Digital weather data is purveyed from national weather organizations in an international standard format, Gridded Binary. This section describes how the program handles such weather files.

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 program supports both GriB Version 1 and GriB Version 2, using the ecCodes library developed by ECMWF (European Centre for Medium-range Weather Forecasting).

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.

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.


Limitations On Grib Files

The GriB format is very general, and BLUEWATER RACING does not support all possible cases, or even a majority of possible cases. BLUEWATER has handled GriB files from SAILDOCS, SAILFLOW, and ZYGRIB.  It supports simple latitude/longitude grids, and weather parameters like surface winds, temperatures, waves, cloud cover, sea-level pressure, and 500mb chart information.

The GriB file must be a simple Lat/Lon grid. It cannot be Gaussian, Lambert Conformal, or any other special projection. 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.


Weather Types (Parameters)

Commonly-used weather parameters, such as wind or pressure, have been given specific support.  (The set of supported parameters 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.

Some GriB files have "missing data" at certain grid points. For example, wave data is missing at grid points that are on land. The program supports “missing values, but it is strongly recommended, 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.