High resolution weather files like you've never seen

We explain what GRIBs are plus why ProGRIB files represent the latest innovation in this technology

Friday October 27th 2006, Author: James Boyd, Location: United Kingdom

Above: a typical medium resolution GRIB file

For some readers the term ‘GRIB’ will be as familiar as ‘mainsail’ or ‘tiller’, while we suspect for others it would be considered at best something one leaves to Steve Hayles, at worst a type of bug that lives on wood. In fact GRIB is a format of weather file (it is a abbreviation of GRIdded Binary) and the very latest incarnation using this technology looks set to revolutionise our forecasting be it for the America’s Cup, the Route du Rhum or even the most modest local dinghy club weekend race. This new technology provides very much more detailed information with, it is said, up to 30% better forecasts being produced even in comparison to the previous highest resolution models and with an interface so straightforward it can be easily picked up by even the most computer-phobic.

So what exactly is a GRIB? Around the world, but mainly in North America, liquid nitrogen-cooled supercomputers whirr night and day number crunching what are known as ‘models’, enormous, highly complex simulation programs used to predict the weather. There are several variations in these models - some global, some more localised and all offering very different resolutions to the data they produce. A problem for the designers of models is the format in which to output the huge quantity of data resulting each time a model is ‘run’.

One solution, conjured up by the World Meteorological Organisation in 1985, is the ‘GRIB’ file format, one of the rawest outputs of a weather model. For the more technically minded, it is a lengthy string of binary data, each segment containing the three dimensional location (lat, long and altitude) of a particular point, at a particular time, and a value for the particular piece of data in question (ie: wind strength, direction, pressure, etc). Since GRIB was introduced more than 20 years ago it is now into its third version,. How v.2 (there was a v.0, v.1 and now a v.2) is better is beyond the scope of this article, but needless to say it IS improved.

Unless you are Neo, the Chosen One from The Matrix binary data is pretty user unfriendly stuff, so a number of GRIB readers are available offering very differing degrees of complexity from basic stand-alone or web-based products that solely offer a visualisation of the GRIB data, to professional weather forecasting software to weather routing software.

GRIB files are regularly used on thedailysail, for example, in conjunction with Nick White’s Expedition routing software in our analysis of offshore races.

To date the way, for example, we use this to pull down a GRIB to use within a routing program, we choose
- the area we want the GRIB for (in the case of the Route du Rhum start or the Velux 5 Oceans for example it would be a large chunk of the eastern North Atlantic)
- the period we want the GRIB for (they’re available up to 14 days into the future, although they are exponentially less accurate the further ahead in time you go, and generally they are not to be trusted more five days into the future). Their accuracy also varies according to which part of the world you want the GRIB data for – ie it is worse in remote parts of the Southern Ocean than, say, the waters immediately off the weather crazy US.
- the amount of data we want within this period – typically hourly, three hourly or six hourly
- The data type we want – typically surface level pressure and wind (strength and direction). Lots of other useful information is available in GRIB format like rain, current, sea and air temperature, significant wave heights, etc)

Every program has a slightly different way of getting you to tell it what you want and also differing ways of delivering it – typically it is emailed to you or a file is made immediately available to you to save to your hard drive. Either way the GRIB file ends up on your hard drive ready to be pulled into whatever program you are using to view it.

Once a wind/pressure GRIB is loaded up in the software being used to view it, then the pressure is always shown as isobars, while wind is shown as vectors, the number of ‘feathers’ on the vector indicating wind speed. Most viewers and navigation programs allow you to choose how these different sets of data are displayed.

For those using this technology in anger at sea rather than their comfy office swivel chair, GRIB files have the significant advantage of being relatively small in size (ie suitable for lethargic/outrageously expensive reception via satcom), although the file size varies and will increase the larger the area you want the GRIB to cover, the longer the period, the more data you want within this period and the more data types you want it to include.

For purposes of weather routing, GRIBs are fantastically versatile thanks to their numeric make-up. Around 20 years ago MacSea (now MaxSea) was introduced allowing the performance polars for a boat (ie a spreadsheet of its speed for various wind strengths and directions) to be compared with the GRIB forecast, to calculate the optimum course to sail between two points taking into account the unique performance characteristics of the boat in question. For reaching machines like multihulls and Open 60s where the fastest way to get downwind was gybing through large angles, this was a god send, the computer verifying what previously navigators had only managed to dictate by feel or trial and error.

Today there is a whole roster of routing software, all with different characteristics, although most typically used on offshore race boats are MaxSea, Deckman and the relatively new kid on the block, Expedition.

A downside of GRIBs is that they don’t show front as clearly as synoptic charts do, however seeing a series of wind vectors on a chart is far easier to interpret than isobars and it is relatively straightforward to determine where fronts are, wherever there is a line along which there is a dramatic change of wind direction over a relatively short distance, which can also be combined with a rainfall display.

So what’s new?

Just two weeks ago a new state of the art subscription GRIB download service called ProGRIB went live, the brainchild of its founder David Young, who runs the London-based company, Kona Ltd.

At present ProGRIB is available as a stand-alone web-based product although Young expects a ProGRIB software application to be built into various GRIB viewers and routing software in the near future – Expedition is shortly to feature this for example.

The ProGRIB GRIBs are special for several reasons. Firstly they are based on the NMM and not the GFS or ETA models. Although the industry standard for many year, GFS is a global model which offers data at 1 degree (ie 60 x 60 mile) resolution. The ETA model took the next step and was capable of producing reliable forecasts up to 0.2 degrees. For the sailing community, the GFS model has the major draw back of not taking into account land effects; okay to a point for the open ocean, but not for Cowes Week. In contrast the NMM model is taking into account all land features (vegetation type, soil type, snow cover and depth, slope and real topography - this is the only model using real topography. Running on up to 0.02 deg resolution the NMM model is able to capture all weather patterns size of 2km and up.

As it takes into account all these topographic features, NMM is by far the most efficient model available and for example will show land/sea effects. Thanks to it being a ‘Meoscale nonhydrostatic scale model’ rather than a ‘hydrostatic model’ (we’re not going to explain these, but at least you have the terminology), is available at a much higher resolution than merely 1 degree. Although it has been around and in development for several years, the NMM model received the ultimate endorsement in June this year when it was adopted by NCEP, the section of NOAA which produces their GRIB files (NOAA GRIBs are the most widely used). Since switching to NMM, NCEP have been producing GRIBs with a resolution of 0.1 deg, but only for the US. The NMM model was also recently adopted by the National Hurricane Centre in Miami.

ProGRIB goes one stage further than what NCEP offer and for the first time super high resolution GRIBs of the English Channel are available with a resolution of just 0.02deg – ie (2 x 2km) higher than what is available for the US! One reason ProGRIB are able to achieve this is that while most organisations, such as NOAA, offering GRIB data, are national authorities and therefore have to offer coverage for their entire country, whereas ProGRIB, being commercially independent, are able to offer much more localised coverage. “It is a massive step,” says David Young. The ProGRIB data is generated by the company’s computing centre in North America where a enormous cluster of computers processes all the data, producing nearly 32,000 GRIB files every 12 hours.

While the process of downloading ProGRIB’s files is similar to the process described above, it offers the addition option of downloading GRIB files in three different levels of resolution, although the super high resolution at present is only available for the English Channel. It should be noted that these super high resolutions are not interpolated but based on real data. Some companies offer GRIB files with a higher resolution than 1 degree but these are interpolated (simply magnified) from 1 degree resolution down to 0.5 degree.

One draw-back of super high resolution models, as ProGRIB are offering, is that although they cover up to three days hence, David Young warns that the further you go ahead in time the greater the decrease in their accuracy compared to their high and medium resolution GRIBs. He advised they are best used for a period of 24 to 36 hours. For day racing – Hamble Winter Series this weekend for example– this is more than satisfactory.

ProGRIB GRIBs also have another usability advantage in that while they can be emailed, most will download them directly and when this happens they arrive automatically ‘zipped’ (compressed) thereby reducing download time and cost. This is quite impressive, bearing in mind it is done on the fly for up to 32,000 files.

To subscribe to ProGRIB costs £20 per month, which as Young puts it enables the average Joe to get data to a degree of detail AC teams might spend 10s or 100s of thousands getting for Valencia. “No one has ever produced GRIB data on the level we are producing it. For a 1deg grid in the Solent we are producing 2,500 times more data on an hourly basis from the most efficient and up to date models ever used compared to the standard 1 degree x 6 hours normally on offer.”

Sold, we feel.

Below - an example of a ProGRIB high resolution GRIB

In fact we are so sold on this, and especially with the Northern Hemisphere bias, and the super-high resolution in the English Channel (which broadly matches our readership) that we been able to secure a special offer discount for DailySail readers who sign up to the ProGRIB service.

This offer is for a 10% discount on a 1 year subscription to the service. This is now priced at £162 (normally £180) for 1GB of data download throughout the year (this is £78 cheaper than the monthly cost and comes with an extra 120Mb of data).

If you want to have a go and see how it works then click through on this link to the ProGRIB site.

You will be able to download a free GRIB viewer - a copy of Expedition LT and use the demo area of the site to have a play around with the service.

The offer is only available via this link .

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