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Guide to terrain in CM1.¶

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The model uses the Gal-Chen and Somerville coordinate transform. The model surfaces follow the terrain at the bottom of the model, and the model top is a surface of constant height. The model surfaces do not move in time ... i.e., they are fixed in space.

When there is no grid stretching, the model top is simply nz x dz. (Both variables are specified in namelist.input)

When vertical grid stretching is used, the model top iz ztop. (ztop is specified in the param6 section of namelist.input)

The terrain profile is specified by the array called "zs". The user should set zs in init_terrain.F

References:

Gal-Chen, T., and R. Somerville, 1975: On the use of a coordinate transformation for the solution of the Navier-Stokes equations. J. Comput. Phys., 17, 209-228.

Durran, D. R., and J. B. Klemp, 1983: A compressible model for the simulation of moist mountain waves. Mon. Wea. Rev., 111, 2341-2361.

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First, a brief overview:

step 1: Set "terrain_flag = .true." in the param0 section of "namelist.input"

step 2: Set a value for "itern" in the param2 section of "namelist.input" This tells the model what analytic terrain profile to use. See details in "README.namelist" for the list of pre-defined profiles.

step 3: Specify the terrain profile (zs) in "init_terrain.F" (or, just double check the settings if you are using one of the pre-defined profiles). The zs array is the height of the topography in m. NOTE: zs is specified at the scalar points (i.e., the same as theta, qv, and pressure).

step 4: If you feel so inclined, set "output_interp = 1" in the file "namelist.output". This will generate an additional set of output files, where the data have been interpolated to the nominal height levels (i.e., the height levels in the absence of topography ... meaning if zs = 0). NOTE: This doubles the amount of output. ALSO NOTE: This is a rather quick-and-dirty method of looking at the output. A more sophisticated plotting scheme/software should be preferred for journal-quality figures.

step 5: Compile.

step 6: Run model.

step 7: View the output. You're on your own here. The output in the regular output files (cm1out_s, cm1out_u, etc.) is on the terrain-following model surfaces. The output in the interp file (cm1out_i) can be rather coarse in the vertical. (I'm working on something better ... when I get the chance.)

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Questions: send email to gbryan@ucar.edu