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Board » Technical Support » NOAA vs SOL weather

Hi all,
I have noticed a difference between TWD/TWS as calculated from NOAA grib data and the ones shown in SOL. Would it be possible to know the way SOL interpolates in space and time wind speed and direction?
Thanks in advance and regards
would love to hear that!

--- Last Edited by Ghibli at 2010-12-03 08:50:51 ---

So, about the interpolation of weather in time and space :-) Maybe some out there is longing for a short one-liner as explanation but I am afraid there is a little more to it… Here is in short the steps we take to get an "observation" (as we call it) at an arbitrary time (called t) and location based on forecasts.

1) Find the two forecast time steps (here called t1 and t2) surrounding the time t.
2) In space we simply use linear interpolation (in the 2 dimensions lat/long) of the forecast-data in the 4 closest forecast-nodes in the grib-file. Since the forecast has separate fields, one for northerly wind and one for easily wind we need to treat these entities separately, in our case by using complex notation. We also need to do this for both frames t1 and t2 from step 1 which thus gives us a set of two complex-valued space-interpolated wind speeds, one at t1 and one at t2.
3) Now we need to interpolate in time as smoothly as we can. We chose to do this using simple shape functions ensuring continuous wind speed and actually in our case continuous time derivatives.

Now, there are naturally many ways of doing this. One draw-back of this kind of interpolation is that we loose energy. Imagine the situation with constant wind speed but rotating wind direction so that at t1 the wind is due easily and at t2 it is due westly. In our interpolation we would (falsely) get that the wind speed at the time (t1+t2)/2 would be zero.

Also note that we do not always use the GFS model from NOAA. In particular around Sweden we have more detailed forecasts.

Hope this helped a little.

...one of the guys behind the game...
Hi Jakob,
to keep in one line, the model smooth and make continue the derivative of a complex 3 variable function to avoid that the function has some jump. [ actually more than one line ;-) ]
need to think about it deeper, may be come back later.
thanks for moment
Would it be possible to differentiate between those wind arrows (highlighted color perhaps) that represent the actual data nodes & those arrows that are derivative which comprise the field.
None so blind
Nice topic indeed.

So, in my own words: two frames of weather grib are interpolated in space using bilinear. This operation gives two different values for the wind vector which are separated in time by the gap between the two frames (3h with GFS or whatever).

Now, in order to have a smooth transition between frames a simple linear approximation is not good because it will give discontinous values of first derivative corresponding to the frames change. And so the need for a proper interpolation function that can smooth the derivative at the beginning and the end of the interval.

One can argue that the only possible value of the derivative at these point is zero. But another important aspect is that, given the shape function, we have steeper gradients in time in order to compensate for the nulling of the derivative at the extremes. Am I right? And this is extremely dependent by the shape of the interpolating function.

my 2 cents
Hi all,

You are basically all correct :-) I am the first to admit that there are many ways of doing this and that we did not spend weeks of thinking when choosing algorithm - but it works. Also (Hubert) the game winds will only correspond exactly with the GFS-forecasts when the time is exactly the time of a certain forecast frame and the point of the "observation" is exactly that of a forecast nodal point. Thus, colour coding would not be of any use at all...

Why this attention? Is it just curiosity or weather-rounting ?

...one of the guys behind the game...
it curiosity driven by weather routing :)

as you remember I was coding my own router and, although I did it for pure fun, I am now checking it against some of the market available competition like maxsea or deckman. All of them share the common feeature of giving different wind values at the same place, same time and same wind data!! So the interpolation is critical and although I can rely on more elaborate intepolation technique, I will definite give a look to this zero-derivative approach.
...well you should be careful with those softwares... especially when there is a Sailplanner available for free ;-)
...one of the guys behind the game...
well... I need to admit that the question from my side was originated because I have notice some differences... and I am using SOL to prepare my sailing licence exams from navigation point of view, but you pass me by far in the discussion...
I appreciate your effort to educate the SOL community and I believe that I must open the old book to follow you now...... thanks!
one of the guys enjoing the game ;-)

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