This section provides some guidance in configuring the model input to perform certain specialized calculations. The default configuration supplied with the test meteorological data is confined to a simple trajectory calculation. More complex scenarios can be configured through the Advanced, Configuration Setup, Trajectory menu tab. The menu is used to modify the SETUP.CFG namelist file. This file is not required, and if not present in the root startup directory, default values are used. These parameters can all be changed without recompilation by modification of the contents of SETUP.CFG and in some cases their modification will substantially change the nature of the simulation. The configuration file should be present in the root directory. An illustration of the menu is shown below.
When using the GUI, the namelist file will be deleted and all variables are returned to their default value by selecting the "Reset" button. The following summarizes the namelist variables and their corresponding section in the GUI menu. Not all variables can be set through the menu.
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.
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.
MGMIN (10) - is the minimum size in grid units of the meteorological sub-grid. The sub-grid is set dynamically during the calculation and depends upon the horizontal distribution of end-points and the wind speed. Larger sub-grids than necessary will slow down the calculation by forcing the processing of meteorological data in regions where no transport or dispersion calculations are being performed. In some situations, such as when the computation is between meteorological data files that have no temporal overlap, the model may try to reload meteorological data with a new sub-grid. This will result in a fatal error. One solution to this error would be to increase the minimum grid size larger than the meteorological grid to force a full-grid data load.
Vertical Grid Coordinate System
KMSL (0) - sets the default for input heights to be relative to the terrain height of the meteorological model. Hence input heights are specified as AGL. Setting this parameter to "1" forces the model to subtract the local terrain height from source input heights before further processing. Hence input heights should be specified as relative to Mean Sea Level (MSL). In concentration simulations, the MSL option also forces the vertical concentration grid heights to be considered relative to mean sea level. The special option (xBL) sets KMSL to 2 and treats the input height as a fraction of the boundary layer (or mixed layer) depth at the trajectory starting location and time. This option is not valid with any multiple trajectory in time configurations. Valid starting heights can be defined as any non-zero fraction less than 2.0.
NSTR (0) - Hours between trajectory starts for multiple trajectory-in-time simulations. When greater than zero, new trajectories will be started from the original starting location every NSTR hours from the initial trajectory starting time. See the special trajectory simulation section for more information.
NVER (0) - Number of vertical levels that trajectories are restarted when trajectories for multiple trajectory-in-time-and-space simulations. When greater than zero, new trajectories will be started at this number of levels from the endpoint position at the NSTR interval. The level heights are set in the CONTROL file and must match the number of starting locations. See the special trajectory simulation section for more information.
MHRS (9999) - Sets the maximum temporal trajectory restart period. For instance if you want to compute one month's worth of two-day trajectories, then the run duration would be 720 hours, MHRS would be set to 672 hours (720-48), and KHMAX (see below) would be set to 48 hours.
TOUT(60) - sets the time interval in minutes at which trajectory end-point positions will be written to the output file. Output intervals of less than 60 minutes can be selected. This will force the internal time step to be an even multiple of the output interval.
KHMAX (9999) - The maximum duration of any one trajectory in hours.
Meteorology Output Along the Trajectory
Sets the option to write the value of certain meteorological variables along the trajectory to the trajectory output file. The marker variables are set to (1) to turn on the option. Multiple variables may be selected for simultaneous output but only one variable may be plotted. If multiple variables are selected in conjunction with the trajectory display option, then only the last variable output will be shown in the graphic. The variable output order is fixed in the program and cannot be changed.
Ensemble Grid Configuration Offset Factors
Sets the dimensions at which the meteorological grid will be offset for the ensemble calculation. Only one offset (+ or -) in either X, Y, or Z is applied per member.>/p>
DXF (1.0) - west to east grid factor for offsetting the meteorological grid in the ensemble calculation.
DYF (1.0) - south to north grid factor for offsetting the meteorological grid in the ensemble calculation
DZF (0.01) - vertical grid factor for ensemble (0.01 ~ 250m)
Variables Not Available through the GUI
AA (30), BB (-25), CC (5) - are the polynomial parameters that control the resolution of the internal HYSPLIT terrain following grid system. Input meteorological data are interpolated to this grid. The polynomial relates height (AGL) to the internal vertical index number where k=1 would be the first level above ground: Z = AA*k**2 + BB*k + CC.
KSFC (2) - defines the vertical index of the internal meteorological data level that is considered to be the top of the surface layer. This value is used for stability scaling computations. The default index value of 2 is consistent with the default polynomial parameters that result in the height of the surface layer top to default to 75 m AGL.
MAXLVL(35) - are the maximum number of meteorological data levels that can be input to the model. This value exceeds all currently available meteorological data sets.