ASCAT winds are satellite scatterometer measurements of wind speed and direction on the surface of the ocean and other large bodies of water, normalized to a 10 m height. The data resolution is 25 km, which is comparable to the resolution of typical global weather models available to the public such as GFS (27.8 km) and ECMWF (44 km). New data are available typically four times a day, from various combinations of two satellites, Metop-B and Metop-C, tracking N-NE (ascending) or S-SW (descending), bringing us data from either the port or starboard side of their split data swaths. For background in ASCAT, see www.starpath.com/ascat. There is a video at the end here demonstrating the use of the data.
Before presenting the specifics of how to use this data, let me back up a moment and put this in perspective. Wind speed and direction are the key factors in marine navigation, in large part because the wind makes the waves, which can be a hazard to any size vessel. For sailors, it is even more primary because wind is their engine. Thus this is the most fundamental information available. It is like having the ocean covered with thousands of buoys measuring wind speed and direction. But unlike isolated real ocean buoys who give us data every 10 min in some cases, hourly at worst, the ASCAT data covers large swaths of the ocean but only give us data 3 or 4 times a day.
In a sense, the ASCAT data have done a major job for us even if we do not look at it, because it is a crucial input to the global weather models whose forecasts we must rely on for routing. But even though the models have assimilated the latest ASCAT data, the model forecasts are not guaranteed to be correct. It is specifically not a goal of the models to reproduce the ASCAT and buoy observations. They have a broader goal to produce the best overall forecast at many levels of the atmosphere. In short, the model forecasts may in fact not be correct in some circumstances, which is why we must compare several models to decide which is likely the best.
And indeed, circling back to check a model forecast with actual ASCAT measurements at a specific place and time we care about is one of the primary reasons for us to access the ASCAT data ourselves. If one model forecast agrees with the ASCAT measurements and another does not, it is most likely the better one to use for our routing computations
Another reason to continually monitor the ASCAT wind measurements for our intended route is the occasional observation of localized anomalous flow. We might spot a notable hole in the wind or a shear line that is not apparent in the global model forecast. In these cases, we have to realize that the ASCAT are real data. That is what the wind was doing at that time, regardless of what the model might imply or not show, and we need to route with that in mind.
The other aspect of "perspective" is to recognize that even though these ASCAT wind measurements are indeed the most fundamental data we care about, the use of this data, which takes several easy, but non-standard procedures, is definitely in the realm of "going the extra mile" to learn the very most we can about the wind ahead of us. For a racing sailor, this is standard operating procedure, but when cruising it would be called on less often, unless we are negotiating a dangerous wind pattern or, more likely, some light air pattern in which we are just looking for enough wind to get there.
We have presented this perspective in the past, and in earlier editions of our text Modern Marine Weather, but then after outlining the basic guides to getting the data by standard procedures, we moved on. Based on discussions with practicing navigators, however, it seems that we needed a more specific procedure for accessing this crucial data, and that is what we have created.
We have semi-empirically compiled graphic indexes of the available data and pass times for the typical cruising and racing waters around North America and Europe, presented below, as well as ways to automatically access the latest available data of interest. (Note that we are not covering here the even more convenient means of obtaining this data in grib format, which can be achieved with LuckGrib or Expedition. Users of those two fine apps, might still find some benefit from the timing structure we present here.)