CAM-SE works with data stored at Gauss-Lobatto-Legendre (GLL) nodes.  These nodes can be interpreted in several ways shown in the three grids below.  In all three grids, the GLL nodes are the same (the green dots in the figure).   Internally, the dycore uses the CAM-SE native grid connectivity, but the rest of CAM and ACME has no knowledge of the grid connectivity and only sees data at GLL nodes and their associated area weight.  The other other types of connectivity are not used within the model, but are sometimes used by pre- and post-processing tools.   

CAM-SE Native Grid

Here is a low-resolution CAM-SE grid.   This is a "ne4np4" grid, meaning that it has 4x4 spectral elements in each cube face (the "ne" value), and each spectral element has a grid of 4x4 (the "np" value) Gauss-Lobatto points inside it.  The blue lines show the edges of the spectral elements.  CAM-SE is collocated, meaning that all variables (U,V,T, etc...) are carried around on the GLL nodes.  CAM physics is computed in columns at GLL nodes.  Each column has a Gauss-Lobatto weight associated with it, which is also that columns area.   The sum of these weights is 4pi (the area of the unit sphere).   One can see from the figure that GLL nodes are not quite equally spaced - they cluster at the edges of the spectral elements.

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If we only allow triangles and quads we get convex polygons, but they are less regular:

Meta data files:  (not used directly by the ACME mode, but used by the noted pre- and post-processing utilities

• <grid>_latlon_<date>.nc      
  • This file contains the Native grid and the subcell grid.  
  • Used by CAM's interpic_new to generate initial condition files
  • Used by Paraview and Visit to plot native grid CAM-SE output
  • Used by TempestRemap to create mapping files
• <grid>_scrip_<date.nc>
  • This file contains the CAM-SE dual grid
  • Used by ESMF to create mapping files.  

Future Plans

We note that the TempestRemap package uses an algorithm that understands both SE and FV grids (Ullrich & Taylor, MWR 2015) and can produce map files directly from the CAM-SE Native grid.  It only needs the SE Native grid (the first grid shown on this page) and does not need any type of dual grid.  These mapping files are more accurate since they take advantage of the SE discretization.  If ACME atmosphere adopted TempestRemap mapping files, we will not need to use the SE dual grid.    

Also, the "physics grid" work, when completed, will allow us to run CAM physics at cell centers of the subcell grid (instead of vertices).  It will also allow us to run the physics on a subcell grid with equally spaced subcell centers.  Either of these options will mean we would no longer need the SE dual grid.  

CAM-FV Grids

We also show some plots of grids used by the Lin-Rood CAM-FV model:  

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