_{k} \left[ {_i^{k+1/2-k_p} {\mathbb{F}_w}_{i_p}^{k_p}} \right] \ T \right\} \\ |
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[3294] | 380 | % &\equiv - \sum_{i,k} \sum_{\substack{i_p,\,k_p}} \left\{ |
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[2282] | 381 | % {_{i+1/2-i_p}^k {\mathbb{F}_u }_{i_p}^{k_p}} \ \delta_{i+1/2} [T] |
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| 382 | % + {_i^{k+1/2-k_p} {\mathbb{F}_w}_{i_p}^{k_p}} \ \delta_{k+1/2} [T] \right\} \\ |
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| 383 | % \end{align*} |
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[10354] | 384 | Each triad slope $_i^k\mathbb{R}_{i_p}^{k_p}$ drives a lateral flux $_i^k{\mathbb{F}_u}_{i_p}^{k_p} (T)$ across |
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| 385 | the $u$-point $i+i_p,k$ and a vertical flux $_i^k{\mathbb{F}_w}_{i_p}^{k_p} (T)$ across the $w$-point $i,k+k_p$. |
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| 386 | The lateral flux drives a net rate of change of variance, |
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[10442] | 387 | summed over the two $T$-points $i+i_p-\fractext{1}{2},k$ and $i+i_p+\fractext{1}{2},k$, of |
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[2282] | 388 | \begin{multline} |
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| 389 | {b_T}_{i+i_p-1/2}^k\left(\frac{\partial T}{\partial t}T\right)_{i+i_p-1/2}^k+ |
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| 390 | \quad {b_T}_{i+i_p+1/2}^k\left(\frac{\partial T}{\partial |
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| 391 | t}T\right)_{i+i_p+1/2}^k \\ |
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[10414] | 392 | \begin{aligned} |
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| 393 | &= -T_{i+i_p-1/2}^k{\;} _i^k{\mathbb{F}_u}_{i_p}^{k_p} (T) \quad + \quad T_{i+i_p+1/2}^k |
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| 394 | {\;}_i^k{\mathbb{F}_u}_{i_p}^{k_p} (T) \\ |
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[11543] | 395 | &={\;} _i^k{\mathbb{F}_u}_{i_p}^{k_p} (T)\,\delta_{i+ i_p}[T^k], \label{eq:TRIADS_dvar_iso_i} |
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[10414] | 396 | \end{aligned} |
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[2282] | 397 | \end{multline} |
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[10354] | 398 | while the vertical flux similarly drives a net rate of change of variance summed over |
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[10442] | 399 | the $T$-points $i,k+k_p-\fractext{1}{2}$ (above) and $i,k+k_p+\fractext{1}{2}$ (below) of |
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[2282] | 400 | \begin{equation} |
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[11543] | 401 | \label{eq:TRIADS_dvar_iso_k} |
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[2282] | 402 | _i^k{\mathbb{F}_w}_{i_p}^{k_p} (T) \,\delta_{k+ k_p}[T^i]. |
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| 403 | \end{equation} |
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[10354] | 404 | The total variance tendency driven by the triad is the sum of these two. |
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| 405 | Expanding $_i^k{\mathbb{F}_u}_{i_p}^{k_p} (T)$ and $_i^k{\mathbb{F}_w}_{i_p}^{k_p} (T)$ with |
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[11543] | 406 | \autoref{eq:TRIADS_latflux-triad} and \autoref{eq:TRIADS_vertflux-triad}, it is |
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[2282] | 407 | \begin{multline*} |
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[10442] | 408 | -{A}_i^k\left \{ |
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[10414] | 409 | { } _i^k{\mathbb{A}_u}_{i_p}^{k_p} |
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| 410 | \left( |
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| 411 | \frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 412 | - {_i^k\mathbb{R}_{i_p}^{k_p}} \ |
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| 413 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} }\right)\,\delta_{i+ i_p}[T^k] \right.\\ |
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| 414 | - \left. { } _i^k{\mathbb{A}_w}_{i_p}^{k_p} |
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| 415 | \left( |
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| 416 | \frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 417 | -{\:}_i^k\mathbb{R}_{i_p}^{k_p} |
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| 418 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
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| 419 | \right) {\,}_i^k\mathbb{R}_{i_p}^{k_p}\delta_{k+ k_p}[T^i] |
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| 420 | \right \}. |
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[2282] | 421 | \end{multline*} |
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[10354] | 422 | The key point is then that if we require $_i^k{\mathbb{A}_u}_{i_p}^{k_p}$ and $_i^k{\mathbb{A}_w}_{i_p}^{k_p}$ to |
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| 423 | be related to a triad volume $_i^k\mathbb{V}_{i_p}^{k_p}$ by |
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[2282] | 424 | \begin{equation} |
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[11543] | 425 | \label{eq:TRIADS_V-A} |
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[2282] | 426 | _i^k\mathbb{V}_{i_p}^{k_p} |
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| 427 | ={\;}_i^k{\mathbb{A}_u}_{i_p}^{k_p}\,{e_{1u}}_{\,i + i_p}^{\,k} |
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| 428 | ={\;}_i^k{\mathbb{A}_w}_{i_p}^{k_p}\,{e_{3w}}_{\,i}^{\,k + k_p}, |
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| 429 | \end{equation} |
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| 430 | the variance tendency reduces to the perfect square |
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| 431 | \begin{equation} |
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[11543] | 432 | \label{eq:TRIADS_perfect-square} |
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[10442] | 433 | -{A}_i^k{\:} _i^k\mathbb{V}_{i_p}^{k_p} |
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[2282] | 434 | \left( |
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| 435 | \frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 436 | -{\:}_i^k\mathbb{R}_{i_p}^{k_p} |
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| 437 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
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| 438 | \right)^2\leq 0. |
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| 439 | \end{equation} |
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[11543] | 440 | Thus, the constraint \autoref{eq:TRIADS_V-A} ensures that the fluxes |
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| 441 | (\autoref{eq:TRIADS_latflux-triad}, \autoref{eq:TRIADS_vertflux-triad}) associated with |
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[10354] | 442 | a given slope triad $_i^k\mathbb{R}_{i_p}^{k_p}$ do not increase the net variance. |
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| 443 | Since the total fluxes are sums of such fluxes from the various triads, this constraint, applied to all triads, |
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| 444 | is sufficient to ensure that the globally integrated variance does not increase. |
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[2282] | 445 | |
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[11543] | 446 | The expression \autoref{eq:TRIADS_V-A} can be interpreted as a discretization of the global integral |
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[2282] | 447 | \begin{equation} |
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[11543] | 448 | \label{eq:TRIADS_cts-var} |
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[10442] | 449 | \frac{\partial}{\partial t}\int\!\fractext{1}{2} T^2\, dV = |
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[3294] | 450 | \int\!\mathbf{F}\cdot\nabla T\, dV, |
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[2282] | 451 | \end{equation} |
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[10354] | 452 | where, within each triad volume $_i^k\mathbb{V}_{i_p}^{k_p}$, the lateral and vertical fluxes/unit area |
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[2282] | 453 | \[ |
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[10414] | 454 | \mathbf{F}=\left( |
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| 455 | \left.{}_i^k{\mathbb{F}_u}_{i_p}^{k_p} (T)\right/{}_i^k{\mathbb{A}_u}_{i_p}^{k_p}, |
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| 456 | \left.{\:}_i^k{\mathbb{F}_w}_{i_p}^{k_p} (T)\right/{}_i^k{\mathbb{A}_w}_{i_p}^{k_p} |
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| 457 | \right) |
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[2282] | 458 | \] |
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| 459 | and the gradient |
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[10414] | 460 | \[ |
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| 461 | \nabla T = \left( |
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| 462 | \left.\delta_{i+ i_p}[T^k] \right/ {e_{1u}}_{\,i + i_p}^{\,k}, |
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| 463 | \left.\delta_{k+ k_p}[T^i] \right/ {e_{3w}}_{\,i}^{\,k + k_p} |
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| 464 | \right) |
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[2282] | 465 | \] |
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[9393] | 466 | |
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[11597] | 467 | %% ================================================================================================= |
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[2282] | 468 | \subsection{Triad volumes in Griffes's scheme and in \NEMO} |
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[10414] | 469 | |
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[10354] | 470 | To complete the discretization we now need only specify the triad volumes $_i^k\mathbb{V}_{i_p}^{k_p}$. |
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[11123] | 471 | \citet{griffies.gnanadesikan.ea_JPO98} identifies these $_i^k\mathbb{V}_{i_p}^{k_p}$ as the volumes of the quarter cells, |
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[10354] | 472 | defined in terms of the distances between $T$, $u$,$f$ and $w$-points. |
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[11543] | 473 | This is the natural discretization of \autoref{eq:TRIADS_cts-var}. |
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[11435] | 474 | The \NEMO\ model, however, operates with scale factors instead of grid sizes, |
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[10354] | 475 | and scale factors for the quarter cells are not defined. |
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| 476 | Instead, therefore we simply choose |
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[2282] | 477 | \begin{equation} |
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[11543] | 478 | \label{eq:TRIADS_V-NEMO} |
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[10442] | 479 | _i^k\mathbb{V}_{i_p}^{k_p}=\fractext{1}{4} {b_u}_{i+i_p}^k, |
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[2282] | 480 | \end{equation} |
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[10354] | 481 | as a quarter of the volume of the $u$-cell inside which the triad quarter-cell lies. |
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| 482 | This has the nice property that when the slopes $\mathbb{R}$ vanish, |
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| 483 | the lateral flux from tracer cell $i,k$ to $i+1,k$ reduces to the classical form |
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[2282] | 484 | \begin{equation} |
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[11543] | 485 | \label{eq:TRIADS_lat-normal} |
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[10442] | 486 | -\overline{A}_{\,i+1/2}^k\; |
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[10414] | 487 | \frac{{b_u}_{i+1/2}^k}{{e_{1u}}_{\,i + i_p}^{\,k}} |
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| 488 | \;\frac{\delta_{i+ 1/2}[T^k] }{{e_{1u}}_{\,i + i_p}^{\,k}} |
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[10442] | 489 | = -\overline{A}_{\,i+1/2}^k\;\frac{{e_{1w}}_{\,i + 1/2}^{\,k}\:{e_{1v}}_{\,i + 1/2}^{\,k}\;\delta_{i+ 1/2}[T^k]}{{e_{1u}}_{\,i + 1/2}^{\,k}}. |
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[2282] | 490 | \end{equation} |
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[10354] | 491 | In fact if the diffusive coefficient is defined at $u$-points, |
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[10442] | 492 | so that we employ ${A}_{i+i_p}^k$ instead of ${A}_i^k$ in the definitions of the triad fluxes |
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[11543] | 493 | \autoref{eq:TRIADS_latflux-triad} and \autoref{eq:TRIADS_vertflux-triad}, |
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[2282] | 494 | we can replace $\overline{A}_{\,i+1/2}^k$ by $A_{i+1/2}^k$ in the above. |
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| 495 | |
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[11597] | 496 | %% ================================================================================================= |
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[2282] | 497 | \subsection{Summary of the scheme} |
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[10414] | 498 | |
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[10354] | 499 | The iso-neutral fluxes at $u$- and $w$-points are the sums of the triad fluxes that |
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[11543] | 500 | cross the $u$- and $w$-faces \autoref{eq:TRIADS_iso_flux}: |
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[10414] | 501 | \begin{subequations} |
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[11543] | 502 | % \label{eq:TRIADS_alltriadflux} |
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[10414] | 503 | \begin{flalign*} |
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[11543] | 504 | % \label{eq:TRIADS_vect_isoflux} |
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[9364] | 505 | \vect{F}_{\mathrm{iso}}(T) &\equiv |
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[3294] | 506 | \sum_{\substack{i_p,\,k_p}} |
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| 507 | \begin{pmatrix} |
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[10414] | 508 | {_{i+1/2-i_p}^k {\mathbb{F}_u}_{i_p}^{k_p} } (T) \\ \\ |
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| 509 | {_i^{k+1/2-k_p} {\mathbb{F}_w}_{i_p}^{k_p} } (T) |
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[3294] | 510 | \end{pmatrix}, |
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[10414] | 511 | \end{flalign*} |
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[11543] | 512 | where \autoref{eq:TRIADS_latflux-triad}: |
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[3294] | 513 | \begin{align} |
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[11543] | 514 | \label{eq:TRIADS_triadfluxu} |
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[10442] | 515 | _i^k {\mathbb{F}_u}_{i_p}^{k_p} (T) &= - {A}_i^k{ |
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[10414] | 516 | \:}\frac{{{}_i^k\mathbb{V}}_{i_p}^{k_p}}{{e_{1u}}_{\,i + i_p}^{\,k}} |
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| 517 | \left( |
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| 518 | \frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 519 | -\ {_i^k\mathbb{R}_{i_p}^{k_p}} \ |
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| 520 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
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| 521 | \right),\\ |
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[3294] | 522 | \intertext{and} |
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| 523 | _i^k {\mathbb{F}_w}_{i_p}^{k_p} (T) |
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[10442] | 524 | &= {A}_i^k{\: }\frac{{{}_i^k\mathbb{V}}_{i_p}^{k_p}}{{e_{3w}}_{\,i}^{\,k+k_p}} |
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[10414] | 525 | \left( |
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| 526 | {_i^k\mathbb{R}_{i_p}^{k_p}}\frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
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| 527 | -\ \left({_i^k\mathbb{R}_{i_p}^{k_p}}\right)^2 \ |
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| 528 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
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[11543] | 529 | \right),\label{eq:TRIADS_triadfluxw} |
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[3294] | 530 | \end{align} |
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[11543] | 531 | with \autoref{eq:TRIADS_V-NEMO} |
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[10414] | 532 | \[ |
---|
[11543] | 533 | % \label{eq:TRIADS_V-NEMO2} |
---|
[10442] | 534 | _i^k{\mathbb{V}}_{i_p}^{k_p}=\fractext{1}{4} {b_u}_{i+i_p}^k. |
---|
[10414] | 535 | \] |
---|
[3294] | 536 | \end{subequations} |
---|
| 537 | |
---|
[11543] | 538 | The divergence of the expression \autoref{eq:TRIADS_iso_flux} for the fluxes gives the iso-neutral diffusion tendency at |
---|
[2282] | 539 | each tracer point: |
---|
[10414] | 540 | \[ |
---|
[11543] | 541 | % \label{eq:TRIADS_iso_operator} |
---|
[10414] | 542 | D_l^T = \frac{1}{b_T} |
---|
[2282] | 543 | \sum_{\substack{i_p,\,k_p}} \left\{ \delta_{i} \left[{_{i+1/2-i_p}^k |
---|
| 544 | {\mathbb{F}_u }_{i_p}^{k_p}} \right] + \delta_{k} \left[ |
---|
| 545 | {_i^{k+1/2-k_p} {\mathbb{F}_w}_{i_p}^{k_p}} \right] \right\} |
---|
[10414] | 546 | \] |
---|
[2282] | 547 | where $b_T= e_{1T}\,e_{2T}\,e_{3T}$ is the volume of $T$-cells. |
---|
| 548 | The diffusion scheme satisfies the following six properties: |
---|
[11598] | 549 | |
---|
[2282] | 550 | \begin{description} |
---|
[11598] | 551 | \item [Horizontal diffusion] The discretization of the diffusion operator recovers the traditional five-point Laplacian |
---|
[11543] | 552 | \autoref{eq:TRIADS_lat-normal} in the limit of flat iso-neutral direction: |
---|
[10414] | 553 | \[ |
---|
[11543] | 554 | % \label{eq:TRIADS_iso_property0} |
---|
[10414] | 555 | D_l^T = \frac{1}{b_T} \ |
---|
[2282] | 556 | \delta_{i} \left[ \frac{e_{2u}\,e_{3u}}{e_{1u}} \; |
---|
[10442] | 557 | \overline{A}^{\,i} \; \delta_{i+1/2}[T] \right] \qquad |
---|
[2282] | 558 | \text{when} \quad { _i^k \mathbb{R}_{i_p}^{k_p} }=0 |
---|
[10414] | 559 | \] |
---|
[11598] | 560 | \item [Implicit treatment in the vertical] Only tracer values associated with a single water column appear in the expression \autoref{eq:TRIADS_i33} for |
---|
[10354] | 561 | the $_{33}$ fluxes, vertical fluxes driven by vertical gradients. |
---|
| 562 | This is of paramount importance since it means that a time-implicit algorithm can be used to |
---|
| 563 | solve the vertical diffusion equation. |
---|
| 564 | This is necessary since the vertical eddy diffusivity associated with this term, |
---|
[10414] | 565 | \[ |
---|
[3294] | 566 | \frac{1}{b_w}\sum_{\substack{i_p, \,k_p}} \left\{ |
---|
[10442] | 567 | {\:}_i^k\mathbb{V}_{i_p}^{k_p} \: {A}_i^k \: \left(_i^k \mathbb{R}_{i_p}^{k_p}\right)^2 |
---|
[3294] | 568 | \right\} = |
---|
| 569 | \frac{1}{4b_w}\sum_{\substack{i_p, \,k_p}} \left\{ |
---|
[10442] | 570 | {b_u}_{i+i_p}^k\: {A}_i^k \: \left(_i^k \mathbb{R}_{i_p}^{k_p}\right)^2 |
---|
[2282] | 571 | \right\}, |
---|
[10414] | 572 | \] |
---|
[2282] | 573 | (where $b_w= e_{1w}\,e_{2w}\,e_{3w}$ is the volume of $w$-cells) can be quite large. |
---|
[11598] | 574 | \item [Pure iso-neutral operator] The iso-neutral flux of locally referenced potential density is zero. |
---|
[11543] | 575 | See \autoref{eq:TRIADS_latflux-rho} and \autoref{eq:TRIADS_vertflux-triad2}. |
---|
[11598] | 576 | \item [Conservation of tracer] The iso-neutral diffusion conserves tracer content, \ie |
---|
[10414] | 577 | \[ |
---|
[11543] | 578 | % \label{eq:TRIADS_iso_property1} |
---|
[10414] | 579 | \sum_{i,j,k} \left\{ D_l^T \ b_T \right\} = 0 |
---|
| 580 | \] |
---|
[10354] | 581 | This property is trivially satisfied since the iso-neutral diffusive operator is written in flux form. |
---|
[11598] | 582 | \item [No increase of tracer variance] The iso-neutral diffusion does not increase the tracer variance, \ie |
---|
[10414] | 583 | \[ |
---|
[11543] | 584 | % \label{eq:TRIADS_iso_property2} |
---|
[10414] | 585 | \sum_{i,j,k} \left\{ T \ D_l^T \ b_T \right\} \leq 0 |
---|
| 586 | \] |
---|
[11543] | 587 | The property is demonstrated in \autoref{subsec:TRIADS_variance} above. |
---|
[10354] | 588 | It is a key property for a diffusion term. |
---|
| 589 | It means that it is also a dissipation term, |
---|
[11435] | 590 | \ie\ it dissipates the square of the quantity on which it is applied. |
---|
[10354] | 591 | It therefore ensures that, when the diffusivity coefficient is large enough, |
---|
| 592 | the field on which it is applied becomes free of grid-point noise. |
---|
[11598] | 593 | \item [Self-adjoint operator] The iso-neutral diffusion operator is self-adjoint, \ie |
---|
[10414] | 594 | \begin{equation} |
---|
[11543] | 595 | \label{eq:TRIADS_iso_property3} |
---|
[10414] | 596 | \sum_{i,j,k} \left\{ S \ D_l^T \ b_T \right\} = \sum_{i,j,k} \left\{ D_l^S \ T \ b_T \right\} |
---|
[2282] | 597 | \end{equation} |
---|
[10354] | 598 | In other word, there is no need to develop a specific routine from the adjoint of this operator. |
---|
| 599 | We just have to apply the same routine. |
---|
| 600 | This property can be demonstrated similarly to the proof of the `no increase of tracer variance' property. |
---|
[11543] | 601 | The contribution by a single triad towards the left hand side of \autoref{eq:TRIADS_iso_property3}, |
---|
| 602 | can be found by replacing $\delta[T]$ by $\delta[S]$ in \autoref{eq:TRIADS_dvar_iso_i} and \autoref{eq:TRIADS_dvar_iso_k}. |
---|
| 603 | This results in a term similar to \autoref{eq:TRIADS_perfect-square}, |
---|
[10414] | 604 | \[ |
---|
[11543] | 605 | % \label{eq:TRIADS_TScovar} |
---|
[10442] | 606 | - {A}_i^k{\:} _i^k\mathbb{V}_{i_p}^{k_p} |
---|
[10414] | 607 | \left( |
---|
| 608 | \frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
---|
| 609 | -{\:}_i^k\mathbb{R}_{i_p}^{k_p} |
---|
| 610 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
---|
| 611 | \right) |
---|
| 612 | \left( |
---|
| 613 | \frac{ \delta_{i+ i_p}[S^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} } |
---|
| 614 | -{\:}_i^k\mathbb{R}_{i_p}^{k_p} |
---|
| 615 | \frac{ \delta_{k+k_p} [S^i] }{{e_{3w}}_{\,i}^{\,k+k_p} } |
---|
| 616 | \right). |
---|
| 617 | \] |
---|
[10354] | 618 | This is symmetrical in $T $ and $S$, so exactly the same term arises from |
---|
[11543] | 619 | the discretization of this triad's contribution towards the RHS of \autoref{eq:TRIADS_iso_property3}. |
---|
[2282] | 620 | \end{description} |
---|
[9393] | 621 | |
---|
[11597] | 622 | %% ================================================================================================= |
---|
[10414] | 623 | \subsection{Treatment of the triads at the boundaries} |
---|
[11543] | 624 | \label{sec:TRIADS_iso_bdry} |
---|
[10414] | 625 | |
---|
[10354] | 626 | The triad slope can only be defined where both the grid boxes centred at the end of the arms exist. |
---|
| 627 | Triads that would poke up through the upper ocean surface into the atmosphere, |
---|
| 628 | or down into the ocean floor, must be masked out. |
---|
[11543] | 629 | See \autoref{fig:TRIADS_bdry_triads}. |
---|
[10442] | 630 | Surface layer triads \triad{i}{1}{R}{1/2}{-1/2} (magenta) and \triad{i+1}{1}{R}{-1/2}{-1/2} (blue) that |
---|
[11543] | 631 | require density to be specified above the ocean surface are masked (\autoref{fig:TRIADS_bdry_triads}a): |
---|
[10354] | 632 | this ensures that lateral tracer gradients produce no flux through the ocean surface. |
---|
| 633 | However, to prevent surface noise, it is customary to retain the $_{11}$ contributions towards |
---|
[10442] | 634 | the lateral triad fluxes \triad[u]{i}{1}{F}{1/2}{-1/2} and \triad[u]{i+1}{1}{F}{-1/2}{-1/2}; |
---|
[10354] | 635 | this drives diapycnal tracer fluxes. |
---|
[11543] | 636 | Similar comments apply to triads that would intersect the ocean floor (\autoref{fig:TRIADS_bdry_triads}b). |
---|
[10442] | 637 | Note that both near bottom triad slopes \triad{i}{k}{R}{1/2}{1/2} and \triad{i+1}{k}{R}{-1/2}{1/2} are masked when |
---|
[11435] | 638 | either of the $i,k+1$ or $i+1,k+1$ tracer points is masked, \ie\ the $i,k+1$ $u$-point is masked. |
---|
[11582] | 639 | The associated lateral fluxes (grey-black dashed line) are masked if \np[=.false.]{ln_botmix_triad}{ln\_botmix\_triad}, |
---|
| 640 | but left unmasked, giving bottom mixing, if \np[=.true.]{ln_botmix_triad}{ln\_botmix\_triad}. |
---|
[2282] | 641 | |
---|
[11582] | 642 | The default option \np[=.false.]{ln_botmix_triad}{ln\_botmix\_triad} is suitable when the bbl mixing option is enabled |
---|
| 643 | (\np[=.true.]{ln_trabbl}{ln\_trabbl}, with \np[=1]{nn_bbl_ldf}{nn\_bbl\_ldf}), or for simple idealized problems. |
---|
| 644 | For setups with topography without bbl mixing, \np[=.true.]{ln_botmix_triad}{ln\_botmix\_triad} may be necessary. |
---|
[10414] | 645 | \begin{figure}[h] |
---|
[11558] | 646 | \centering |
---|
[11690] | 647 | \includegraphics[width=0.66\textwidth]{TRIADS_GRIFF_bdry_triads} |
---|
[11558] | 648 | \caption[Boundary triads]{ |
---|
| 649 | (a) Uppermost model layer $k=1$ with $i,1$ and $i+1,1$ tracer points (black dots), |
---|
| 650 | and $i+1/2,1$ $u$-point (blue square). |
---|
| 651 | Triad slopes \triad{i}{1}{R}{1/2}{-1/2} (magenta) and |
---|
| 652 | \triad{i+1}{1}{R}{-1/2}{-1/2} (blue) poking through the ocean surface are masked |
---|
| 653 | (faded in figure). |
---|
| 654 | However, |
---|
| 655 | the lateral $_{11}$ contributions towards \triad[u]{i}{1}{F}{1/2}{-1/2} and |
---|
| 656 | \triad[u]{i+1}{1}{F}{-1/2}{-1/2} (yellow line) are still applied, |
---|
| 657 | giving diapycnal diffusive fluxes. |
---|
| 658 | \newline |
---|
| 659 | (b) Both near bottom triad slopes \triad{i}{k}{R}{1/2}{1/2} and |
---|
| 660 | \triad{i+1}{k}{R}{-1/2}{1/2} are masked when |
---|
| 661 | either of the $i,k+1$ or $i+1,k+1$ tracer points is masked, |
---|
| 662 | \ie\ the $i,k+1$ $u$-point is masked. |
---|
| 663 | The associated lateral fluxes (grey-black dashed line) are masked if |
---|
[11582] | 664 | \protect\np[=.false.]{ln_botmix_triad}{ln\_botmix\_triad}, but left unmasked, |
---|
| 665 | giving bottom mixing, if \protect\np[=.true.]{ln_botmix_triad}{ln\_botmix\_triad}} |
---|
[11558] | 666 | \label{fig:TRIADS_bdry_triads} |
---|
[10414] | 667 | \end{figure} |
---|
[9393] | 668 | |
---|
[11597] | 669 | %% ================================================================================================= |
---|
[10414] | 670 | \subsection{ Limiting of the slopes within the interior} |
---|
[11543] | 671 | \label{sec:TRIADS_limit} |
---|
[10414] | 672 | |
---|
[10354] | 673 | As discussed in \autoref{subsec:LDF_slp_iso}, |
---|
| 674 | iso-neutral slopes relative to geopotentials must be bounded everywhere, |
---|
[11123] | 675 | both for consistency with the small-slope approximation and for numerical stability \citep{cox_OM87, griffies_bk04}. |
---|
[11435] | 676 | The bound chosen in \NEMO\ is applied to each component of the slope separately and |
---|
[10354] | 677 | has a value of $1/100$ in the ocean interior. |
---|
[3294] | 678 | %, ramping linearly down above 70~m depth to zero at the surface |
---|
[10354] | 679 | It is of course relevant to the iso-neutral slopes $\tilde{r}_i=r_i+\sigma_i$ relative to geopotentials |
---|
| 680 | (here the $\sigma_i$ are the slopes of the coordinate surfaces relative to geopotentials) |
---|
[11543] | 681 | \autoref{eq:MB_slopes_eiv} rather than the slope $r_i$ relative to coordinate surfaces, so we require |
---|
[10406] | 682 | \[ |
---|
[3294] | 683 | |\tilde{r}_i|\leq \tilde{r}_\mathrm{max}=0.01. |
---|
[10406] | 684 | \] |
---|
[3294] | 685 | and then recalculate the slopes $r_i$ relative to coordinates. |
---|
| 686 | Each individual triad slope |
---|
[10354] | 687 | \begin{equation} |
---|
[11543] | 688 | \label{eq:TRIADS_Rtilde} |
---|
[10354] | 689 | _i^k\tilde{\mathbb{R}}_{i_p}^{k_p} = {}_i^k\mathbb{R}_{i_p}^{k_p} + \frac{\delta_{i+i_p}[z_T^k]}{{e_{1u}}_{\,i + i_p}^{\,k}} |
---|
| 690 | \end{equation} |
---|
| 691 | is limited like this and then the corresponding $_i^k\mathbb{R}_{i_p}^{k_p} $ are recalculated and |
---|
| 692 | combined to form the fluxes. |
---|
| 693 | Note that where the slopes have been limited, there is now a non-zero iso-neutral density flux that |
---|
| 694 | drives dianeutral mixing. |
---|
| 695 | In particular this iso-neutral density flux is always downwards, |
---|
| 696 | and so acts to reduce gravitational potential energy. |
---|
[9393] | 697 | |
---|
[11597] | 698 | %% ================================================================================================= |
---|
[10414] | 699 | \subsection{Tapering within the surface mixed layer} |
---|
[11543] | 700 | \label{sec:TRIADS_taper} |
---|
[10414] | 701 | |
---|
[10354] | 702 | Additional tapering of the iso-neutral fluxes is necessary within the surface mixed layer. |
---|
| 703 | When the Griffies triads are used, we offer two options for this. |
---|
[9393] | 704 | |
---|
[11597] | 705 | %% ================================================================================================= |
---|
[10414] | 706 | \subsubsection{Linear slope tapering within the surface mixed layer} |
---|
[11543] | 707 | \label{sec:TRIADS_lintaper} |
---|
[10414] | 708 | |
---|
[11582] | 709 | This is the option activated by the default choice \np[=.false.]{ln_triad_iso}{ln\_triad\_iso}. |
---|
[10354] | 710 | Slopes $\tilde{r}_i$ relative to geopotentials are tapered linearly from their value immediately below |
---|
[11543] | 711 | the mixed layer to zero at the surface, as described in option (c) of \autoref{fig:LDF_eiv_slp}, to values |
---|
[10414] | 712 | \begin{equation} |
---|
[11543] | 713 | \label{eq:TRIADS_rmtilde} |
---|
[10414] | 714 | \rMLt = -\frac{z}{h}\left.\tilde{r}_i\right|_{z=-h}\quad \text{ for } z>-h, |
---|
| 715 | \end{equation} |
---|
| 716 | and then the $r_i$ relative to vertical coordinate surfaces are appropriately adjusted to |
---|
| 717 | \[ |
---|
[11543] | 718 | % \label{eq:TRIADS_rm} |
---|
[10414] | 719 | \rML =\rMLt -\sigma_i \quad \text{ for } z>-h. |
---|
| 720 | \] |
---|
[3294] | 721 | Thus the diffusion operator within the mixed layer is given by: |
---|
[10414] | 722 | \[ |
---|
[11543] | 723 | % \label{eq:TRIADS_iso_tensor_ML} |
---|
[11151] | 724 | D^{lT}=\nabla {\mathrm {\mathbf .}}\left( {A^{lT}\;\Re \;\nabla T} \right) \qquad |
---|
[10414] | 725 | \mbox{with}\quad \;\;\Re =\left( {{ |
---|
| 726 | \begin{array}{*{20}c} |
---|
| 727 | 1 \hfill & 0 \hfill & {-\rML[1]}\hfill \\ |
---|
| 728 | 0 \hfill & 1 \hfill & {-\rML[2]} \hfill \\ |
---|
| 729 | {-\rML[1]}\hfill & {-\rML[2]} \hfill & {\rML[1]^2+\rML[2]^2} \hfill |
---|
| 730 | \end{array} |
---|
| 731 | }} \right) |
---|
| 732 | \] |
---|
[3294] | 733 | |
---|
[10354] | 734 | This slope tapering gives a natural connection between tracer in the mixed-layer and |
---|
| 735 | in isopycnal layers immediately below, in the thermocline. |
---|
| 736 | It is consistent with the way the $\tilde{r}_i$ are tapered within the mixed layer |
---|
[11543] | 737 | (see \autoref{sec:TRIADS_taperskew} below) so as to ensure a uniform GM eddy-induced velocity throughout the mixed layer. |
---|
[10354] | 738 | However, it gives a downwards density flux and so acts so as to reduce potential energy in the same way as |
---|
[11543] | 739 | does the slope limiting discussed above in \autoref{sec:TRIADS_limit}. |
---|
| 740 | |
---|
| 741 | As in \autoref{sec:TRIADS_limit} above, the tapering \autoref{eq:TRIADS_rmtilde} is applied separately to |
---|
[10354] | 742 | each triad $_i^k\tilde{\mathbb{R}}_{i_p}^{k_p}$, and the $_i^k\mathbb{R}_{i_p}^{k_p}$ adjusted. |
---|
| 743 | For clarity, we assume $z$-coordinates in the following; |
---|
| 744 | the conversion from $\mathbb{R}$ to $\tilde{\mathbb{R}}$ and back to $\mathbb{R}$ follows exactly as |
---|
[11543] | 745 | described above by \autoref{eq:TRIADS_Rtilde}. |
---|
[3294] | 746 | \begin{enumerate} |
---|
[11596] | 747 | \item Mixed-layer depth is defined so as to avoid including regions of weak vertical stratification in |
---|
[10354] | 748 | the slope definition. |
---|
[11543] | 749 | At each $i,j$ (simplified to $i$ in \autoref{fig:TRIADS_MLB_triad}), |
---|
[10354] | 750 | we define the mixed-layer by setting the vertical index of the tracer point immediately below the mixed layer, |
---|
| 751 | $k_{\mathrm{ML}}$, as the maximum $k$ (shallowest tracer point) such that |
---|
| 752 | the potential density ${\rho_0}_{i,k}>{\rho_0}_{i,k_{10}}+\Delta\rho_c$, |
---|
| 753 | where $i,k_{10}$ is the tracer gridbox within which the depth reaches 10~m. |
---|
[11543] | 754 | See the left side of \autoref{fig:TRIADS_MLB_triad}. |
---|
[11435] | 755 | We use the $k_{10}$-gridbox instead of the surface gridbox to avoid problems \eg\ with thin daytime mixed-layers. |
---|
[10354] | 756 | Currently we use the same $\Delta\rho_c=0.01\;\mathrm{kg\:m^{-3}}$ for ML triad tapering as is used to |
---|
| 757 | output the diagnosed mixed-layer depth $h_{\mathrm{ML}}=|z_{W}|_{k_{\mathrm{ML}}+1/2}$, |
---|
| 758 | the depth of the $w$-point above the $i,k_{\mathrm{ML}}$ tracer point. |
---|
[11596] | 759 | \item We define `basal' triad slopes ${\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p}$ as |
---|
[10354] | 760 | the slopes of those triads whose vertical `arms' go down from the $i,k_{\mathrm{ML}}$ tracer point to |
---|
| 761 | the $i,k_{\mathrm{ML}}-1$ tracer point below. |
---|
| 762 | This is to ensure that the vertical density gradients associated with |
---|
| 763 | these basal triad slopes ${\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p}$ are representative of the thermocline. |
---|
[11543] | 764 | The four basal triads defined in the bottom part of \autoref{fig:TRIADS_MLB_triad} are then |
---|
[10414] | 765 | \begin{align*} |
---|
| 766 | {\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p} &= |
---|
| 767 | {\:}^{k_{\mathrm{ML}}-k_p-1/2}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p}, |
---|
[11543] | 768 | % \label{eq:TRIADS_Rbase} |
---|
[10414] | 769 | \\ |
---|
[11435] | 770 | \intertext{with \eg\ the green triad} |
---|
[10414] | 771 | {\:}_i{\mathbb{R}_{\mathrm{base}}}_{1/2}^{-1/2}&= |
---|
| 772 | {\:}^{k_{\mathrm{ML}}}_i{\mathbb{R}_{\mathrm{base}}}_{\,1/2}^{-1/2}. |
---|
| 773 | \end{align*} |
---|
[10354] | 774 | The vertical flux associated with each of these triads passes through |
---|
| 775 | the $w$-point $i,k_{\mathrm{ML}}-1/2$ lying \emph{below} the $i,k_{\mathrm{ML}}$ tracer point, so it is this depth |
---|
[10414] | 776 | \[ |
---|
[11543] | 777 | % \label{eq:TRIADS_zbase} |
---|
[3294] | 778 | {z_\mathrm{base}}_{\,i}={z_{w}}_{k_\mathrm{ML}-1/2} |
---|
[10414] | 779 | \] |
---|
[10354] | 780 | one gridbox deeper than the diagnosed ML depth $z_{\mathrm{ML}})$ that sets the $h$ used to taper the slopes in |
---|
[11543] | 781 | \autoref{eq:TRIADS_rmtilde}. |
---|
[11596] | 782 | \item Finally, we calculate the adjusted triads ${\:}_i^k{\mathbb{R}_{\mathrm{ML}}}_{\,i_p}^{k_p}$ within |
---|
[10354] | 783 | the mixed layer, by multiplying the appropriate ${\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p}$ by |
---|
| 784 | the ratio of the depth of the $w$-point ${z_w}_{k+k_p}$ to ${z_{\mathrm{base}}}_{\,i}$. |
---|
| 785 | For instance the green triad centred on $i,k$ |
---|
[10414] | 786 | \begin{align*} |
---|
| 787 | {\:}_i^k{\mathbb{R}_{\mathrm{ML}}}_{\,1/2}^{-1/2} &= |
---|
| 788 | \frac{{z_w}_{k-1/2}}{{z_{\mathrm{base}}}_{\,i}}{\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,1/2}^{-1/2} \\ |
---|
| 789 | \intertext{and more generally} |
---|
| 790 | {\:}_i^k{\mathbb{R}_{\mathrm{ML}}}_{\,i_p}^{k_p} &= |
---|
| 791 | \frac{{z_w}_{k+k_p}}{{z_{\mathrm{base}}}_{\,i}}{\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p}. |
---|
[11543] | 792 | % \label{eq:TRIADS_RML} |
---|
[10414] | 793 | \end{align*} |
---|
[3294] | 794 | \end{enumerate} |
---|
| 795 | |
---|
| 796 | \begin{figure}[h] |
---|
[11558] | 797 | \centering |
---|
[11690] | 798 | \includegraphics[width=0.66\textwidth]{TRIADS_GRIFF_MLB_triads} |
---|
[11558] | 799 | \caption[Definition of mixed-layer depth and calculation of linearly tapered triads]{ |
---|
[10354] | 800 | Definition of mixed-layer depth and calculation of linearly tapered triads. |
---|
[11558] | 801 | The figure shows a water column at a given $i,j$ (simplified to $i$), |
---|
| 802 | with the ocean surface at the top. |
---|
[10354] | 803 | Tracer points are denoted by bullets, and black lines the edges of the tracer cells; |
---|
[10414] | 804 | $k$ increases upwards. |
---|
| 805 | \newline |
---|
[11558] | 806 | We define the mixed-layer by setting the vertical index of the tracer point immediately below |
---|
| 807 | the mixed layer, $k_{\mathrm{ML}}$, as the maximum $k$ (shallowest tracer point) such that |
---|
[10354] | 808 | ${\rho_0}_{i,k}>{\rho_0}_{i,k_{10}}+\Delta\rho_c$, |
---|
| 809 | where $i,k_{10}$ is the tracer gridbox within which the depth reaches 10~m. |
---|
| 810 | We calculate the triad slopes within the mixed layer by linearly tapering them from zero |
---|
| 811 | (at the surface) to the `basal' slopes, |
---|
| 812 | the slopes of the four triads passing through the $w$-point $i,k_{\mathrm{ML}}-1/2$ (blue square), |
---|
| 813 | ${\:}_i{\mathbb{R}_{\mathrm{base}}}_{\,i_p}^{k_p}$. |
---|
| 814 | Triads with different $i_p,k_p$, denoted by different colours, |
---|
[11435] | 815 | (\eg\ the green triad $i_p=1/2,k_p=-1/2$) are tapered to the appropriate basal triad.} |
---|
[11558] | 816 | \label{fig:TRIADS_MLB_triad} |
---|
[3294] | 817 | \end{figure} |
---|
| 818 | |
---|
[11597] | 819 | %% ================================================================================================= |
---|
[9393] | 820 | \subsubsection{Additional truncation of skew iso-neutral flux components} |
---|
[11543] | 821 | \label{subsec:TRIADS_Gerdes-taper} |
---|
[10414] | 822 | |
---|
[11577] | 823 | The alternative option is activated by setting \np{ln_triad_iso}{ln\_triad\_iso} = true. |
---|
[10354] | 824 | This retains the same tapered slope $\rML$ described above for the calculation of the $_{33}$ term of |
---|
| 825 | the iso-neutral diffusion tensor (the vertical tracer flux driven by vertical tracer gradients), |
---|
| 826 | but replaces the $\rML$ in the skew term by |
---|
[3294] | 827 | \begin{equation} |
---|
[11543] | 828 | \label{eq:TRIADS_rm*} |
---|
[3294] | 829 | \rML^*=\left.\rMLt^2\right/\tilde{r}_i-\sigma_i, |
---|
| 830 | \end{equation} |
---|
| 831 | giving a ML diffusive operator |
---|
[10414] | 832 | \[ |
---|
[11543] | 833 | % \label{eq:TRIADS_iso_tensor_ML2} |
---|
[11151] | 834 | D^{lT}=\nabla {\mathrm {\mathbf .}}\left( {A^{lT}\;\Re \;\nabla T} \right) \qquad |
---|
[10414] | 835 | \mbox{with}\quad \;\;\Re =\left( {{ |
---|
| 836 | \begin{array}{*{20}c} |
---|
| 837 | 1 \hfill & 0 \hfill & {-\rML[1]^*}\hfill \\ |
---|
| 838 | 0 \hfill & 1 \hfill & {-\rML[2]^*} \hfill \\ |
---|
| 839 | {-\rML[1]^*}\hfill & {-\rML[2]^*} \hfill & {\rML[1]^2+\rML[2]^2} \hfill \\ |
---|
| 840 | \end{array} |
---|
| 841 | }} \right). |
---|
| 842 | \] |
---|
[3294] | 843 | This operator |
---|
[10354] | 844 | \footnote{ |
---|
| 845 | To ensure good behaviour where horizontal density gradients are weak, |
---|
[11123] | 846 | we in fact follow \citet{gerdes.koberle.ea_CD91} and |
---|
[10414] | 847 | set $\rML^*=\mathrm{sgn}(\tilde{r}_i)\min(|\rMLt^2/\tilde{r}_i|,|\tilde{r}_i|)-\sigma_i$. |
---|
| 848 | } |
---|
[10354] | 849 | then has the property it gives no vertical density flux, and so does not change the potential energy. |
---|
| 850 | This approach is similar to multiplying the iso-neutral diffusion coefficient by |
---|
| 851 | $\tilde{r}_{\mathrm{max}}^{-2}\tilde{r}_i^{-2}$ for steep slopes, |
---|
[11123] | 852 | as suggested by \citet{gerdes.koberle.ea_CD91} (see also \citet{griffies_bk04}). |
---|
[3294] | 853 | Again it is applied separately to each triad $_i^k\mathbb{R}_{i_p}^{k_p}$ |
---|
| 854 | |
---|
[10354] | 855 | In practice, this approach gives weak vertical tracer fluxes through the mixed-layer, |
---|
| 856 | as well as vanishing density fluxes. |
---|
| 857 | While it is theoretically advantageous that it does not change the potential energy, |
---|
| 858 | it may give a discontinuity between the fluxes within the mixed-layer (purely horizontal) and |
---|
| 859 | just below (along iso-neutral surfaces). |
---|
[3294] | 860 | % This may give strange looking results, |
---|
| 861 | % particularly where the mixed-layer depth varies strongly laterally. |
---|
[11597] | 862 | %% ================================================================================================= |
---|
[10414] | 863 | \section{Eddy induced advection formulated as a skew flux} |
---|
[11543] | 864 | \label{sec:TRIADS_skew-flux} |
---|
[2282] | 865 | |
---|
[11597] | 866 | %% ================================================================================================= |
---|
[10414] | 867 | \subsection{Continuous skew flux formulation} |
---|
[11543] | 868 | \label{sec:TRIADS_continuous-skew-flux} |
---|
[3294] | 869 | |
---|
[10354] | 870 | When Gent and McWilliams's [1990] diffusion is used, an additional advection term is added. |
---|
| 871 | The associated velocity is the so called eddy induced velocity, |
---|
| 872 | the formulation of which depends on the slopes of iso-neutral surfaces. |
---|
| 873 | Contrary to the case of iso-neutral mixing, the slopes used here are referenced to the geopotential surfaces, |
---|
[11543] | 874 | \ie\ \autoref{eq:LDF_slp_geo} is used in $z$-coordinate, |
---|
| 875 | and the sum \autoref{eq:LDF_slp_geo} + \autoref{eq:LDF_slp_iso} in $z^*$ or $s$-coordinates. |
---|
[2282] | 876 | |
---|
[3294] | 877 | The eddy induced velocity is given by: |
---|
[10414] | 878 | \begin{subequations} |
---|
[11543] | 879 | % \label{eq:TRIADS_eiv} |
---|
[10414] | 880 | \begin{equation} |
---|
[11543] | 881 | \label{eq:TRIADS_eiv_v} |
---|
[10414] | 882 | \begin{split} |
---|
| 883 | u^* & = - \frac{1}{e_{3}}\; \partial_i\psi_1, \\ |
---|
| 884 | v^* & = - \frac{1}{e_{3}}\; \partial_j\psi_2, \\ |
---|
| 885 | w^* & = \frac{1}{e_{1}e_{2}}\; \left\{ \partial_i \left( e_{2} \, \psi_1\right) |
---|
| 886 | + \partial_j \left( e_{1} \, \psi_2\right) \right\}, |
---|
| 887 | \end{split} |
---|
| 888 | \end{equation} |
---|
| 889 | where the streamfunctions $\psi_i$ are given by |
---|
| 890 | \begin{equation} |
---|
[11543] | 891 | \label{eq:TRIADS_eiv_psi} |
---|
[10414] | 892 | \begin{split} |
---|
| 893 | \psi_1 & = A_{e} \; \tilde{r}_1, \\ |
---|
| 894 | \psi_2 & = A_{e} \; \tilde{r}_2, |
---|
| 895 | \end{split} |
---|
| 896 | \end{equation} |
---|
[3294] | 897 | \end{subequations} |
---|
[10354] | 898 | with $A_{e}$ the eddy induced velocity coefficient, |
---|
| 899 | and $\tilde{r}_1$ and $\tilde{r}_2$ the slopes between the iso-neutral and the geopotential surfaces. |
---|
[2282] | 900 | |
---|
[10354] | 901 | The traditional way to implement this additional advection is to add it to the Eulerian velocity prior to |
---|
| 902 | computing the tracer advection. |
---|
[11435] | 903 | This is implemented if \texttt{traldf\_eiv?} is set in the default implementation, |
---|
[11577] | 904 | where \np{ln_traldf_triad}{ln\_traldf\_triad} is set false. |
---|
[10354] | 905 | This allows us to take advantage of all the advection schemes offered for the tracers |
---|
| 906 | (see \autoref{sec:TRA_adv}) and not just a $2^{nd}$ order advection scheme. |
---|
| 907 | This is particularly useful for passive tracers where |
---|
| 908 | \emph{positivity} of the advection scheme is of paramount importance. |
---|
[2282] | 909 | |
---|
[11577] | 910 | However, when \np{ln_traldf_triad}{ln\_traldf\_triad} is set true, |
---|
[11435] | 911 | \NEMO\ instead implements eddy induced advection according to the so-called skew form \citep{griffies_JPO98}. |
---|
[10354] | 912 | It is based on a transformation of the advective fluxes using the non-divergent nature of the eddy induced velocity. |
---|
| 913 | For example in the (\textbf{i},\textbf{k}) plane, |
---|
| 914 | the tracer advective fluxes per unit area in $ijk$ space can be transformed as follows: |
---|
[2282] | 915 | \begin{flalign*} |
---|
[10414] | 916 | \begin{split} |
---|
| 917 | \textbf{F}_{\mathrm{eiv}}^T = |
---|
| 918 | \begin{pmatrix} |
---|
| 919 | {e_{2}\,e_{3}\; u^*} \\ |
---|
| 920 | {e_{1}\,e_{2}\; w^*} |
---|
| 921 | \end{pmatrix} \; T |
---|
| 922 | &= |
---|
| 923 | \begin{pmatrix} |
---|
| 924 | { - \partial_k \left( e_{2} \,\psi_1 \right) \; T \;} \\ |
---|
| 925 | {+ \partial_i \left( e_{2} \, \psi_1 \right) \; T \;} |
---|
| 926 | \end{pmatrix} \\ |
---|
| 927 | &= |
---|
| 928 | \begin{pmatrix} |
---|
| 929 | { - \partial_k \left( e_{2} \, \psi_1 \; T \right) \;} \\ |
---|
| 930 | {+ \partial_i \left( e_{2} \,\psi_1 \; T \right) \;} |
---|
| 931 | \end{pmatrix} |
---|
| 932 | + |
---|
| 933 | \begin{pmatrix} |
---|
| 934 | {+ e_{2} \, \psi_1 \; \partial_k T} \\ |
---|
| 935 | { - e_{2} \, \psi_1 \; \partial_i T} |
---|
| 936 | \end{pmatrix} |
---|
| 937 | \end{split} |
---|
[2282] | 938 | \end{flalign*} |
---|
[10354] | 939 | and since the eddy induced velocity field is non-divergent, |
---|
| 940 | we end up with the skew form of the eddy induced advective fluxes per unit area in $ijk$ space: |
---|
[10414] | 941 | \begin{equation} |
---|
[11543] | 942 | \label{eq:TRIADS_eiv_skew_ijk} |
---|
[10414] | 943 | \textbf{F}_\mathrm{eiv}^T = |
---|
| 944 | \begin{pmatrix} |
---|
| 945 | {+ e_{2} \, \psi_1 \; \partial_k T} \\ |
---|
| 946 | { - e_{2} \, \psi_1 \; \partial_i T} |
---|
| 947 | \end{pmatrix} |
---|
[2282] | 948 | \end{equation} |
---|
[3294] | 949 | The total fluxes per unit physical area are then |
---|
[10414] | 950 | \begin{equation} |
---|
[11543] | 951 | \label{eq:TRIADS_eiv_skew_physical} |
---|
[10414] | 952 | \begin{split} |
---|
| 953 | f^*_1 & = \frac{1}{e_{3}}\; \psi_1 \partial_k T \\ |
---|
| 954 | f^*_2 & = \frac{1}{e_{3}}\; \psi_2 \partial_k T \\ |
---|
| 955 | f^*_3 & = -\frac{1}{e_{1}e_{2}}\; \left\{ e_{2} \psi_1 \partial_i T + e_{1} \psi_2 \partial_j T \right\}. |
---|
[3294] | 956 | \end{split} |
---|
| 957 | \end{equation} |
---|
[11543] | 958 | Note that \autoref{eq:TRIADS_eiv_skew_physical} takes the same form whatever the vertical coordinate, |
---|
| 959 | though of course the slopes $\tilde{r}_i$ which define the $\psi_i$ in \autoref{eq:TRIADS_eiv_psi} are relative to |
---|
[10354] | 960 | geopotentials. |
---|
| 961 | The tendency associated with eddy induced velocity is then simply the convergence of the fluxes |
---|
[11543] | 962 | (\autoref{eq:TRIADS_eiv_skew_ijk}, \autoref{eq:TRIADS_eiv_skew_physical}), so |
---|
[10414] | 963 | \[ |
---|
[11543] | 964 | % \label{eq:TRIADS_skew_eiv_conv} |
---|
[10414] | 965 | \frac{\partial T}{\partial t}= -\frac{1}{e_1 \, e_2 \, e_3 } \left[ |
---|
| 966 | \frac{\partial}{\partial i} \left( e_2 \psi_1 \partial_k T\right) |
---|
| 967 | + \frac{\partial}{\partial j} \left( e_1 \; |
---|
| 968 | \psi_2 \partial_k T\right) |
---|
| 969 | - \frac{\partial}{\partial k} \left( e_{2} \psi_1 \partial_i T |
---|
| 970 | + e_{1} \psi_2 \partial_j T \right) \right] |
---|
| 971 | \] |
---|
[10354] | 972 | It naturally conserves the tracer content, as it is expressed in flux form. |
---|
| 973 | Since it has the same divergence as the advective form it also preserves the tracer variance. |
---|
[2282] | 974 | |
---|
[11597] | 975 | %% ================================================================================================= |
---|
[9393] | 976 | \subsection{Discrete skew flux formulation} |
---|
[10414] | 977 | |
---|
[11543] | 978 | The skew fluxes in (\autoref{eq:TRIADS_eiv_skew_physical}, \autoref{eq:TRIADS_eiv_skew_ijk}), |
---|
| 979 | like the off-diagonal terms (\autoref{eq:TRIADS_i13c}, \autoref{eq:TRIADS_i31c}) of the small angle diffusion tensor, |
---|
| 980 | are best expressed in terms of the triad slopes, as in \autoref{fig:TRIADS_ISO_triad} and |
---|
| 981 | (\autoref{eq:TRIADS_i13}, \autoref{eq:TRIADS_i31}); |
---|
[10354] | 982 | but now in terms of the triad slopes $\tilde{\mathbb{R}}$ relative to geopotentials instead of |
---|
| 983 | the $\mathbb{R}$ relative to coordinate surfaces. |
---|
[11543] | 984 | The discrete form of \autoref{eq:TRIADS_eiv_skew_ijk} using the slopes \autoref{eq:TRIADS_R} and |
---|
[3294] | 985 | defining $A_e$ at $T$-points is then given by: |
---|
[2282] | 986 | |
---|
[10414] | 987 | \begin{subequations} |
---|
[11543] | 988 | % \label{eq:TRIADS_allskewflux} |
---|
[10414] | 989 | \begin{flalign*} |
---|
[11543] | 990 | % \label{eq:TRIADS_vect_skew_flux} |
---|
[10414] | 991 | \vect{F}_{\mathrm{eiv}}(T) &\equiv \sum_{\substack{i_p,\,k_p}} |
---|
[3294] | 992 | \begin{pmatrix} |
---|
[10414] | 993 | {_{i+1/2-i_p}^k {\mathbb{S}_u}_{i_p}^{k_p} } (T) \\ \\ |
---|
[3294] | 994 | {_i^{k+1/2-k_p} {\mathbb{S}_w}_{i_p}^{k_p} } (T) \\ |
---|
| 995 | \end{pmatrix}, |
---|
[10414] | 996 | \end{flalign*} |
---|
[11543] | 997 | where the skew flux in the $i$-direction associated with a given triad is (\autoref{eq:TRIADS_latflux-triad}, |
---|
| 998 | \autoref{eq:TRIADS_triadfluxu}): |
---|
[3294] | 999 | \begin{align} |
---|
[11543] | 1000 | \label{eq:TRIADS_skewfluxu} |
---|
[10442] | 1001 | _i^k {\mathbb{S}_u}_{i_p}^{k_p} (T) &= + \fractext{1}{4} {A_e}_i^k{ |
---|
[10414] | 1002 | \:}\frac{{b_u}_{i+i_p}^k}{{e_{1u}}_{\,i + i_p}^{\,k}} |
---|
| 1003 | \ {_i^k\tilde{\mathbb{R}}_{i_p}^{k_p}} \ |
---|
| 1004 | \frac{ \delta_{k+k_p} [T^i] }{{e_{3w}}_{\,i}^{\,k+k_p} }, \\ |
---|
| 1005 | \intertext{ |
---|
[11543] | 1006 | and \autoref{eq:TRIADS_triadfluxw} in the $k$-direction, changing the sign |
---|
| 1007 | to be consistent with \autoref{eq:TRIADS_eiv_skew_ijk}: |
---|
[10414] | 1008 | } |
---|
[3294] | 1009 | _i^k {\mathbb{S}_w}_{i_p}^{k_p} (T) |
---|
[10442] | 1010 | &= -\fractext{1}{4} {A_e}_i^k{\: }\frac{{b_u}_{i+i_p}^k}{{e_{3w}}_{\,i}^{\,k+k_p}} |
---|
[11543] | 1011 | {_i^k\tilde{\mathbb{R}}_{i_p}^{k_p}}\frac{ \delta_{i+ i_p}[T^k] }{ {e_{1u}}_{\,i + i_p}^{\,k} }.\label{eq:TRIADS_skewfluxw} |
---|
[3294] | 1012 | \end{align} |
---|
| 1013 | \end{subequations} |
---|
[2282] | 1014 | |
---|
[10354] | 1015 | Such a discretisation is consistent with the iso-neutral operator as it uses the same definition for the slopes. |
---|
| 1016 | It also ensures the following two key properties. |
---|
[9393] | 1017 | |
---|
[11597] | 1018 | %% ================================================================================================= |
---|
[3294] | 1019 | \subsubsection{No change in tracer variance} |
---|
[10414] | 1020 | |
---|
[11435] | 1021 | The discretization conserves tracer variance, \ie\ it does not include a diffusive component but is a `pure' advection term. |
---|
[10354] | 1022 | This can be seen %either from Appendix \autoref{apdx:eiv_skew} or |
---|
| 1023 | by considering the fluxes associated with a given triad slope $_i^k{\mathbb{R}}_{i_p}^{k_p} (T)$. |
---|
[11543] | 1024 | For, following \autoref{subsec:TRIADS_variance} and \autoref{eq:TRIADS_dvar_iso_i}, |
---|
[10354] | 1025 | the associated horizontal skew-flux $_i^k{\mathbb{S}_u}_{i_p}^{k_p} (T)$ drives a net rate of change of variance, |
---|
[10442] | 1026 | summed over the two $T$-points $i+i_p-\fractext{1}{2},k$ and $i+i_p+\fractext{1}{2},k$, of |
---|
[3294] | 1027 | \begin{equation} |
---|
[11543] | 1028 | \label{eq:TRIADS_dvar_eiv_i} |
---|
[3294] | 1029 | _i^k{\mathbb{S}_u}_{i_p}^{k_p} (T)\,\delta_{i+ i_p}[T^k], |
---|
| 1030 | \end{equation} |
---|
[10354] | 1031 | while the associated vertical skew-flux gives a variance change summed over |
---|
[10442] | 1032 | the $T$-points $i,k+k_p-\fractext{1}{2}$ (above) and $i,k+k_p+\fractext{1}{2}$ (below) of |
---|
[3294] | 1033 | \begin{equation} |
---|
[11543] | 1034 | \label{eq:TRIADS_dvar_eiv_k} |
---|
[3294] | 1035 | _i^k{\mathbb{S}_w}_{i_p}^{k_p} (T) \,\delta_{k+ k_p}[T^i]. |
---|
| 1036 | \end{equation} |
---|
[11543] | 1037 | Inspection of the definitions (\autoref{eq:TRIADS_skewfluxu}, \autoref{eq:TRIADS_skewfluxw}) shows that |
---|
| 1038 | these two variance changes (\autoref{eq:TRIADS_dvar_eiv_i}, \autoref{eq:TRIADS_dvar_eiv_k}) sum to zero. |
---|
[10354] | 1039 | Hence the two fluxes associated with each triad make no net contribution to the variance budget. |
---|
[2282] | 1040 | |
---|
[11597] | 1041 | %% ================================================================================================= |
---|
[3294] | 1042 | \subsubsection{Reduction in gravitational PE} |
---|
[10414] | 1043 | |
---|
[10354] | 1044 | The vertical density flux associated with the vertical skew-flux always has the same sign as |
---|
| 1045 | the vertical density gradient; |
---|
| 1046 | thus, so long as the fluid is stable (the vertical density gradient is negative) |
---|
| 1047 | the vertical density flux is negative (downward) and hence reduces the gravitational PE. |
---|
[2282] | 1048 | |
---|
[3294] | 1049 | For the change in gravitational PE driven by the $k$-flux is |
---|
| 1050 | \begin{align} |
---|
[11543] | 1051 | \label{eq:TRIADS_vert_densityPE} |
---|
[3294] | 1052 | g {e_{3w}}_{\,i}^{\,k+k_p}{\mathbb{S}_w}_{i_p}^{k_p} (\rho) |
---|
| 1053 | &=g {e_{3w}}_{\,i}^{\,k+k_p}\left[-\alpha _i^k {\:}_i^k |
---|
| 1054 | {\mathbb{S}_w}_{i_p}^{k_p} (T) + \beta_i^k {\:}_i^k |
---|
| 1055 | {\mathbb{S}_w}_{i_p}^{k_p} (S) \right]. \notag \\ |
---|
[11543] | 1056 | \intertext{Substituting ${\:}_i^k {\mathbb{S}_w}_{i_p}^{k_p}$ from \autoref{eq:TRIADS_skewfluxw}, gives} |
---|
[10414] | 1057 | % and separating out |
---|
| 1058 | % $\rtriadt{R}=\rtriad{R} + \delta_{i+i_p}[z_T^k]$, |
---|
| 1059 | % gives two terms. The |
---|
| 1060 | % first $\rtriad{R}$ term (the only term for $z$-coordinates) is: |
---|
[10442] | 1061 | &=-\fractext{1}{4} g{A_e}_i^k{\: }{b_u}_{i+i_p}^k {_i^k\tilde{\mathbb{R}}_{i_p}^{k_p}} |
---|
[10414] | 1062 | \frac{ -\alpha _i^k\delta_{i+ i_p}[T^k]+ \beta_i^k\delta_{i+ i_p}[S^k]} { {e_{1u}}_{\,i + i_p}^{\,k} } \notag \\ |
---|
[10442] | 1063 | &=+\fractext{1}{4} g{A_e}_i^k{\: }{b_u}_{i+i_p}^k |
---|
[10414] | 1064 | \left({_i^k\mathbb{R}_{i_p}^{k_p}}+\frac{\delta_{i+i_p}[z_T^k]}{{e_{1u}}_{\,i + i_p}^{\,k}}\right) {_i^k\mathbb{R}_{i_p}^{k_p}} |
---|
| 1065 | \frac{-\alpha_i^k \delta_{k+ k_p}[T^i]+ \beta_i^k\delta_{k+ k_p}[S^i]} {{e_{3w}}_{\,i}^{\,k+k_p}}, |
---|
[3294] | 1066 | \end{align} |
---|
[11543] | 1067 | using the definition of the triad slope $\rtriad{R}$, \autoref{eq:TRIADS_R} to |
---|
[10354] | 1068 | express $-\alpha _i^k\delta_{i+ i_p}[T^k]+\beta_i^k\delta_{i+ i_p}[S^k]$ in terms of |
---|
| 1069 | $-\alpha_i^k \delta_{k+ k_p}[T^i]+ \beta_i^k\delta_{k+ k_p}[S^i]$. |
---|
[2282] | 1070 | |
---|
[3294] | 1071 | Where the coordinates slope, the $i$-flux gives a PE change |
---|
| 1072 | \begin{multline} |
---|
[11543] | 1073 | \label{eq:TRIADS_lat_densityPE} |
---|
[10414] | 1074 | g \delta_{i+i_p}[z_T^k] |
---|
| 1075 | \left[ |
---|
| 1076 | -\alpha _i^k {\:}_i^k {\mathbb{S}_u}_{i_p}^{k_p} (T) + \beta_i^k {\:}_i^k {\mathbb{S}_u}_{i_p}^{k_p} (S) |
---|
| 1077 | \right] \\ |
---|
[10442] | 1078 | = +\fractext{1}{4} g{A_e}_i^k{\: }{b_u}_{i+i_p}^k |
---|
[10414] | 1079 | \frac{\delta_{i+i_p}[z_T^k]}{{e_{1u}}_{\,i + i_p}^{\,k}} |
---|
| 1080 | \left({_i^k\mathbb{R}_{i_p}^{k_p}}+\frac{\delta_{i+i_p}[z_T^k]}{{e_{1u}}_{\,i + i_p}^{\,k}}\right) |
---|
| 1081 | \frac{-\alpha_i^k \delta_{k+ k_p}[T^i]+ \beta_i^k\delta_{k+ k_p}[S^i]} {{e_{3w}}_{\,i}^{\,k+k_p}}, |
---|
[3294] | 1082 | \end{multline} |
---|
[11543] | 1083 | (using \autoref{eq:TRIADS_skewfluxu}) and so the total PE change \autoref{eq:TRIADS_vert_densityPE} + |
---|
| 1084 | \autoref{eq:TRIADS_lat_densityPE} associated with the triad fluxes is |
---|
[10414] | 1085 | \begin{multline*} |
---|
[11543] | 1086 | % \label{eq:TRIADS_tot_densityPE} |
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[3294] | 1087 | g{e_{3w}}_{\,i}^{\,k+k_p}{\mathbb{S}_w}_{i_p}^{k_p} (\rho) + |
---|
[10414] | 1088 | g\delta_{i+i_p}[z_T^k] {\:}_i^k {\mathbb{S}_u}_{i_p}^{k_p} (\rho) \\ |
---|
[10442] | 1089 | = +\fractext{1}{4} g{A_e}_i^k{\: }{b_u}_{i+i_p}^k |
---|
[10414] | 1090 | \left({_i^k\mathbb{R}_{i_p}^{k_p}}+\frac{\delta_{i+i_p}[z_T^k]}{{e_{1u}}_{\,i + i_p}^{\,k}}\right)^2 |
---|
| 1091 | \frac{-\alpha_i^k \delta_{k+ k_p}[T^i]+ \beta_i^k\delta_{k+ k_p}[S^i]} {{e_{3w}}_{\,i}^{\,k+k_p}}. |
---|
| 1092 | \end{multline*} |
---|
[3294] | 1093 | Where the fluid is stable, with $-\alpha_i^k \delta_{k+ k_p}[T^i]+ |
---|
| 1094 | \beta_i^k\delta_{k+ k_p}[S^i]<0$, this PE change is negative. |
---|
[2282] | 1095 | |
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[11597] | 1096 | %% ================================================================================================= |
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[10414] | 1097 | \subsection{Treatment of the triads at the boundaries} |
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[11543] | 1098 | \label{sec:TRIADS_skew_bdry} |
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[10414] | 1099 | |
---|
[11543] | 1100 | Triad slopes \rtriadt{R} used for the calculation of the eddy-induced skew-fluxes are masked at the boundaries |
---|
| 1101 | in exactly the same way as are the triad slopes \rtriad{R} used for the iso-neutral diffusive fluxes, |
---|
| 1102 | as described in \autoref{sec:TRIADS_iso_bdry} and \autoref{fig:TRIADS_bdry_triads}. |
---|
| 1103 | Thus surface layer triads $\triadt{i}{1}{R}{1/2}{-1/2}$ and $\triadt{i+1}{1}{R}{-1/2}{-1/2}$ are masked, |
---|
| 1104 | and both near bottom triad slopes $\triadt{i}{k}{R}{1/2}{1/2}$ and $\triadt{i+1}{k}{R}{-1/2}{1/2}$ are masked when |
---|
| 1105 | either of the $i,k+1$ or $i+1,k+1$ tracer points is masked, \ie\ the $i,k+1$ $u$-point is masked. |
---|
[11577] | 1106 | The namelist parameter \np{ln_botmix_triad}{ln\_botmix\_triad} has no effect on the eddy-induced skew-fluxes. |
---|
[2282] | 1107 | |
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[11597] | 1108 | %% ================================================================================================= |
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[10414] | 1109 | \subsection{Limiting of the slopes within the interior} |
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[11543] | 1110 | \label{sec:TRIADS_limitskew} |
---|
[10414] | 1111 | |
---|
[11543] | 1112 | Presently, the iso-neutral slopes $\tilde{r}_i$ relative to geopotentials are limited to be less than $1/100$, |
---|
| 1113 | exactly as in calculating the iso-neutral diffusion, \S \autoref{sec:TRIADS_limit}. |
---|
[10354] | 1114 | Each individual triad \rtriadt{R} is so limited. |
---|
[2282] | 1115 | |
---|
[11597] | 1116 | %% ================================================================================================= |
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[10414] | 1117 | \subsection{Tapering within the surface mixed layer} |
---|
[11543] | 1118 | \label{sec:TRIADS_taperskew} |
---|
[10414] | 1119 | |
---|
[11543] | 1120 | The slopes $\tilde{r}_i$ relative to geopotentials (and thus the individual triads \rtriadt{R}) |
---|
| 1121 | are always tapered linearly from their value immediately below the mixed layer to zero at the surface |
---|
| 1122 | \autoref{eq:TRIADS_rmtilde}, as described in \autoref{sec:TRIADS_lintaper}. |
---|
| 1123 | This is option (c) of \autoref{fig:LDF_eiv_slp}. |
---|
| 1124 | This linear tapering for the slopes used to calculate the eddy-induced fluxes is unaffected by |
---|
[11577] | 1125 | the value of \np{ln_triad_iso}{ln\_triad\_iso}. |
---|
[2282] | 1126 | |
---|
[10354] | 1127 | The justification for this linear slope tapering is that, for $A_e$ that is constant or varies only in |
---|
| 1128 | the horizontal (the most commonly used options in \NEMO: see \autoref{sec:LDF_coef}), |
---|
| 1129 | it is equivalent to a horizontal eiv (eddy-induced velocity) that is uniform within the mixed layer |
---|
[11543] | 1130 | \autoref{eq:TRIADS_eiv_v}. |
---|
[11123] | 1131 | This ensures that the eiv velocities do not restratify the mixed layer \citep{treguier.held.ea_JPO97,danabasoglu.ferrari.ea_JC08}. |
---|
[10354] | 1132 | Equivantly, in terms of the skew-flux formulation we use here, |
---|
| 1133 | the linear slope tapering within the mixed-layer gives a linearly varying vertical flux, |
---|
| 1134 | and so a tracer convergence uniform in depth |
---|
| 1135 | (the horizontal flux convergence is relatively insignificant within the mixed-layer). |
---|
[3294] | 1136 | |
---|
[11597] | 1137 | %% ================================================================================================= |
---|
[10414] | 1138 | \subsection{Streamfunction diagnostics} |
---|
[11543] | 1139 | \label{sec:TRIADS_sfdiag} |
---|
[10414] | 1140 | |
---|
[11582] | 1141 | Where the namelist parameter \np[=.true.]{ln_traldf_gdia}{ln\_traldf\_gdia}, |
---|
[10354] | 1142 | diagnosed mean eddy-induced velocities are output. |
---|
| 1143 | Each time step, streamfunctions are calculated in the $i$-$k$ and $j$-$k$ planes at |
---|
| 1144 | $uw$ (integer +1/2 $i$, integer $j$, integer +1/2 $k$) and $vw$ (integer $i$, integer +1/2 $j$, integer +1/2 $k$) |
---|
[11543] | 1145 | points (see Table \autoref{tab:DOM_cell}) respectively. |
---|
[11123] | 1146 | We follow \citep{griffies_bk04} and calculate the streamfunction at a given $uw$-point from |
---|
[10354] | 1147 | the surrounding four triads according to: |
---|
[10414] | 1148 | \[ |
---|
[11543] | 1149 | % \label{eq:TRIADS_sfdiagi} |
---|
[10442] | 1150 | {\psi_1}_{i+1/2}^{k+1/2}={\fractext{1}{4}}\sum_{\substack{i_p,\,k_p}} |
---|
[3294] | 1151 | {A_e}_{i+1/2-i_p}^{k+1/2-k_p}\:\triadd{i+1/2-i_p}{k+1/2-k_p}{R}{i_p}{k_p}. |
---|
[10414] | 1152 | \] |
---|
[3294] | 1153 | The streamfunction $\psi_1$ is calculated similarly at $vw$ points. |
---|
[11543] | 1154 | The eddy-induced velocities are then calculated from the straightforward discretisation of \autoref{eq:TRIADS_eiv_v}: |
---|
[10414] | 1155 | \[ |
---|
[11543] | 1156 | % \label{eq:TRIADS_eiv_v_discrete} |
---|
[10414] | 1157 | \begin{split} |
---|
| 1158 | {u^*}_{i+1/2}^{k} & = - \frac{1}{{e_{3u}}_{i}^{k}}\left({\psi_1}_{i+1/2}^{k+1/2}-{\psi_1}_{i+1/2}^{k+1/2}\right), \\ |
---|
| 1159 | {v^*}_{j+1/2}^{k} & = - \frac{1}{{e_{3v}}_{j}^{k}}\left({\psi_2}_{j+1/2}^{k+1/2}-{\psi_2}_{j+1/2}^{k+1/2}\right), \\ |
---|
| 1160 | {w^*}_{i,j}^{k+1/2} & = \frac{1}{e_{1t}e_{2t}}\; \left\{ |
---|
| 1161 | {e_{2u}}_{i+1/2}^{k+1/2} \,{\psi_1}_{i+1/2}^{k+1/2} - |
---|
| 1162 | {e_{2u}}_{i-1/2}^{k+1/2} \,{\psi_1}_{i-1/2}^{k+1/2} \right. + \\ |
---|
| 1163 | \phantom{=} & \qquad\qquad\left. {e_{2v}}_{j+1/2}^{k+1/2} \,{\psi_2}_{j+1/2}^{k+1/2} - {e_{2v}}_{j-1/2}^{k+1/2} \,{\psi_2}_{j-1/2}^{k+1/2} \right\}, |
---|
| 1164 | \end{split} |
---|
| 1165 | \] |
---|
| 1166 | |
---|
[11693] | 1167 | \subinc{\input{../../global/epilogue}} |
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[10414] | 1168 | |
---|
[6997] | 1169 | \end{document} |
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