2.1. Dry neutral boundary layer

2.1.1. Background

This is a canonical neutral boundary layer scenario. The case is broadly based upon Sauer and Munoz-Esparza (2020) but is not identical. A geostrophic wind is prescribed over ground with a set aerodynamic roughness length under a neutrally stratified boundary layer. The purpose of this test case is to visualize and analyze the resultant flow and turbulence characteristics that develop when the LES reaches statistical steady-state.

2.1.2. Input parameters

  • Number of grid points: \([N_x,N_y,N_z]=[640,634,58]\)

  • Isotropic grid spacings in the horizontal directions: \([dx,dy]=[15,15]\) m, vertical grid is \(dz=15\) m at the surface and stretched with verticalDeformFactor \(=0.75\)

  • Domain size: \([9.6 \times 9.51 \times 1.08]\) km

  • Model time step: \(0.04\) s

  • Advection scheme: 5th-order upwind

  • Time scheme: 3rd-order Runge Kutta

  • Geostrophic wind: \([U_g,V_g]=[10,0]\) m/s

  • Latitude: \(54.0^{\circ}\) N

  • Surface potential temperature: \(300\) K

  • Potential temperature profile:

\[\begin{split}\partial{\theta}/\partial z = \begin{cases} 0 & \text{if $z$ $\le$ 500 m}\\ 0.08 & \text{if 500 m < $z$ $\le$ 650 m}\\ 0.003 & \text{if $z$ > 650 m} \end{cases}\end{split}\]

  • Surface heat flux: \(0.0\) Km/s

  • Surface roughness length: \(z_0=0.1\) m

  • Rayleigh damping layer: uppermost \(400\) m of the domain

  • Initial perturbations: \(\pm 0.25\) K

  • Depth of perturbations: \(375\) m

  • Top boundary condition: free slip

  • Lateral boundary conditions: periodic

  • Time period: \(7\) h

2.1.3. Execute FastEddy

  1. Create a working directory to run the FastEddy tutorials and change to that directory.

  2. Create a Example01_NBL subdirectory and change to that directory.

  3. The FastEddy code will write its output to an output subdirectory. Create an output directory, if one does not already exist.

  4. Run FastEddy using the input parameters file Example01_NBL.in located in the tutorials/examples/ subdirectory of the FastEddy repository.

See Build and Run for instructions on how to build and run FastEddy on NSF NCAR’s High Performance Computing machines.

2.1.4. Visualize the output

  1. Open the Jupyter notebook entitled MAKE_FE_TUTORIAL_PLOTS.ipynb.

  2. Under the “Define parameters” section, modify path_base, specifying the full path to the Example01_NBL subdirectory, but don’t include Example01_NBL subdirectory. Be sure to include a trailing slash /).

  3. Under the “Define parameters” section, modify case to set its value to neutral.

  4. Run the Jupyter notebook.

  5. The resulting XY cross section png plots will be placed in a FIGS subdirectory of the Example01_NBL directory.

XY-plane views of instantaneous velocity components at \(t=7\) h (FE_NBL.630000):

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XZ-plane views of instantaneous velocity components at \(t=7\) h (FE_NBL.630000):

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Mean (domain horizontal average) vertical profiles of state variables at \(t=7\) h (FE_NBL.630000):

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Horizontally-averaged vertical profiles of turbulence quantities at \(t=6-7\) h [perturbations are computed at each time instance from horizontal-slab means, then averaged horitontally and over the previous 1-hour mean]:

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2.1.5. Analyze the output

  • Using the XY and XZ cross sections, discuss the characteristics (scale and magnitude) of the resolved turbulence.

  • What is the boundary layer height in the neutral case?

  • Using the vertical profile plots, explain why the boundary layer is neutral.