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I believe some of the Global Forecast System (GFS) parameters are point values while others are average values over an interval. For example, the parameter DSWRF (Downward Short-Wave Radiation Flux) is labeled as a "0-3 hour ave". Does this mean that for forecast hour 3 from the GFS, the DSWRF value is the average for the interval of hour 3 to hour 6? Is anything not labeled as an average a point value? Is there documentation that answers these questions (I can't find any)? I ask because the documentation for solarforecastarbiter-core claims that "Cloud cover from GFS is an interval average with ending label" (line 88). If this were true for my DSWRF example above, the forecast for hour 3 would be the average for the interval of hour 0 to hour 3 rather than from hour 3 to hour 6.

Thank you for your help!

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These GFS parameters are stored as a running average, reset every 6 hours. This is a bit unusual and requires some special handling to get back to hourly forecasts.

The value you have, time-tagged 03:00 (3h) and labeled as "0-3 hour ave" in GRIB is indeed the average of the forecasts from 0-3 hours ahead, time-tagged interval ending. Or could be understood as the average of what one would typically like to have as three one-hour averages labelled 01:00 (0-1 hour ahead average), 02:00 (1-2 hour ahead average) and 03:00 (2-3 hour ahead average), time-tagged interval ending.

This is as follows for each the different time steps found in GFS data:

  • 01:00 (labeled as "0-1 hour ave"): the forecast for 01:00 (tagged interval ending, 1 hour average).
  • 02:00 (labeled as "0-2 hour ave"): the average of the 01:00 and 02:00 forecasts (interval ending 1 hour averages).
  • 03:00 (labeled as "0-3 hour ave"): the average of the 01:00, 02:00, 03:00 forecasts.
  • 04:00 (labeled as "0-4 hour ave"): the average of the 01:00, 02:00, 03:00, 04:00 forecasts.
  • 05:00 (labeled as "0-5 hour ave"): the average of the 01:00, 02:00, 03:00, 04:00, 05:00 forecasts.
  • 06:00 (labeled as "0-6 hour ave"): the average of the 01:00, 02:00, 03:00, 04:00, 05:00, 06:00 forecasts.
  • 07:00 (labeled as "6-7 hour ave"): the 07:00 forecast. The averaging has been reset after 6h.
  • 08:00 (labeled as "6-8 hour ave"): the average of the 07:00 and 08:00 forecasts.
  • and so on.

To get back to hourly forecasts (or 3-hourly for 3-hourly datasets), you either need to do the math yourself or you can use the wgrib2 option -ncep_norm. See https://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/ncep_norm.html .

To convert back yourself, you need to use the previous average and substract it with proper weighting. Here is short instructions by Shrinivas Moorthi from this thread http://gradsusr.org/pipermail/gradsusr/2016-November/040397.html :

Let us denote the data from 6 hourly output as C1, C2, C3, C4, C5, C6.

To get hourly mean, then
for hour 1, the mean is C1,
for hour 2, the mean is 2C2-C1
for hour 3, the mean is 3C3-2C2
for hour 4, the mean is 4C4-3C2
for hour 5, the mean is 5C5-4C4
and for hour 6, it is 6C6-5C5

This pattern repeats every six ours.

There also is an NCEP/NOAA paper on the model data, where this is explained somewhere deep down, but I couldn't find it right now. The two links above however give a bit of explanation/guidance as well.

And always a good idea to compare your results to the output of a tool doing the conversion automatically.

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  • $\begingroup$ Thank you for your thorough and very helpful answer! I really appreciate your help! $\endgroup$
    – Jordan
    Dec 16, 2022 at 20:50
  • $\begingroup$ Thanks for your comment, Jordan, pleasure. I have come across this a few times over the last years with colleagues asking/being confused... When I searched for this once more now, I still didn't find good documentation, but found your question, so I thought I'd write this down properly once for everyone. $\endgroup$
    – FelixD
    Dec 19, 2022 at 11:05

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