The Global Eulerian Model (GEM) is invoked as a series of subroutines within the main HYSPLIT transport and dispersion code. Particles or puffs are always first released in the Lagrangian framework and carried within HYSPLIT until they exceed a certain age at which point their mass is transferred to the GEM routines. Particles can be released and transferred for the entire duration of the simulation. The only requirement is that at one pressure-level global meteorological grid (2.5 degree reanalysis or 1 degree GFS) needs to be defined for the HYSPLIT calculation. Additional finer resolution regional meteorological grids can be defined for the Lagrangian portion of the transport calculation. Details of the global model have been previously published: Draxler, R.R., 2007, Demonstration of a global modeling methodology to determine the relative importance of local and long-distance sources, Atmospheric Environment, 41:776-789, doi:10.1016/j.atmosenv.2006.08.052.
General Instructions
Although the HYSPLIT-GEM version has its own executable, without the required namelist file GEMPARM.CFG the calculation will proceed as a standard Lagrangian HYSPLIT simulation. In this situation, the model will write the default file gemparm.bak as a reminder that a GEM configuration file is required to invoke the global subroutines. This file can then be edited and renamed if not using the GUI interface. GEM and HYSPLIT each create their own concentration output files and if the total concentration is required, the results from the two calculations must be added together. The program concadd is included in the distribution and can be accessed from the global display menu by selecting the plume add checkbox. Furthermore, the GEM concentration grid is always a snapshot concentration. There is no averaging option. In general, the model simulation is configured like any other HYSPLIT simulation, through the CONTROL and SETUP.CFG files. Additional GEM specific options are set in the GEMPARM.CFG file. Not all GEM options are available through the GUI and can only be modified by editing the file directly.
SETUP.CFG Namelist Configuration
There is only one parameter in the HYSPLIT namelist that is required for the GEM subroutines:
GEMPARM.CFG GUI Namelist Options
Only a subset of the namelist options are available to be set through the GUI. Note that concentration (mass/m3) output will always be written to gemconc.bin as individual or averaged layers, but the integrated output (mass/m2) will only be written to gemzint.bin. The bottom and top values are given as index numbers where 1 is the bottom layer. Layer thickness corresponds with the input meteorological data and cannot be modified. The GUI interface is shown below:
GEMPARM.CFG Namelist Description