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

Run FastEddy using the input parameters file /examples/Example01_NBL.in. To execute FastEddy, follow the instructions here: https://github.com/NCAR/FastEddy-model/blob/main/README.md.

2.1.4. Visualize the output

Open the Jupyter notebook entitled “MAKE_FE_TUTORIAL_PLOTS.ipynb” and execute it using setting: case = ‘neutral’.

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.