Note that the NCEP models are run at much higher spatial resolution than what is archived on the ARL server. If calculations using the higher resolution date are desired, then the original GRIB encoded files must be obtained from the NCEP server and converted to the HYSPLIT compatible format.
The procedure under this menu tab accesses the NCEP server for the GDAS to convert the GFS GRIB data files to the HYSPLIT compatible format. The GRIB decoder (same as the AVN decoder) interpolates global one-degree latitude-longitude data to a conformal map projection. The default HYSPLIT file is a 100 x 100 grid-point domain at 100-km resolution, centered about the point selected by the slider bars on the lower portion of the menu. GDAS data consist of one GRIB file per forecast hour (0, +3, +6, and +9 hours). The size of each GRIB file is about 24Mb. The decoder is run for each downloaded file, which creates a single-time forecast-file called DATA.AVN. This file is appended to GDAS{YY}{MM}{DD} to create one file that contains all the forecast hours. The size of the final file is about 970 Kb per forecast time.
The menu is divided into two parts. The upper portion is used to select the initialization time and the duration of the forecast. The lower portion is used to set the output directory and center latitude-longitude of the extracted grid. If the "Save input global GRIB files" option is checked, the downloaded GRIB files are saved rather than deleted after each forecast hour is processed. In this way a different regional grid can be extracted from the global files without going through the FTP process again. The GRIB data files can be reprocessed through the "Convert to ARL" menu tab, which has more extensive conversion options available. Note that only the forecast files from the previous 24-h are available from the NCEP server. A longer-term archive file must be created manually be processing only the initialization times and +3 hour forecasts with each new 6-h forecast cycle.