Showing posts from August, 2019

Compact Day of the Year Calendar

There are several occasions in navigation when we need to know the total number of days between two dates, or a new date after so many days from another date.

One example is when planning a long distance coastal or ocean route which yields the total duration, and we know the starting date and time, and want to know the date of arrival.

Another example is when figuring the watch error in cel nav for a watch set some time ago. We might know the watch gains 0.335 seconds every 10 days and it was set on April 14. It is now July 22 and we want to know the watch error.

We could of course start out with "Thirty days has September, April, June and November..." to find number of days for each month (if we did not know this already), and then count out the days. But we don't want to make mistakes at the Nav Table, so it is much easier to look up that (in say 2019) April 14 is day 104 and July 22 is day 203. This difference is  99 days and (99 /10) x 0.335 = 3.3 seconds.  The watch…

Compare ASCAT and WindSAT Scatterometer Wind Data

We have two sources of scatterometer data these days, the 3 Metop satellites with ASCAT data and the US Navy Coriolis satellite with WindSAT data. The latter does not get much mention in official NWS forecast discussions, whereas ASCAT is referred to frequently. The reason for this is not clear, so in  a first step toward trying to understand this, we will compare the data whenever we can spot passes at about the same time, over the same region.  It could be we can calculate that conjunction of passes, which if so we will add to our satellite prediction time article, which is underway.

The OSWT site presents the ASCAT data in a 10º x 15º Lat-Lon grid and the WindSAT in a 20 x 30 grid. They both have in principle 25-km resolution, so the reason for this is not completely clear. It could be simply that the single WindSAT data swath is about twice as wide as one of the halves of the two ASCAT swaths, so it would likely take a different file layout to account for it.

If the WindSAT data a…

Tools for Crucial Weather Routing: With an ongoing comparison of GFS and FV3-GFS.

We will add new data at the end here until we figure out which model wins out!
It can happen that crucial decisions depend on details of a forecast. Usually this will be for a longer term forecast, since one or two days out are generally pretty good.  We have a specific case at hand now, which is a classic, real-world example.  The vessel has two optional routes, on each a crucial turn must be  made roughly 96 hr from now, meaning we are in interested in what happens on Friday, May 31.

The two main primary sources we have are the GFS and the new FV3 GFS, which will replace the former on June 12.  We see below that these two do not agree at all for Friday.

Again, Friday shown here is 4 days out from these  Monday 18z forecasts.  The displays are the meteograms from LuckGrib.  The standard GFS we have used for years shows roughly 20 kts from the east, or just above east.  The in-principle-better model FV3 GFS shows 30 kts from the SE.  For the vessel at hand here, and almost all vessels…

FV3-GFS v. GFS—A comparison of real forecasts

On June 12, the GFS will be updated to the new FV3-GFS, which promises to be a big improvement in global forecasting.  We have been following the winds off the coast of Queensland, Australia in a real world example of needing the best possible long-range forecast. See Tools for Crucial Weather Routing: With an ongoing comparison of GFS and FV3-GFS. This note here is a summary of those results, which we can do now that we know what the winds turned out to be.

We started with a 90h forecast, using Friday May 31 at 12z as our test case.  

The boat was travelling at about 1.5 kts at the time, so the location did not effectively change relative to ocean wind patterns. But we see that at that location, the wind did indeed change a lot with time, so we stick with our 12z reference. Below are the winds for that day and location.

Observed wind from Emerson on Fri May 31 located 154 nmi from Cairns, in direction 123T.

00z to 08z  12 @ 070

08z to 14z20 @ 130-140

14z  to 20z15 @ 130-140

20z  to 00z20 @ …

Another Interactive Test of the Oceanic National Blend of Models

We recently completed a comparison of several models looking for the best extended forecast. The apparent winner in that case was the NBM. There are arguments on why this might in fact be expected to be best for extended forecasts; we will come back to that.

For now we have another, equally crucial test at hand. And in this case it looks like an ideal way to make this test, because the NBM is notably different at 78 hr out than the "competitors," namely GFS, FV3-GFS, and GEFS, and to the best we can estimate it, the ECMWF.

The vessel we are following is moored off of Flynn Reef in the Great Barrier Reef, 30 nmi east of Cairns in Queensland.  He needs winds solidly less than 20 kts to make progress on the last leg of his 9,000 mile row (actual miles rowed, not great circle) from Neah Bay, WA to Cairns AU.

We will use Sat Jun 8, 12z as our test point, which is 78h from the most recent model runs at 06z Wed, Jun 5.  Below are the meteograms for this period at that location that we…