>255 hrs simulation WPP error

Questions relating to the use of the WRF Post Processor V3

>255 hrs simulation WPP error

Postby estatio » Thu Jun 19, 2008 12:39 am


In time step 256, the grib file' date return to the initial time. This happens every 255 time step interval, for example in time step 512 the date is returned again to the initial time. Any ideas?
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Re: >255 hrs simulation WPP error

Postby The_Samuelson » Mon Jul 14, 2008 1:57 pm


I have seen this problem also. I wanted to have GRIB output in minutes and needed to extend beyond 255minutes. However after 255, the counter would reset to 0. The issue as far as I can tell is that due to the byte maximum in a GRIB file and its inherent compression, one can only have a maximum of 255 in the forecast step. I googled the problem and found it to be a recurring topic in GRIB communities. I am not sure if this is true for GRIB2 format, but for GRIB1, 255 is the max forecast step. After 255, the counter resets to 0 again.

I would welcome any more technical explanations, though... it is troublesome!

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Location: Burlington, Massachusetts, USA

Re: >255 hrs simulation WPP error

Postby smasters » Tue Aug 05, 2008 3:29 pm

Check carefully the grib1 documentation on this. The forecast hour parameter is actually a 2-octet field. Normally, we (including me) write grib-reading code that only reads the first one, which can only hold values up to 255. I don't have my full grib documentation in front of me, but looking at a document on the web, it looks like the forecast hour is in octets 14 AND 15. The high-order octet is usually zero, but will be 1 (or larger) once your forecast hour counter goes above 255.

One way to verify this...check out one of the GFS forecast files at hour 300.

Steve Masters
Melbourne, FL
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Re: >255 hrs simulation WPP error

Postby darkvieder » Wed Jul 22, 2009 6:15 pm

interesting.. im going to check this one out.. thanks a lot

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