2.2. Dry convective boundary layer

This is the convective boundary layer scenario described by Sauer and Munoz-Esparza (2020). This case represents the boundary layer conditions at the SWiFT facility near Lubbock, Texas at 4 July 2012 during the period of 18Z-20Z (12:00–14:00 local time), the strongest period of convection on the day.

2.2.1. Input parameters

  • Number of grid points: \([N_x,N_y,N_z]=[600,594,122]\)

  • Isotropic grid spacings in the horizontal directions: \([dx,dy]=[20,20]\) m, vertical grid is \(dz=20\) m at the surface and stretched with verticalDeformFactor \(=0.80\)

  • Domain size: \([12.0 \times 11.9 \times 3.0]\) km

  • Model time step: \(0.05\) s

  • Geostrophic wind: \([U_g,V_g]=[9,0]\) m/s

  • Advection scheme: Hybrid 5th order upwind

  • Time scheme: 3rd-order Runge Kutta

  • Latitude: \(33.5^{\circ}\) N

  • Surface potential temperature: \(309\) K

  • Potential temperature profile:

\[\begin{split}\partial{\theta}/\partial z = \begin{cases} 0 & \text{if $z$ $\le$ 600 m}\\ 0.004 & \text{if $z$ > 600 m} \end{cases}\end{split}\]

  • Surface heat flux: \(0.35\) Km/s

  • Surface roughness length: \(z_0=0.05\) m

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

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

  • Depth of perturbations: \(400\) m

  • Top boundary condition: free slip

  • Lateral boundary conditions: periodic

  • Time period: \(4\) h

2.2.2. Execute FastEddy

Run FastEddy using the input parameters file /examples/Example02_CBL.in. To execute FastEddy, follow the instructions here: https://github.com/NCAR/FastEddy-model/blob/main/README.md.

2.2.3. Visualize the output

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

XY-plane views of instantaneous velocity components at \(t=4\) h (FE_CBL.288000):

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XZ-plane views of instantaneous velocity components at \(t=4\) h (FE_CBL.288000):

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Mean (domain horizontal average) vertical profiles of state variables at \(t=4\) h (FE_CBL.288000):

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Horizontally-averaged vertical profiles of turbulence quantities \(t=3-4\) h [perturbations are computed at each point relative to the previous 1-hour mean, and then horizontally averaged]:

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2.2.4. 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 convective case?

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