Marine Glossary

|| || || || ||

GRIB

An acronym for gridded binary. A WMO digital file format used to convey data on weather, waves, and currents, as well as other parameters such as ASCAT wind measurements. It puts one data point at each grid point, the spacing of which depends on the grib resolution of the data. The datum at each point can be a vector such as a wind speed and direction, or it can be a scaler value such as water temperature.

GRIB files can be viewed in most charting programs (ECS), or in dedicated GRIB viewers. A popular free GRIB viewer for Mac and PC is XyGrib. Popular free charting programs that displays GRIB files are OpenCPN and qtVlm. We use the former in our Inland and Coastal Navigation Course and we use the latter in our Marine Weather Course and in our Electronic Chart Navigation Course. See Starpath Online Courses.

Many options for GRIB file downloads (requested by email) are offered by Saildocs. There are in use now two GRIB formats: grib1 and grib2. The latter is the new WMO standard and essentially all data are in that format now. However, there are various allowed structures in the grib2 format. One of them is essentially the same as grib1, so that type of grib2 file can be displayed in older viewers that only show grib1.

A few ECS programs that say they will accept grib2, actually mean they show grib2 as long as they are in the old grib1 format, which indeed many of our popular data sources are. This has a lot to do with the map projection used in the grib2 format. Those that use a straight Lat-Lon grid or one using a mercator grid, translate easily to grib1, so providers can make the conversion easily. On the other hand, some of the newest data are in a lambert projection, and only a few viewers can display those files. LuckGrib and Expedition are examples of a GRIB viewer and a nav program that accept all grib2 formats. There are also other subtitles that come into play on some files. Any file downloaded from Saildocs or XyGrib, will be in a grib1 compatible format.

The weather maps we get in GRIB format are always raw model output such as GFS or NAM. These model forecasts have not been modified by human forecasters, so it is up to us to compare the pure model forecasts with the actual NWS official forecasts to decide the extent to which we can rely on the models alone. A major role of our textbook Modern Marine Weather, 3rd ed. is directed toward that comparison.