Difference between revisions of "GFS"

Revision as of 15:35, 22 September 2009

This article is about GFS and is under construction

The Global Forecasting System (GFS) files are packed under the GRIB protocol, following World Meteorological Organization (WMO) protocol and is the primary format at the operational centers (ECMWF,NCEP). There are two different types of GRIB files: GRIB1 and GRIB2. GRIB2 is the most recent format, so that historical simulation files (e.g. reanalysis) are usually found in GRIB1 while the forecasts and recent analysis files are in GRIB2.

Reanalysis data in GRIB1 format can be collected from the NCEP FNL Operational Model Global Tropospheric Analyses http://dss.ucar.edu/datasets/ds083.2/, from July 1999. To collect the data you must become a registered user.

GFS forecasts in GRIB2 format can be downloaded from ftp://ftpprd.ncep.noaa.gov/pub/data/nccf/com/gfs/prod/. This ftp has a different folder for each simulation. The name indicates the begining of the simulation (gfs.yYYYYMMDDHH). For example, the folder named gfs.2009081406 indicated results produced at 6 a.m. (UTC) of the 14th of August 2009. Inside each folder, there can be found different files, but our interest is mainly in the files named with the following format gfs.tHHz.pgrb2bfHZ that indicate that are gfs files, produced at the simulation hour HH and that correspond to the HZ forecast hour (after the beginning of the simulation). Results are produced every 3 hours and the size of each file is around 40 MB.

More information on GFS files can be found on http://www.nco.ncep.noaa.gov/pmb/products/gfs/ .

Ungrib

GRIB1 files can be converted into ASCII or binary files with wgrib.exe while GRIB2 are converted into ASCII, binary or NETCDF with wgrib2.exe Once our files are converted into ASCII text files they would be converted into HDF5 files through a Matlab code.

The wgrib and wgrib2 programs can be downloaded at the NOAA´s Climate Prediction Center website http://www.cpc.noaa.gov/products/wesley/

wgrib2.exe

The wgrib2.exe is a command-line program and presents a wide list of options. In order to reduce the computing space, the extracted domain can be limited through the option -small_grib. A typical command-line using this option would be:

wgrib2 20090101/gfs.t00z.pgrb2f00 -small_grib -10:11 30:45 Smallergrib00.grib

where -10:11 is the longitude interval, 30:45 is the latitude interval and Smallergrib00.grib correspond to the new grib file.

Once executed, the outputs are .dat files each containing a different property with the following correspondences:

• HR2m - Relative Humidity 2 m above ground [%]
• LAND - Land Cover (1=land, 0=sea) [Proportion]
• MSLP - Pressure Reduced to Mean Sea Level [Pa]
• TCDC - Total Cloud Cover [%]
• TMP2m - Temperature 2 m above ground [K]
• U - U-Component of Wind 10 m above ground [m/s]
• V - V-Component of Wind 10 m above ground [m/s]

Binary to HDF5 format

Once obtained, the ASCII results outcoming from the wrib have to be converted into HDF5 to serve as input for the Mohid atmosphere module. This operation is performed through a Matlab code found in:

\\einstein\www\ftp.mohid.com\operational\meteoIST\GFSGRIB2\Matlab

The Matlab version for using this code have to be the R2008b version or previous due to the uncompatibility found with new versions (i.e. R2009a) with the HDF5 libraries version. The new versions work with HDF5 libraries 1.8.1 while the previous work with 1.6.5 libraries that are compatible with the rest of the Mohid modelling system.