Code structure for outer threading

timestep_number needs to be PRIVATE and timestep_ref needs to be SHARED. Consider three timesteps per hour.

DO timestep_ref=1,total_timesteps,3
  timestep_number=timestep_ref

!$OMP PARALLEL
!$OMP SECTION
  CALL rad_ctl(...)
!$OMP END SECTION
!$OMP SECTION
 CALL atm_step_4a(...)
!$OMP END SECTION
!$OMP END PARALLEL

!$OMP PARALLEL
!$OMP SECTION
  timestep_number=timestep_ref+1
  call ukca_main_big_dt(...)
!$OMP END SECTION
!$OMP SECTION
  DO timestep_number=timestep_ref+1, &
                     timestep_ref+2
    CALL atm_step_4a(...)
  END DO
!$OMP END SECTION
!$OMP END PARALLEL

! Probably need to write output 
! from ukca_main_big_dt to a central 
! location
CALL write_ukca_output_to_central_loc(...)