# Changeset 11445

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Timestamp:
2019-08-19T09:18:20+02:00 (8 months ago)
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#2216 revision of LBC chapter: main ones in bdy description and geometry

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 r11435 \end{description} Note that when the bottom topography is entirely represented by the $s$-coor-dinates (pure $s$-coordinate), Note that when the bottom topography is entirely represented by the $s$-coordinates (pure $s$-coordinate), the lateral boundary condition on tangential velocity is of much less importance as it is only applied next to the coast where the minimum water depth can be quite shallow. Options are defined through the \nam{bdy} \nam{bdy\_dta} namelist variables. The BDY module is the core implementation of open boundary conditions for regional configurations on temperature, salinity, barotropic and baroclinic velocities, as well as ice concentration, ice and snow thicknesses). temperature, salinity, barotropic and baroclinic velocities, as well as ice concentration, ice and snow thicknesses. The BDY module was modelled on the OBC module (see \NEMO\ 3.4) and shares many features and a similar coding structure \citep{chanut_rpt05}. The specification of the location of the open boundary is completely flexible and allows for example the open boundary to follow an isobath or other irregular contour. allows any type of setup, from regular boundaries to irregular contour (it includes the possibility to set an open boundary able to follow an isobath). Boundary data files used with versions of \NEMO\ prior to Version 3.4 may need to be re-ordered to work with this version. See the section on the Input Boundary Data Files for details. If external boundary data is required then the \nam{bdy\_dta} namelist must be defined. One \nam{bdy\_dta} namelist is required for each boundary set in the order in which the boundary sets are defined in nambdy. One \nam{bdy\_dta} namelist is required for each boundary set, adopting the same order of indexes in which the boundary sets are defined in nambdy. In the example given, two boundary sets have been defined. The first one is reading data file in the \nam{bdy\_dta} namelist shown above and the second one is using data from intial condition (no namelist bloc needed). and the second one is using data from intial condition (no namelist block needed). The boundary data is read in using the fldread module, so the \nam{bdy\_dta} namelist is in the format required for fldread. For each variable required, the filename, the frequency of the files and the frequency of the data in the files is given. For each required variable, the filename, the frequency of the files and the frequency of the data in the files are given. Also whether or not time-interpolation is required and whether the data is climatological (time-cyclic) data.\\ the latter used to make any adjustment to the velocity fields due to differences in the total water depths between the two vertical grids.\\ In the example namelists given, two boundary sets are defined. In the example of given namelists, two boundary sets are defined. The first set is defined via a file and applies FRS conditions to temperature and salinity and Flather conditions to the barotropic variables. No condition specified for the baroclinic velocity and sea-ice. Each open boundary set is defined as a list of points. The information is stored in the arrays $nbi$, $nbj$, and $nbr$ in the $idx\_bdy$ structure. The $nbi$ and $nbj$ arrays define the local $(i,j)$ indices of each point in the boundary zone and the $nbr$ array defines the discrete distance from the boundary with $nbr=1$ meaning that the point is next to the edge of the model domain and $nbr>1$ showing that the point is increasingly further away from the edge of the model domain. The $nbi$ and $nbj$ arrays define the local $(i,j)$ indexes of each point in the boundary zone and the $nbr$ array defines the discrete distance from the boundary: $nbr=1$ means that the boundary point is next to the edge of the model domain, while $nbr>1$ means that the boundary point is increasingly further away from the edge of the model domain. A set of $nbi$, $nbj$, and $nbr$ arrays is defined for each of the $T$, $U$ and $V$ grids. Figure \autoref{fig:LBC_bdy_geom} shows an example of an irregular boundary. by reading in a \ifile{coordinates.bdy}'' file. The nambdy\_index namelist defines a series of straight-line segments for north, east, south and west boundaries. One nambdy\_index namelist bloc is needed for each boundary condition defined by indexes. One nambdy\_index namelist block is needed for each boundary condition defined by indexes. For the northern boundary, \texttt{nbdysegn} gives the number of segments, \jp{jpjnob} gives the $j$ index for each segment and \jp{jpindt} and The boundary geometry may also be defined from a \ifile{coordinates.bdy}'' file. Figure \autoref{fig:LBC_nc_header} gives an example of the header information from such a file. Figure \autoref{fig:LBC_nc_header} gives an example of the header information from such a file, based on the description of geometrical setup given above. The file should contain the index arrays for each of the $T$, $U$ and $V$ grids. The arrays must be in order of increasing $nbr$. For some choices of irregular boundary the model domain may contain areas of ocean which are not part of the computational domain. For example if an open boundary is defined along an isobath, say at the shelf break, For example, if an open boundary is defined along an isobath, say at the shelf break, then the areas of ocean outside of this boundary will need to be masked out. This can be done by reading a mask file defined as \np{cn\_mask\_file} in the nam\_bdy namelist.