# Changeset 11316 for NEMO/trunk/doc/latex/NEMO/subfiles/chap_ASM.tex

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Timestamp:
2019-07-19T19:20:02+02:00 (14 months ago)
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#2297 Updates to OBS and ASM documentation

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• ## NEMO/trunk/doc/latex/NEMO/subfiles/chap_ASM.tex

 r11151 \label{chap:ASM} Authors: D. Lea,  M. Martin, K. Mogensen, A. Weaver, ...   % do we keep \minitoc \minitoc \vfill \begin{figure}[b] \subsubsection*{Changes record} \begin{tabular}{l||l|m{0.65\linewidth}} Release   & Author        & Modifications \\ {\em 4.0} & {\em D. J. Lea} & {\em \NEMO 4.0 updates}  \\ {\em 3.4} & {\em D. J. Lea, M. Martin, K. Mogensen, A. Weaver} & {\em Initial version}  \\ \end{tabular} \end{figure} \newpage The ASM code adds the functionality to apply increments to the model variables: temperature, salinity, sea surface height, velocity and sea ice concentration. sea surface height, velocity and sea ice concentration. These are read into the model from a NetCDF file which may be produced by separate data assimilation code. The code can also output model background fields which are used as an input to data assimilation code. Typically the increments are spread evenly over the full window. In addition, two different weighting functions have been implemented. The first function employs constant weights, The first function (namelist option \np{niaufn} = 0) employs constant weights, \begin{align} \label{eq:F1_i} 0     &    {\mathrm if} \; \; \; t_{i} > t_{n} \end{array} \right. \right. \end{align} where $M = m-n$. The second function employs peaked hat-like weights in order to give maximum weight in the centre of the sub-window, The second function (namelist option \np{niaufn} = 1) employs peaked hat-like weights in order to give maximum weight in the centre of the sub-window, with the weighting reduced linearly to a small value at the window end-points: \begin{align} \right. \end{align} where $\alpha^{-1} = \sum_{i=1}^{M/2} 2i$ and $M$ is assumed to be even. where $\alpha^{-1} = \sum_{i=1}^{M/2} 2i$ and $M$ is assumed to be even. The weights described by \autoref{eq:F2_i} provide a smoother transition of the analysis trajectory from one assimilation cycle to the next than that described by \autoref{eq:F1_i}. \label{sec:ASM_div_dmp} The velocity increments may be initialized by the iterative application of a divergence damping operator. In iteration step $n$ new estimates of velocity increments $u^{n}_I$ and $v^{n}_I$ are updated by: It is quite challenging for data assimilation systems to provide non-divergent velocity increments. Applying divergent velocity increments will likely cause spurious vertical velocities in the model. This section describes a method to take velocity increments provided to \NEMO ($u^0_I$ and $v^0_I$) and adjust them by the iterative application of a divergence damping operator. The method is also described in \citet{dobricic.pinardi.ea_OS07}. In iteration step $n$ (starting at $n=1$) new estimates of velocity increments $u^{n}_I$ and $v^{n}_I$ are updated by: \label{eq:asm_dmp} \right., where where the divergence is defined as $% \label{eq:asm_div} +\delta_j \left[ {e_{1v}\,e_{3v}\,v^{n-1}_I} \right]} \right).$ By the application of \autoref{eq:asm_dmp} and \autoref{eq:asm_dmp} the divergence is filtered in each iteration, By the application of \autoref{eq:asm_dmp} the divergence is filtered in each iteration, and the vorticity is left unchanged. In the presence of coastal boundaries with zero velocity increments perpendicular to the coast The divergence damping is activated by assigning to \np{nn\_divdmp} in the \textit{nam\_asminc} namelist a value greater than zero. By choosing this value to be of the order of 100 the increments in the vertical velocity will be significantly reduced. This specifies the number of iterations of the divergence damping. Setting a value of the order of 100 will result in a significant reduction in the vertical velocity induced by the increments. \label{sec:ASM_details} Here we show an example \ngn{namasm} namelist and the header of an example assimilation increments file on Here we show an example \ngn{nam\_asminc} namelist and the header of an example assimilation increments file on the ORCA2 grid. %------------------------------------------namasm----------------------------------------------------- %------------------------------------------nam_asminc----------------------------------------------------- % \nlst{nam_asminc}
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