## Advanced / Configuration Setup / Define subgrid and MSL/AGL units

### Meteorological Sub-grid Size

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.

### Height Unit for Input and Output

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 or any of the concentration-dispersion calculations. Valid starting heights can be defined as any non-zero fraction less than 2.0.

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