A difference between reference time and update time

[vsinceac]

There is no 1 grib file for all validities: each of them comes in a single, dedicated grib file. There are 209 grib files for each runtime, from f000 to f384, where the number is in hours related to runtime. From f000 to f120, validities are provided every 1h, then every 3h up to f384.

You can check e.g. this one (runtime today at 06hUTC, validity for 07hUTC, 515.1 GB, all params/levels): https://nomads.ncep.noaa.gov/cgi-bin/filter_gfs_0p25.pl?file=gfs.t06z.pgrb2.0p25.f001&dir=%2Fgfs.20211014/06/atmos.
Here you can select the day, the runtime, the forecast, then optionally only partial parameters/levels/coverage to download: https://nomads.ncep.noaa.gov/cgi-bin/filter_gfs_0p25.pl?

Download time of course depends on many factors:

• internet speed/latency/bandpass/etc. (it may be also regulated sometimes by NCEP/NOAA itself)
• one can download only part of available grids (parameter/level) in a grib2 file (size to download would be smaller)
• I don't know if Windy downloads all 209 validities (size to download would be smaller)
• full size of 209 grib2 files for a runtime is about 110 GB
• for an average download speed of 30 MB/s from NCEP/NOAA servers (which is not easy to obtain e.g. from Europe), you should download all grib files of a runtime in about 1h

Then I guess Windy servers spend also some time to ingest the downloaded grib2 files before being able to serve map data layers to client side.

[Paspox]

@vsinceac Thanks a lot for such a thorough reply. Slowly, thanks to people like you, I understand more and more about the details of these weather updates.

FYI, my interest in the "precise" timing of Windy is because I play an e-sailing game (called Virtual Regatta) and it uses real world weather data (GFS model, only wind speed and direction).
So, for example, when the 00z file is released (@3h40 UTC), the game's weather starts to update (takes around 2h) and often, it is interesting to look at Windy but I wanted to know if what Windy shows is still the old (18z) data or already the new one (00z) and when/how it goes from one to the other.

Again, thanks for your reply.

[Paspox]

@vsinceac Me again, with a few more questions, still concerning the GFS model.

In your message, you said "runtime today at 06hUTC, validity for 07hUTC, 515.1 GB, all params/levels"
I believe you meant 515.1 MB, right?
515MB x 209 validities = 110GB for a full run.

Do you know of a website where I can follow live the release of the different validities by the NOAA?
I know about this one (https://www.nco.ncep.noaa.gov/pmb/nwprod/prodstat/index.html#TARGET).
But is there a way to follow the release progress at the file level?

One last question:
Is there a difference in accuracy between the 4 daily updates?
I read in an old article (from 2012) that the 00z and the 12z were better than the other two.
I was surprised to read that and maybe it was the case back then but do you know if that is still the case?

Thanks

[itblocksg]

@matijamet said in A difference between reference time and update time:

@Korina ,

if you look on the "reference time" for NEMS, for example hier, https://www.windy.com/multimodel/44.107/15.326?44.108,14.248,8
you'll see for METEOBLUE (NEMS) :

Updated: 2020-02-27T07:27:42000Z
Reference time: 2020-02-25

Or, I don't understand something?

Thanks!

weird

[idefix37]

@itblocksg
This is not weird and @Korina explained the difference.
NEMS is a model provided by the Meteoblue company.
But this model is different from the model called here METEOBLUE (more exactly Meteoblue AI) in the point forecast:

Additional information:
https://community.windy.com/topic/10316/metoblue-wildly-inaccurate-in-the-sw-usa/8?_=1637514782709

[Korina]

@itblocksg Hello, it would be nice if you could explain why it is weird.

[Zancle10]

Salve, se i tempi di riferimento ufficiali per il modello ECMWF sono 0.00 e 12.00, i due aggiornamenti in più con premium su cosa si basano? Grazie

[Korina]

@zancle10 Hello, this is an old article. You can find the update times at Model updates times.

[ColtonAxel]

I think Reference time is the time when a model starts a new forecast block. The process consists of the following: Initialisation - data are entered into the model, this creates Initial conditions (initialisation time) and then model starts calculating conditions.

[youtubevanced55]

@ColtonAxel thank you so much..

[imran512]

@Korina said in A difference between reference time and update time:

A difference between reference time and update time
Frequently Asked Questions ARTICLE 13 26 12.9k

Reply

Korina
21 Feb 2020, 18:28

Each model has a different update time and it could be confusing what connection does it have with the update time.

Here is the explanation:

Reference time - is the time when a model starts a new forecast block.
It consists of:

Initialisation - data are entered into the model, this creates Initial conditions (initialisation time) and then model starts calculating conditions.
Assimilation - normalisation on data and assembling data for computation.
Computation - calculating the future atmospheric rates of change in time increments.
Data extraction - extracting data into regular forecast intervals.
Data storage - data are stored and written in accessible format.
Data postprocessing - special postprocessing routines

For example model ECMWF has an update interval 12 hours (or 6 hours interval for Premium users), that means the first reference time will be 00:00:00Z (Zulu time) and second one 12:00:00Z.

Update time is time between the reference time and the actual update.
The whole process of data computation takes approximately from 8 to 9 hours.
So for ECMWF model the update times are 07:15:00Z and 19:15:00Z.
See the exact update times here - When and how often are the weather data updated?.

Where to find info about reference and update time:

Thanks for the clarification! The distinction between reference time and update time makes a lot more sense now. It’s interesting to see how much processing goes into generating accurate forecasts.