## Advanced / Configuration Setup / Time Step Selection Criteria

Set Ratio - *TRATIO* (0.75) - defines the fraction of a grid
cell that a particle or trajectory is permitted to transit in one
advection time step. Reducing this value will reduce the time step and
increase computational times. Smaller time steps result in less
integration error. Integration errors can be estimated by computing
a backward trajectory from the forward trajectory end position and
computing the ratio of the distance between that endpoint and the
original starting point divided by the total forward and backward
trajectory distance.

Set Value - *DELT* (0.0) - is used to set the integration time step
to a fixed value in minutes from 1 to 60. It should be evenly divisible
into 60. The default value of zero causes the program to compute
the time step each hour according to the maximum wind speed,
meteorological and concentration grid spacing, and the
*TRATIO* parameter. The fixed time step option should only be
used when strong winds in regions not relevant to the dispersion
simulation (perhaps the upper troposphere) are causing the model to
run with small time steps. Improper specification of the time step
could cause aliasing errors in advection and underestimation of air
concentrations. An alternate approach is to set the a negative value
which will result in that value (abs) being used as the minimum time step.

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