How winds are calculated in VR and eSail4VR ?

Let's try to understand the information given by eSail4VR and Virtual Regatta (VR).

In eSail4VR

A GRIB file is identified by an identifier such as:
GFS025 20241209 12Z +384

That means :
1. GFS025: The weather model used is GFS, with a resolution 0.25°.
2. 20241209: The model initialization date is  Decembre the 9th, 2024.
3. 12Z: The last available run (simulation) is that at 12h UTC.
4. +384: This file includes 384 hours of weather forecasts from this run.

However, this GRIB file is actually made up of several time steps from different runs. In the following example, we have the following time steps:

  • 20241209.06.009: wind for the 9th hour of the 06Z run
  • 20241209.06.012: wind for the 12th hour of the 06Z run
  • 20241209.12.009: wind for the 9th hour of the 12Z run
  • 20241209.12.012: wind for the 12th hour of the 12Z run
  • ...
  • 20241209.12.384: wind for the 384th hour of the 12Z run

But how to calculate the boat's wind at 19h52 UTC (20:52 local time in Paris)? ⏱️

We use a linear interpolation between the 2 time steps closest to 19:52 UTC in our grib file.

  • 20241209.06.012 :  it is the 12th hour of the 06Z run, so the wind forecast at 12 + 6 = 18h00 UTC
  • 20241209.12.009:  it is the 9th hour of the 12Z run, so the wind forecast at 9 + 12 = 21h00 UTC

The length of each time step depends on the current time:
At 19h52 UTC, we are 112 minutes after 18h00 and 68 minutes before 21h00.

  • the weight of time step 20241209.06.012 is therefore 68
  • the weight of time step 20241209.06.009 is therefore 112/180

In short, at that time, the wind used in eSail4VR is an interpolation between:

  • 38 % of the data from the 06Z run (time step 20241209.06.012)
  • 62 % of the data from the 12Z run (time step 20241209.12.009)

🌊 In Virtual Regatta

Virtual Regatta's interface shows this interpolation graphically:

[caption id="attachment_34307" align="alignnone" width="350"] Screenshot of VR where we have added the percentages of each time step[/caption]

It is 8:52pm (French local time), so 19h52 UTC.

  • The blue section of the left-hand gauge represents the weight of the 06Z run, equal to 38 %
  • The green section of the left-hand gauge represents the weight of the 12Z run, equal to 62 %
  • The gauge on the right is full green, meaning that from 21h00 UTC (10pm local), the boat's wind will come from interpolations between time steps all issued from the 12Z run.

N.B. Percentages numbers are not displayed in Virtual Regatta, only gauges.

✅ Conclusion

Although the information displayed is different, eSail4VR and Virtual Regatta use strictly the same data and the same interpolation logic to calculate winds from GRIB files. These interpolations make it possible to calculate weather conditions at any time, by combining data from the various available runs.

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