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% $Header: /u/gcmpack/manual/part2/shap.tex,v 1.4 2001/11/06 15:07:32 adcroft Exp $ |
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% $Name: $ |
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\section{Shapiro Filter} |
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\label{sect:shapiro-filter} |
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|
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The Shapiro filter \cite{Shapiro_70} is a high order horizontal |
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filter that efficiently remove small scale grid noise |
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without affecting the physical structures of a field. |
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It is applied at the end of the time step %(the\_correction\_step), |
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on both velocity and tracer fields. |
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|
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Three different space operators are considered here (S1,S2 and S4). |
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They differs essentially by the sequence of derivative in |
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both X and Y directions. Consequently they show different |
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damping response function specially in the diagonal directions |
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X+Y and X-Y. |
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|
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Space derivatives can be computed in the real space, |
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taken into account the grid spacing. |
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Alternatively, a pure computational filter can be defined, |
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using pure numerical differences and ignoring |
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grid spacing. |
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This later form is stable whatever the grid is, and therefore |
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specially useful for highly anisotropic grid such as spherical |
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coordinate grid. |
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A damping time-scale parameter $\tau_{shap}$ |
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defines the strength of the filter damping. |
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|
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The 3 computational filter operators are : |
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$$ |
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\mathrm{S1c:}\hspace{2cm} |
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[1 - 1/2 \frac{\Delta t}{\tau_{shap}} |
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\{ (\frac{1}{4}\delta_{ii})^n |
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+ (\frac{1}{4}\delta_{jj})^n \} ] |
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$$ |
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|
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$$ |
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\mathrm{S2c:}\hspace{2cm} |
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[1 - \frac{\Delta t}{\tau_{shap}} |
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\{ \frac{1}{8} (\delta_{ii} + \delta_{jj}) \}^n] |
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$$ |
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|
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$$ |
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\mathrm{S4c:}\hspace{2cm} |
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[1 - \frac{\Delta t}{\tau_{shap}} (\frac{1}{4}\delta_{ii})^n] |
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[1 - \frac{\Delta t}{\tau_{shap}} (\frac{1}{4}\delta_{jj})^n] |
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$$ |
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|
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In addition, the S2 operator can easily be extended to |
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a physical space filter: |
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$$ |
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\mathrm{S2g:}\hspace{2cm} |
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[1 - \frac{\Delta t}{\tau_{shap}} |
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\{ \frac{L_{shap}^2}{8} \overline{\nabla}^2 \}^n] |
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$$ |
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|
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with the Laplacian operator $\overline{\nabla}^2 $ |
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and a length scale parameter $L_{shap}$. |
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The stability of this S2g filter requires |
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$L_{shap} < \mathrm{Min}^{(Global)}(\Delta x,\Delta y)$. |
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|
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\marginpar{Add Response functions and figures} |