== A minimum parameter set for the CBL ==

In this chapter a brief, simple and complete parameter set is described, which can be used to simulate a quasi-stationary, convective, atmospheric boundary layer with zero mean horizontal wind. For evaluation purposes, cross sections and horizontally averaged vertical profiles of typical boundary layer variables are output at the end of the run. \\\\
The convective boundary layer parameter file necessary to carry out a run must be provided to the model as an input file under the local name [../../iofiles#PARIN PARIN] (see [source:palm/trunk/TESTS/cases/example_cbl/INPUT/example_cbl_p3d here] for parameter file).

The initialization parameters ([../../initialization_parameters &initialization_parameters]) are located at the beginning of the file. For analysis of a convective boundary layer of approx. 1000 m thickness the horizontal size of the model domain should amount to at least ''2 km x 2 km''. In order to resolve the convective structures a grid spacing of [../../initialization_parameters#dx dx]'' = ''[../../initialization_parameters#dy dy]'' = ''[../../initialization_parameters#dz dz]'' = 50 m'' is enough, since the typical diameter of convective plumes is more than ''100 m.'' Thereby the upper array index in the two horizontal directions needs to be [../../initialization_parameters#nx nx]'' = ''[../../initialization_parameters#ny ny]'' = 39'' . Since in each case the lower array index has the value ''0'', ''40'' grid points are used along both horizontal directions. In the vertical direction the domain must be high enough to include the entrainment processes at the top of the boundary layer as well as the propagation of gravity waves, which were stimulated by the convection. However, in the stably stratified region above the boundary layer the grid resolution has not necessarily to be as high as within the boundary layer. This can be obtained by a vertical stretching of the grid starting from ''1225 m'' via [../../initialization_parameters#dz_stretch_level dz_stretch_level]''= 1225.0 m.'' 

Fast Fourier transformations ([../../initialization_parameters#fft_method fft_method]) are calculated using the Temperton-algorithm. The initial profiles for wind and temperature can be assigned via [../../initialization_parameters#initializing_actions initializing_actions]'' = 'set_constant_profiles'.'' The wind speed, constant with height, amounts to [../../initialization_parameters#ug_surface ug_surface]'' = ''[../../initialization_parameters#vg_surface vg_surface]'' = 0.0 m/s.'' In order to allow for a fast onset of convection, a neutral stratified layer up to ''z = 800 m'' capped by an inversion with ''dtheta/dz = 1K/100 m'' is given: [../../initialization_parameters#pt_vertical_gradient pt_vertical_gradient]'' = 0.0, 1.0,'' [../../initialization_parameters#pt_vertical_gradient_level pt_vertical_gradient_level]'' = 0.0, 800.0.'' The surface temperature, which by default amounts to ''300 K'', provides the fixed point for the temperature profile (see [../../initialization_parameters#pt_surface pt_surface]). Convection is driven by a given, near-surface sensible heat flux via [../../initialization_parameters#surface_heatflux surface_heatflux]'' = 0.1 K m/s''. A given surface sensible heta flux requires the bottom boundary condition for potential temperature to be [../../initialization_parameters#bc_pt_b bc_pt_b]'' = 'neumann'.'' Thus all initialization parameters are determined. These can not be changed during the run (also not for restart runs).

Now the run parameters ([../../runtime_parameters &runtime_parameters] ) must be specified. To produce a quasi stationary boundary layer the simulated time should be at least one hour, i.e. [../../runtime_parameters#end_time end_time]'' = 3600 s.'' To stimulate convection, the initially homogeneous (zero) wind field must be disturbed ([../../runtime_parameters#create_disturbances create_disturbances]'' = .T.''). These perturbations should be repeated in a temporal interval of [../../runtime_parameters#dt_disturb dt_disturb]'' = 150.0 s'' until the energy of the perturbations exceeds the value [../../runtime_parameters#disturbance_energy_limit disturbance_energy_limit]'' = 0.01 m^2^/s^2^''. After each time step run time information (e.g. size of the timestep, maximum velocities, etc.) are to be written to the local file [../../iofiles#RUN_CONTROL RUN_CONTROL] ([../../runtime_parameters#dt_run_control dt_run_control]'' = 0.0 s'').

Instantaneous cross section data of vertical velocity (''w'') and potential temperature (''pt'') are to be output for horizontal (''xy'') and vertical (''xz'') cross sections, and additionally, time averaged (''av'') vertical cross section data are to be output for the vertical velocity: [../../runtime_parameters#data_output data_output]'' = 'w_xy', 'w_xz', 'w_xz_av', 'theta_xy', 'theta_xz'.'' Output of instantaneous (time averaged) data is done after each ''900 (1800)s'': [../../runtime_parameters#dt_data_output dt_data_output]'' = 900.0,'' [../../runtime_parameters#dt_data_output_av  dt_data_output_av]'' = 1800.0.'' The averaged data are time averaged over the last ''900.0 s'', where the temporal interval of data entering the average is ''10 s'': [../../runtime_parameters#averaging_interval averaging_interval]'' = 900.0,'' [../../runtime_parameters#dt_averaging_input dt_averaging_input]'' = 10.0.'' Horizontal cross sections are output for vertical levels with grid index ''k=2'' and ''k=10,'' vertical cross sections are output for index ''j=20'': [../../runtime_parameters#section_xy section_xy]'' = 2, 10, '' [../../runtime_parameters#section_xz section_xz]'' = 20.'' For runs on more than one processor, cross section data are collected and output on PE0: [../../runtime_parameters#data_output_2d_on_each_pe data_output_2d_on_each_pe]'' = .F.''. 

Output of vertical profiles is to be done after each ''900 s''. The profiles shall be temporally averaged over the last ''600 s'', whereby the temporal interval of the profiles entering the average has to be ''10 s'': [../../runtime_parameters#dt_dopr dt_dopr]'' = 900.0 s,'' [../../runtime_parameters#averaging_interval_pr averaging_interval_pr]'' = 600.0 s'', [../../runtime_parameters#dt_averaging_input_pr dt_averaging_input_pr]'' = 10.0 s.'' The temperature profile including the initial temperature profile (therefore'' '#theta' ''), the subgrid scale, resolved and total vertical sensible heat flux as well as the variances of the vertical velocity and the potential temperature are to be output:  [../../runtime_parameters#data_output_pr data_output_pr]'' = '#theta', 'w"theta”', 'w*theta*', 'wtheta', 'w*2', 'theta*2'.''