# Parameters related to time steps¶

The user must specify a number of parameters to enable CHIMERE to determine how many time steps must be used in various routines. See CHIMERE plugin for information on which parameters are to be provided and how this is done in the yaml.

## nphour¶

The number of physical time steps per hour i.e. the number of times routines such as `zenith`

, `locvalues`

, `physics2`

are called per hour.

The user provides a reference value for nphour which is called `nphour_ref`

. CHIMERE then computes the actual nphour that will be used to ensure that the CFL is satisfied. For this, nphour_ref is compared to nphourm, the number of time steps per hour required to satisfy the CFL as computed by the meteo pre-processors and read in METEO.nc.

if nphour_ref is larger than nphourm, then nphour is taken equal to nphour_ref

if nphour_ref is smaller than nphourm, then nphour is taken equal to nphourm.

See routines `checkcfl.f90, iniphys.f90, iniread.f90`

for details.

## ichemstep¶

The number of chemical refined iterations, i.e. the number of times routines such as `twostep`

is called inside a physical time step. The time step duration for an integration of the chemistry in a given hour is then dtr=3600./(nphour*ichemstep).
With a chemical scheme such as MELCHIOR2 for gases: down to 1/4 degree resolution, nphour=6 and ichemstep=1 is fine; for finer grids (5-10 kms), take at least ichemstep=4; for grids of 2-5 kms, try ichemstep=6 or more.

## ihoursu¶

The number of hours for the spin-up. During spin-up, the number of Gauss-Seidel iterations is adapted (see nitgssu) so that computation time can be saved (see `twostep_mod.f90`

).

## nsavedepos¶

The cumulated depositions are saved every nsavedepos hour in the output netcdf file for depositions dep.nc.

## nitgs¶

The number of Gauss-Seidel iterations in the TWOSTEP solver outside of the spin-up (see `twostep_mod.f90`

for more details). WARNING when using the adjoint.

## nitgssu¶

The number of Gauss-Seidel iterations in the TWOSTEP solver during the spin-up (see also ihoursu).

## hpulse¶

The hour of an emission pulse. All emissions before this hour from the beginning of the simulations are ignored, and all transport/chemistry computations are skipped. To be used when computing response functions of a given hour