Atmospheric Topography Generation
- Mark Taylor
- Ben Hillman
Requirements:
High resolution USGS topography file in a “pg1” cubed sphere grid.
E3SM has traditionally used USGS-topo-cube3000.nc (located in the CESM inputdata server here; note this is on a 3 km cubed sphere grid, derived from 1 km resolution source data)
New as of 2024/3: @Jishi Zhang has produced a USGS-topo-cube12000.nc (750m resolution) from a 500m / 250m USGS GMTED2010 source DEM dataset. This data set should be used if ones target E3SM resolution is finer than 3 km.
topography tool (
components/eam/tools/topo_tool/cube_to_target)homme_tool ( components/homme/test/tool, used for np4/pgN mapping).
Topography needs to be interpolated from a high resolution USGS file, and then doctored up a bit to allow the model to run stably with the new topography. The tool chain used to compute topography is documented in the following paper:
There are three main steps in this process:
Step (a) Remap (with cell averaging) USGS topography data to the atmosphere grid
Step (b) apply dycore-specific smoothing to reduce the amount of gravity wave noise if the topography is too rough. This is a tunable parameter, and a rigorous analysis probably needs to be performed to determine the optimal tuning parameter for future very-high-resolution atmosphere grids. For physgrid configurations (pg1, pg2, pg3) there is the extra step of generating consistent topo data on both the GLL (dynamics) and pgN (physics) grid.
Step (c) based on the smoothed topography, compute the SGH and SGH30 surface roughness fields used by some parameterizations
Step (b) requires running the homme_tool's smoothing algorithm. The namelist should be edited to specify the resolution (NE value) or grid file (RRM) grid and the amount of smoothing
Detailed instructions depend on which version of the model is being used:
E3SM V1. V1 Topography: GLL grids
V1 uses topography on a GLL grid with a laplacian smoothing. The smoothing reduces the effective resolution of the topography from 2dx to 6.4dx. (effective resolution is determined by the first wavelength to be damped by 50% from the unsmoothed topography, computed via spherical harmonic spectra)
V1 only needs data on the GLL grid.
Tool chain: Requires cube_to_target, homme_tool, and a SCRIP file for the grid.
V1 has poor treatment of topography and compensates in part with increased divergence damping.
E3SM V2:V2 Topography: GLL/PG2 grids
V2 needs topography on the GLL grid, and then a consistent topography on the PG2 grid (created with the same GLL->PG2 algorithm used internally when running EAM)
V2 uses the same smoothing as V1: 50% damping of the 6.4dx wavelength.
V2 has improved topography treatment over V1. V2 configurations do not need to run with increased divergence damping.
E3SM V3 and EAMxx/SCREAM: V3 Topography: GLL/PG2 grids
E3SM V3 and EAMxx have further improved topography treatments ( namelist varaiables pgrad_correction=1, hv_ref_profiles=6, see ( https://acme-climate.atlassian.net/wiki/spaces/NGDNA/pages/2411397876 ) which allow for rougher topography
The V3 tool chain is similar to V2, with minor modifications to in the smoothing algorithm. We further recommend less smoothing, producing rougher topography.
The rougher topography datasets are the ones with “x6t” in the name - 6 iterations of the tensor-laplace operator.