Notes and Information on Atlantic RTOFS Model Data (Text)

Contents

1. General Notes
2. GRIB
3. netCDF
4. NOMADS/DODS

1. General Notes

Latitude-longitude points on the native grid for the Atlantic RTOFS model are on a curvilinear orthogonal grid. The latitude-longitude point files for correct interpretation of the data files as well as other data files and software for processing Atlantic RTOFS data is available here. See the README and the Readme.RTOFS files for more information on the files and software. Vertical coordinates are interpolated to 40 fixed-depth positions, as specified here.

2. GRIB

The operational data file format is GRiB (GRIdded Binary). Additional information on the use of GRiB at NCEP is available here.

To work with Atlantic RTOFS GRiB, any standard tools which read GRiB will work. One such tool is the freely available wgrib utility. The wgrib home page and software is here.

Another free GRiB file reader is the read_grib utility that can decode GRiB data directly into MATLAB (MATLAB is a registered trademark of The Mathworks, Inc.).

• The home page for read_grib is here.
• An updated version of read_grib can be downloaded here (latest version is read_grib_1.5.tar.gz).
  The updates to read_grib include the latest changes to the MMAB optional table 128 and the NCO table 129 of GRiB parameter variable definitions. Included with the read_grib files is the getgrib.m utility for simplified extraction of three dimensional data from the Atlantic RTOFS grib files. The wgrib2 utility is required if you wish to decode GRiB2 files.

3. NetCDF

We realize that many users of Atlantic RTOFS output may be more familiar with NetCDF (network Common Data Form). This is not a format available from NCEP operational systems.

To convert GRiB to netCDF, you will need a utility such at gribtonc, described here. Source code for gribtonc is available here as part of the UNIDATA decoders package.

To build the converter to work with the large Atlantic RTOFS files, you will need to change the file grib1.h in decoders-3.1.2/src/gribtonc/

• Change :
•    #define MAX_GRIB_SIZE 200000
• to
•    #define MAX_GRIB_SIZE 2000000
• Then build and install as instructed.

The netCDF files are approximately 6 times the size of the GRiB files, up to 1.2 GB for a netCDF file containing a single 3-dimensional field as compared to 250 MB in a GRiB file containing all available fields.

You will need the lat-long files described in the General Notes. You may need the file ofs3z.cdl to carry out the conversion as gribtocdl does not generate a correct cdl.

4. NOMADS/DODS

The NOMADS (NOAA Operational Model Archive Distribution System) data server offers daily full-fields and hourly surface fields. The server can be accessed at http://nomads6.ncdc.noaa.gov/ncep_data/index.html, and offers interactive graphics as well as direct data downloads and DODS/OpenDAP access.

More information on NOMADS is available here, and DODS/OpenDAP client software can be found here.