Wednesday, February 10, 2021

Applications of Weather Data in GRIB Format

We have several articles and videos on getting started with GRIB files cited at the end of this note. Here we take a look at various applications of this powerful technology. It would be too much to cover in any useful manner in one place, so we present this index to specific topics with linked video demos. We initially focus on a couple free viewers that we use in our online course, and then at the end show how these analyses can be expanded upon with several outstanding commercial products.

1. Global model weather forecasts for ocean navigation

This was the original application of the data, which was spearheaded by Saildocs using Airmail for HF radio communications and the PC program ViewFax for viewing the grib data. ViewFax remains a versatile way to view and obtain the files, along with other marine weather data.

We started our introduction to grib files using the grib viewer app XyGrib, so we will show samples from it whenever possible. We also look at GRIB files in the nav app OpenCPN that we use in both the coastal nav and marine weather courses. Our goal is then to also include qtVlm which is a powerful program that does both navigation, grib viewing, and grib file sourcing. All three apps have both a PC and a Mac version. We have an introductory article on XyGrib that includes a short video on loading a global model

The sample below looks at more details of using a global forecast of an ocean route.

We set up a global model display to look like the OPC maps. By comparing surface and 500 mb maps we get an insight into the dependability of a forecast [13m:32s].

The same files we download from XyGrib or by email request from Saildocs, can also be shown in  OpenCPN—recall that OpenCPN has a way to view the files, but does not directly download the files, although it does assist with preparing an email request for some of the saildocs files. See models available at Saildocs. For the most part, XyGrib offers a more informative way to display most GRIB files than does OpenCPN, but OpenCPN has a super convenient way to load georeferenced weather map images, upon which we can overlay the GRIB model forecasts for improved insight into both the model forecast and the graphic map image from a weather agency such as OPC. This process is illustrated in Section 5 below. 

There are  some 14 global weather models internationally, about half of which offer free access to their data in grib format. A couple of the popular ones are:


NAVGEM (US Navy, see also)

ARPEGE (France)

ICON (Germany)

GDPS (Canada)

For historical analysis we can use re-analyzed model forecasts. These global data are available every 6 hr from ECMWF and GFS. When you use this data starting at say18z, July 10, 1990 for 10 days, you can get one map every 6 hr, but they are not forecasts, but the equivalent of surface analyses valid at those synoptic times. It is an excellent way to study past races or voyages.

Here is another example of viewing grib files in a free app with both Mac and PC versions called qtVlm.

Loading and viewing grib files in qtVlm [11m:35s].

2. Global ocean model forecasts of wind waves and swells

We can access the US WW3 model in almost any navigation  and grib viewer apps. The model has an extensive set of parameters (see definitions in the Background article.)  The German weather service, Deutsche Wetterdienst (DWD),  also have a high-quality ocean program with a global wave model (GWAM), European wave model (EWAM) and a higher resolution coastal wave model (CWAM).  XyGrib will download WW3, GWAN, and EWAM. Only WW3 is available from Saildocs. (All are available from Expedition or LuckGrib.)

Below we compare two global wave models in XyGrib. Recall the grib2 wave parameters listed below, which shows that both models use the same parameters, but they have different abbreviations for them.

Sample page from Panoply showing content of two grib files from XyGrib

Here I have loaded both files into Panoply, which not only displays the data graphically but also shows exactly what is included. We have videos on the use of Panoply.

A video comparison of WW3 and GWAN in XyGrib with notes on wave display [12m:13s].

3. Ocean model for currents and water temperature

The most popular ocean model for global currents is the US RTOFS model. It is available from Saildocs.  Other global ocean current models include the US Navy models HYCOM and NCOM, the latter of which is a regional model, generally offering the best current data when available. 

XyGrib v1.2.6 does not offer ocean current data download, but it does have a currents parameter to be shown if the data are obtained elsewhere. To get RTOFS grib file, send this in the body of a mail to

send RTOFS:42.1N,24.2N,34.3W,3.4W|0.08,0.08|0,6..144|CUR,WTMP

Change the Lat Lon box to what you want. This is for every 6h out to h144 (6 days). It provides current direction and speed as well as sea surface temperature (called WTM). The 0.08ยบ (4.8 nmi, 8.9 km) is the resolution. 

The model is run once a day, completing at about 1700Z. Each run starts with a 24 hr hindcast and produces ocean surface forecasts starting at 12z the previous day for every 3h to h48, then every 6h till h48, then every 12h to h192 (8 days).

A video display of RTOFS data  in XyGrib [7m:43s].

4. Regional model forecasts for inland and coastal sailing

The main data sources for US regional models are HRRR, and the CONUS or Regional versions of NBM, NAM, and NDFD. In other parts of the world, the best regional data will typically be a WRF model run by a local university, institute, or commercial company. Samples can be seen at OpenSkiron and within XyGrib for Europe, and at Expedition Marine for other parts of the world, especially near AU and NZ.

HRRR is the only hi-res US dataset  that is readily available to free GRIB viewers, because we can get it from Saildocs. You might note that NAM and NDFD are also available from Saildocs, but you will discover that the NAM they offer is only 12-km resolution (we need the 3 km version) and the NDFD they offer is only the oceanic version (10 km). XyGrib also has only 12 km data for their version of NAM CONUS.  See our note on NDFD: Oceanic, CONUS, and Regional.

The HRRR model (High Resolution Rapid Refresh) is a 3-km model, updated every hour. The model is updated every hour with forecasts that go out 18h.  The runs that occur at the synoptic times are special in that they then go out 48h, with hourly forecasts to 18h, then 3h forecasts to 48h. We can get these extended HRRR forecasts from saildocs.  (The 2.5-km NAM data goes out for 2.5 days, but we need special sources such as LuckGrib to get these data.) 

For now, we can practice with regional forecasting using HRRR data in US waters (inland and coastal) or any of several hi-res models available from within XyGrib for European inland and coastal waters.  For HRRR data we can use this request to

send HRRR:47.9N,48.5N,123.4W,122.3W|0.03,0.03|0,1..18|WIND,PRMSL,REFC

It is important to use the decimals on the position, as these can be very large files at 3-km resolution. Even this area maybe more than we want.  Modify the Lat Lon coordinates for your location. For minimum files omit the PRMSL and REFC

A video demo of displaying HRRR data from Saildocs in the XyGrib viewer [9m:45s].

5. Overlay model forecast winds on weather map and cloud photo images

A major contribution that OpenCPN makes to weather work at sea is its ability to show selected model wind forecasts in grib format overlaid onto official, up-to-date weather maps made by the OPC or their counterparts around the world (UK Met, DWD, BOM, and others). Once the grib file is in hand, this overlay can be accomplished  in a few seconds.  It is a powerful way to evaluate the model forecasts. The procedure is implemented with the weatherfax plugin, which can be downloaded from within the program. We have several videos on the use of this tool and a note on how to add your own files to the list. Here is another example.

Overlay of GFS winds and pressure on an OPC map and on a GOES West satellite image [7m:40s].

The program qtVlm also has a sophisticated means of importing and displaying georeferenced weather maps and other related images. They are not shipped with the program, but they are very easy to import and save so that each run brings the latest maps. Here is a video of the process using qtVlm. Expedition also has a similar functionality.

Loading and viewing weather map images in qtVlm [11m:48s].

6. Probabilistic forecasts from ensemble forecasts and model blends

We get now to the modern aspect of using grib format of model forecasts, namely the use of standard deviations in the forecasts and probabilistic wind forecasts. In the past we would say "There will always be a forecast, and they are not marked good or bad," but that is no longer true—providing we have access to all the newest data.

Ensemble models, such as the US GEFS and the Canadian GEPS, compute 20 or so solutions (see Background notes) at each synoptic time and then provide us with an average of those solutions, along with the standard deviation of the set, which is in practice an uncertainty level. A wind forecast of 12 kts ± 2 kts is obviously much better forecast than 12 kts ± 10 kts.

The National Blend of Models (NBM CONUS) also includes similar standard deviation parameters. On the other hand, the NBM Oceanic domain includes actual wind probabilities deduced from the very many sources they take into account. 

This dataset includes these probabilistic winds: P25 is the wind at the high end of the lowest 25% of all wind solutions at that point; P75 is wind at the low end of the highest 25% of all wind solutions; and the mean wind called P50 is the average of all solutions between P25 and P75. The P50 wind represents the middle 50% of all wind solutions for that point. Likewise there is a P10 and P90. A P90 wind speed of 20 kts means that 10% of all solutions at that point were higher than 20 kts, and so on. Or you could say, there is only a 10% chance that the wind will be greater than 20 kts at that point and time. This is precisely the type of forecasting we need when we have crucial routing decisions to make.  See Evaluating a Weather Forecast.

These data are presented in standard grib2 format, but they are special parameters and not many apps have adapted to them yet. To my knowledge the only readily available source and display of this full range of data is the app called LuckGrib which is app for Mac computers and for iOS tablets and phones. Each app is a one time fee of $25 in their respective app stores; there is no charge for data downloads. The data can be obtained directly from NOAA and the viewed in a free program like Panoply, but this is very time consuming process.  LuckGrib has a very convenient interface and certainly state of the art file handling and graphic display. Below is a brief demo of these probabilistic forecasting features.  I am sure that more apps will adapt to these parameters once the utility of the data becomes better understood and tested. Several apps show the ensemble runs but not yet the standard deviations.

A quick look at GFS compared to GEFS standard deviations and NBM Oceanic probabilistic wind forecasts [12m:38s].

7. View sea ice coverage from RTOFS in LuckGrib

If you plan to travel to high latitudes, RTOFS as of this year also provides sea ice coverage. We can obtain and view this new ice data in LuckGrib—Saildocs has RTOFS currents but not yet the ice data. See the notes above on RTOFS model runs.

Looking at several northern waterways for sea ice coverage with LuckGrib [3m:44s].

8. Compute optimal sailing route

We come now to what is in one sense the ultimate goal of working with model forecasts in GRIB format, namely using that data along with the known performance of our boat in various wind conditions (polar diagrams) to compute the mathematically optimum route across the forecasted wind field, taking into account forecasted ocean currents and waves as they might apply. There is a section of Modern Marine Weather devoted to this topic that highlights the precautions we must keep in mind when relying on such results. It is crucial to go over those points before using such data, computed yourself or provided by third party services. Recall the joke of being told when asked for directions that "you cannot get there from here." For a sailor who does this routing incorrectly from the beginning (often traced to inadequate polar data), this might no longer be a joke!

There are several apps that provide optimum routing. The historic leader in this field has been the PC navigation program Expedition, which not only provides the GRIB files and does the routing computations, but has state of the art procedures for collecting and analyzing performance data so the best polar diagrams can be made. It also has many other custom features, which makes the investment worth it for the majority of racing sailors, worldwide. Cruising or day sailors may not need the full power of that app. There are several books on the use of Expedition

A short overview of optimum routing using Expedition [18m: 23m].

Another sophisticated routing solution is the add-on routing option to LuckGrib for Mac or iOS devices. That link also includes a detailed discussion of the routing process. The key to any good routing solution is the ability it offers to study the proposed route to learn why it was presented as the "optimum" and now sensitive that solution is to other factors. Luckgrib has several unique approaches to this function.

A short overview of optimum routing using LuckGrib [12m: 34m].

All good routing apps include numerous filters to be applied, such as avoid winds greater than a specified speed, or waves bigger than a specified height, motor if the wind speed is below something, or sailing speed below something, or scale the polar boat speeds up or down by a factor, or scale the model winds by some factor, or scale the effect of current by some factor, add so many seconds for every tack or jibe, don't sail closer upwind than some value or downwind by some value, and so on. There are many, depending on the app.  

We do not get into any of these details here, but leave those to our textbook and online courses, and to the several other books and online seminars on the topic. The job at hand is just to show a few solutions to see what they look like, and to note a couple options for practicing with this on your own. 

Also to consider is the Time Zero app, which is another popular commercial app for Windows. It  evolved from the collaboration of Nobeltec and the MaxSea app, which was one of the pioneers in optimum weather routing.  

As for open source or free products,  both OpenCPN and qtVlm have both Mac and PC routing functions. Below is an article and video demo on the OpenCPN version. 

A video demo of OpenCPN routing procedure [22m:55s]. Details are presented in a separate article.

Optimum routing is a key part of qtVlm, which is a popular app for following along and taking part in ocean races online. It is a free program for both Mac and PC, with grib file sourcing as well as routing. Even though this is a free program, its routing function is among the most sophisticated of the offerings. 

A video demo of qtVlm routing procedure [17m:33s]. Details are presented in a separate article.

Optimum sailboat routing in local coastal or inland waters is the exciting frontier of this technology. This must be done with the regional models, with special adjustments of the routing app routines. We will add notes on this process shortly.

9. View ASCAT scatterometer near-live satellite wind measurements

This, it turns out, is not a new technology. The Ocens company in Seattle had a grant to distribute the QuikSCAT data in GRIB format during the last few years of that program, which ended in 2009. The European program ASCAT has replaced that now, and these new data are available in graphic format and in GRIB format.  To my knowledge the only two packaged apps that can both download and display this data are Expedition and LuckGrib. Without these apps, you can download the files yourself and view them in Panoply. We have a video on how to do that. The latest data files are not large as they only include the latest passes. ASCAT wind is the truth meter for any forecast.

We do not have a video demo of this powerful tool because at the moment (early Feb, 2021) the data are not available. There is some technical snag going on. This is the first time we have seen this in years. We should note, however, that the GRIBed ASCAT data, since its inception nearly 15 years ago, has always been labeled "experimental," which distinguishes it from "operational."  The latter are intended to be guaranteed; the former are specifically not.  I will update this when it gets fixed.

We have ways to get this data onto your boat at sea by email request, but before rejuvenating that technology we will wait and hope that the NWS gets this sorted out. 


That concludes this brief overview of GRIB file applications. Please stand by the announcement of a new short online course we will offer on this topic with hands on practical details. 

Related topics:  The series of articles we have on related topics are in our blog index for Jan and Feb of 2021.

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