Introduction to Gridded Binary (GriB) Files
"Gridded Binary" is an international format for the exchange of numeric data specified on a grid. It was designed to compactly encode weather data at regular points on the Earth’s surface, but in principle one can encode anything in GriB format. The simplest possible GriB file would contain exactly one record (also called a message). A record gives the grid values for a single weather parameter (e.g., sea-level pressure) at a single forecast time (e.g., 72 hours from the computation time.) The record has a header section that describes the weather parameter, the grid layout, the reference time of the record (roughly, when it was created), and a multitude of other stuff. Please see the WMO Manual on Codes, or see "A Guide to Grib Version 1" for more info (for example, http://badc.nerc.ac.uk/help/formats/grib/gribdoc.txt). (WMO = World Meteorological Organization.)
Typically, a weather file you download from a source such as Saildocs.com will contain multiple parameter types (perhaps winds, sea level pressure, and 500mb heights.) Furthermore, for each parameter there will be multiple forecast times. BLUEWATER (like pretty much every other GriB display program) aggregates all the records for a single parameter into a "forecast sequence". You view and manipulate sequences.
BLUEWATER also produces various special-purpose GriB sequences that are accessed through the Grib Manager. These contain data produced during optimization runs, and can be viewed like any other Grib record. The programs GriB interface is built on top of the freeware MEL library developed by the US Navy
Limitations On Grib Files
BLUEWATER RACING is only intended to handle weather data useful to mariners. The GriB format is very general, and there is no guarantee that BLUEWATER RACING can handle all possible cases, or even a majority of possible cases. BLUEWATER has handled GriB files from SAILDOCS, SAILFLOW, and ZYGRIB. It provides general support for the standard WMO weather parameters on simple latitude/longitude grids, and custom support for weather parameters like surface winds, temperatures, waves, cloud cover, sea-level pressure, and 500mb chart information.
The program supports only GriB Version 1 files. The GriB file must be a simple Lat/Lon grid. It cannot be Gaussian, Lambert Conformal, or any other special projection. A GDS (Grid Descriptor Section) must be present. (The program does not know about the WMO’s standard grids that can be specified without a GDS). The grid cannot be thinned or quasi-regular. Vector data must contain both U and V data. If the data spans the International Date Line, it will be broken into two subsets, one for each hemisphere. Fortunately, it seems that most of the weather data you can download satisfies these restrictions.
Weather Types (Parameters)
The program will only display parameter types given in WMO Table 2. The parameters that people use most often, such as wind or pressure, have been given specific support. (The parameters with specific support can be seen by examining the file “gribDescription.tcl”.) They include all the parameters accessible through Saildocs, (10m and surface winds, 500mb heights, sea-level pressure), the Wave Watch 3 parameters (sea-level winds, significant wave height, wave directions and wave periods), surface temperatures, and cloud cover. Beyond that, the program makes a best effort to display the parameter in a reasonably sensible fashion. There are no promises, and you may want to check out the feature for viewing raw data described below.
Various weather centers employ local extensions to WMO Table 2, using indices higher than 127 to indicate additional custom parameters. Bluewater Racing knows nothing about those extensions, and in addition it has its own custom local parameters. If you attempt to load a Grib file containing parameter indices higher than 127, the program behavior is undetermined but probably bad.
The program supports “missing values” indicated in the BMS (Bit Map Section). I strongly recommend, however, that you do not use wind data sets with missing values for performance calculations.
Contour lines for Sea Level pressure are labeled by the last two digits of their value in millibars E.g., 1016 becomes "16" and 986 is shown as "86". Contour lines for 500 mb charts are labeled by meter heights divided by 10. E.g. the 5640 m contour line is labeled 564. (The special 5640 line is also shown bold.) These display choices follow standard NOAA style. Wind barbs follow the convention that feathers are drawn pointing towards low pressure.