Re: [idvusers] Time in GRIB2 files from CFSR

On Mon, 15 Mar 2010, Guilherme Chagas wrote:

> Hi Robb,
>
> > I fixed the Time problem but I was comparing the results with wgrib2. It
> > seems that wgrib2 calculates the forecast time differently then our grib
> > library. Our library figures out the valid time that is the end of the
> > accumulation interval verses the wgrib2 that uses the start of the interval.
>
> Since the accumulation interval is 1 month the difference should be no
> bigger than 1 month... The way I see it, for the monthly average of
> January 1979 instead of 1979010100 one would see 1979013123 (or
> 1979020100).


Guilherme,

I was interpreting the time ranges wrong for the forecast time. Since 
there are two ranges, the first has value 124 and second has value 6. The
forecast time is calculated as the
reference time + forecast time + ( 124 * 6 ) which equals

 1979-02-01 00:00:00Z
 1979-03-01 00:00:00Z
 1979-04-01 00:00:00Z
 1979-05-01 00:00:00Z
 1979-06-01 00:00:00Z
 1979-07-01 00:00:00Z
 ...

These are the forecast times, the reference time is 1979010100.  The 
definition of forecast time for accumulations is at the end of the 
interval, so if it's a month interval then the end would be the start of 
the next month. These calculations seem correct to me.

Robb...

> > hours since 1979-01-01T00:00:00Z
> > time = 124, 856, 1540, 2280, 3004, 3744, ....
> >  1979-01-06 04:00:00Z
> >  1979-02-05 16:00:00Z
> >  1979-03-06 04:00:00Z
> >  1979-04-06 00:00:00Z
> >  ...
> >
> > wgrib2:
> >
> > 1.1:0:d=1979010100
> > 2.1:149612:d=1979020100
> >
> > Does this make sense?
>
> While it appears that there is a 1 month time-step between records the
> first one starts at 124 hours, when it *should* start at 00Z
> (1979010100). It does get the months right, but perhaps this
> difference on the first record might wrongly increase the gaps between
> each record, eventually leading to differences in the Time (month)
> itself after several steps. Nevertheless the 124 hours at the first
> record makes no sense since it's supposed to be a monthly average!
>
> Do you believe that, despite the wrong initial time, the months will
> correspond to what it's supposed to be, or that eventually it will
> skip a month?
>
> Best Regards,
> ___________________________
>                Guilherme O. Chagas
>                               address@hidden

===============================================================================
Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
address@hidden             WWW: http://www.unidata.ucar.edu/
===============================================================================