WIP update developer's guide

docs/developers_guide/ocean/tasks/nondivergent_2d.md

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+(dev-ocean-nondivergent-2d)=
+
+# nondivergent_2d
+
+The {py:class}`polaris.ocean.tasks.sphere_transport.SphereTransport`
+`nondivergent_2d` test performs a 12-day run on the sphere that has a periodic
+deforming flow which affects tracer distributions. The resolution of the
+sphere varies (by default, between 60 and 240 km). After one period, the
+tracer distributions are compared the initial condition to evaluate numerical
+errors associated with the horizontal advection scheme and determine the rate
+of convergence.
+
+## framework
+
+The config options for the `nondivergent_2d` test is described in 
+{ref}`ocean-nondivergent-2d` in the User's Guide.
+
+Additionally, the test uses a `forward.yaml` file with a few common
+model config options related to drag and default horizontal and
+vertical momentum and tracer diffusion, as well as defining `mesh`, `input`,
+`restart`, and `output` streams.  This file has Jinja templating that is
+used to update model config options based on Polaris config options, see
+{ref}`dev-ocean-spherical-convergence`.
+
+### base_mesh
+
+Sphere transport tasks use shared `base_mesh` steps for creating
+{ref}`dev-ocean-spherical-meshes` at a sequence of resolutions.
+
+### init
+
+The class {py:class}`polaris.ocean.tasks.sphere_transport.init.Init`
+defines a step for setting up the initial state at each resolution. The
+initial state is differentiated between `nondivergent_2d` and the other tests
+set up by the class.
+
+### forward
+
+The class {py:class}`polaris.ocean.tasks.sphere_transport.forward.Forward`
+descends from {py:class}`polaris.ocean.convergence.spherical.SphericalConvergenceForward`,
+and defines a step for running MPAS-Ocean from an initial condition produced in
+an `init` step. See {ref}`dev-ocean-spherical-convergence` for some relevant
+discussion of the parent class. The time step is determined from the resolution
+based on the `dt_per_km` config option in the `[spherical_convergences]` 
+section.  Other model config options are taken from `forward.yaml`.
+
+### analysis
+
+The class {py:class}`polaris.ocean.tasks.sphere_transport.analysis.Analysis`
+descends from
+{py:class}`polaris.ocean.convergence.spherical.SphericalConvergenceAnalysis`,
+and defines a step for computing the error norm (L2) for the results
+at each resolution for tracers and layer thickness, saving them in
+`convergence_*.csv` and plotting them in `convergence_*.png`.
+
+### viz
+
+Visualization steps are available only in the `nondivergent_2d/with_viz`
+tasks.  They are not included in the `nondivergent_2d` in order to keep regression
+as fast as possible when visualization isn't needed.
+
+The class {py:class}`polaris.ocean.tasks.sphere_transport.viz.VizMap`
+defines a step for creating a mapping file from the MPAS mesh at a given
+resolution to a lon-lat grid at a resolution and interpolation method 
+determined by config options.
+
+```cfg
+# options for visualization for the cosine bell convergence test case
+[sphere_transport_viz]
+
+# visualization latitude and longitude resolution
+dlon = 0.5
+dlat = 0.5
+
+# remapping method ('bilinear', 'neareststod', 'conserve')
+remap_method = conserve
+```
+
+The class {py:class}`polaris.ocean.tasks.sphere_transport.viz.Viz`
+is a step for plotting the initial and final states of the advection test for
+each resolution, mapped to the common lat-lon grid.  The colormap is controlled
+by these options:
+
+```cfg
+# options for visualization for the cosine bell convergence test case
+[sphere_transport_viz_*]
+
+# colormap options
+# colormap
+colormap_name = viridis
+
+# the type of norm used in the colormap
+norm_type = linear
+
+# A dictionary with keywords for the norm
+norm_args = {'vmin': 0., 'vmax': 1.}
+
+# We could provide colorbar tick marks but we'll leave the defaults
+# colorbar_ticks = np.linspace(0., 1., 9)
+```
+
+See {ref}`dev-visualization-global` for more details.
+
+TODO filament_analysis