| [2759] | 1 | The ideal em_les test case: |
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| 2 | |
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| 3 | This test case produces a large-eddy simulation (LES) of a free |
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| 4 | convective boundary layer (CBL). The environmental wind (or |
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| 5 | the initial wind profile) is set to zero in this default case. |
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| 6 | The turbulence of the free CBL is driven/maintained by the surface |
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| 7 | heat flux, which is specified in the namelist as tke_heat_flux=0.24 |
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| 8 | (in MKS units). |
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| 9 | |
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| 10 | A random perturbation is imposed initially on the mean temperature |
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| 11 | field at the lowest four grid levels to kick off the |
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| 12 | turbulent motion. Double periodic boundary condition |
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| 13 | is used in x and y. |
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| 14 | |
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| 15 | The default version uses a grid resolution of |
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| 16 | dx=dy=100m and dz=50m, which is considered to be rather coarse |
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| 17 | for an LES of the CBL. A typical grid mesh for an LES |
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| 18 | of the CBL is dx=dy=50m and dz=20m. An LES flow field is more |
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| 19 | accurate when the grid resolution is finer because it resolves |
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| 20 | more turbulent scales. |
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| 21 | |
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| 22 | This LES version uses the Deardorff's TKE scheme to compute the SGS |
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| 23 | eddy viscosity and eddy diffusivity for turbulent mixing, that is |
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| 24 | diff_opt=2 and km_opt = 2 in the namelist. The Coriolis parameter |
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| 25 | is set to f = 10^-4/s. |
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| 26 | |
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| 27 | It takes at least 30 minutes of simulation time to spin up the |
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| 28 | turbulent flow field; only after the spin-up, the turbulence inside |
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| 29 | the CBL is considered well established. A sign of well-established |
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| 30 | turbulence is that the total (i.e., the resolved-scale plus the |
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| 31 | subgrid-scale) heat flux profile should decrease linearly with height |
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| 32 | within the CBL. |
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| 33 | |
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| 34 | To simulate a CBL with a mean wind, change the initial wind |
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| 35 | profile in the input sounding. |
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| 36 | When pert_coriolis= true is set in the namelist, there is no need to |
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| 37 | include the geostrophic wind terms in the right-hand sides of |
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| 38 | the u and v equations for LESs with non-zero geostrophic wind. |
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| 39 | |
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| 40 | Note, parameterization constants, c_s and c_k in this namelist are |
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| 41 | different from the defaults and are the ones recommended to use with LES. |
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| 42 | |
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