CAM-SE works with data stored at Gauss-Lobatto-Legendre (GLL) nodes. These nodes can be interpreted in several ways shown in various grids below. In the "SE native grid", "SE subcell grid" and "SE dual grid", 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 E3SM 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.
E3SM v2 adds the option to run the physics on a "physgrid", also shown below. The physgrid is a conventional finite volume (FV) grid, where each physics column is located in the center of the FV cell and represents the cell averaged quantities. The physgrid gives E3SM the flexibility to run the physics at lower or higher resolution than the dynamics.
Most native grid atmosphere output for E3SM V2 will be on the pg2 physgrid. Native grid atmosphere output from E3SM V1 is on the np4 GLL grid.
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 can also be interpreted as an area associated with each node. 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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- <grid>_latlon_<date>.nc
- This file contains the GLL node locations and the native grid and the subcell grid connectivity
- Used by CAM's interpic_new to generate initial condition files
- Used by Paraview and Visit to plot native grid CAM-SE outputUsed by TempestRemap to create mapping files (when specifying the Jacobian to override Tempests internal reference element map)
- <grid>_scrip_<date.nc>
- This file contains the GLL node locations, the dual grid cell vertices and the dual grid connectivity
- Used by ESMF to create mapping files.
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- For quasi-uniform cubed-sphere grids, E3SM currently uses the "pentagons" option to construct the dual grid for making ESMF mapping files. To create these meta data grid files, we use a utility distributed with the E3SM code - see e3sm/components/homme/test/tool.
- For RRM grids, E3SM v1 used the "cheveron" option to construct the dual grid for making ESMF mapping files. To create these meta data grid files we use a Matlab program that performs a Newton interation. This matlab code is in the E3SM PreAndPostProcessingScripts repo under regridding/spectral_elements_grid_utilities/
See also CAM-FV Grid Overview
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