only one time_step for nested-domain running

[sd16121]

Dear Office,

I have one question about the time_step in the follwoing link:
https://www2.mmm.ucar.edu/wrf/users/namelist_best_prac_wrf.html#time_step

&domains
time_step = 180,
time_step_fract_num = 0,
time_step_fract_den = 1,
max_dom = 1,
e_we = 74, 112, 94
e_sn = 61, 97, 91,
e_vert = 30, 30, 30,
p_top_requested = 5000,
num_metgrid_levels = 27
num_metgrid_soil_levels = 4
dx = 30000, 10000, 3333.33,
dy = 30000, 10000, 3333.33,
grid_id = 1, 2, 3,
parent_id = 0, 1, 2,
i_parent_start = 1, 31, 30,
j_parent_start = 1, 17, 30,
parent_grid_ratio = 1, 3, 3,
feedback = 1,
smooth_option = 0,
/

It only has one time_step for all three domains, is it possible to have three different time_steps for different domains? And why the time_step should follow the coarsest domain's dx, namely dx × 6 = 30km × 6 = 180s?

Looking forward to hearing from you soon. Many thanks.

Best wishes,
Stella

 

[Ming Chen]

Stella,

We only specify time_step for D01. When running a nested case, time_setp for other domains will be determined inside WRF based on the nesting ratio. For example, if you run a nested case with the parent_grid_ratio = 1, 3, 3, and you set time_sstep =180, then the time_step for D02 will be 180/3 =60.
You don't need to explicitly specify time_setp for D02.