Re: time_counter as axis

[Elizabeth Dobbins <Elizabeth.Dobbins@xxxxxxxx>]

Hi;

Don't know if this is the problem, but climatologies work better if the
data is in the middle of the month, rather than on the 1st.  The user's
guide has a good description of the issue at the end of Ch4 Sec2.5.
(Modulo regridding), and an example of using linear interpolation to fix
it.

Liz


On Wed, 1 Sep 2004, Arthur M. Greene wrote:

> Hi Jason,
>
> Thanks for the reply! Here's what I've been able to do (There are 5
> years of monthly variable v; we wish to construct a climatology for the
> last three years):
>
> let v_3to5yr = tsequence(v[L=25:60])   ! Extract desired time slice
>
> define axis/T=0:1095.7275/npoints=36/edges/units=days/\
> T0="01-jan-1974:00:00" t3to5yr
>
> let v_3to5yr_t = v_3to5yr[GT=t3to5yr@asn]
>
> So far, so good. I now have three years of data on a real-time axis,
> beginning with jan-1974. The plot looks OK. Now for the climatology:
>
> define axis/T=0:365.2425/EDGES/NPOINTS=12/\
> T0=1-JAN-0001/UNITS=DAYS/MODULO tclim
>
> let v_clim_12mon = v_3to5yr_t[GT=tclim@MOD]
>
> This does produce a 12-month plot that _looks_ like a climatology (i.e.,
> x-axis reads Jan, Feb, Mar...), but the values are not equal to the
> averages of the corresponding month's values in v_3to5yr_t, i.e.,
> v(Jan,v_clim_12mon) <> (Sum of Jan vals in v_3to5mon)/3. So perhaps I
> don't understand how the MOD (or ASN, for that matter) climatologies are
> actually computed. Even if the 36 months of data were interpolated into
> daily values, and the appropriate daily values averaged to produce the
> monthly climatology, the resultant would not appear to be correct. The
> docs do not seem to clarify exactly how these climatologies are computed
> (or maybe I need a refresher!)
>
> Your comments greatly appreciated!
>
> Thanks,
>
> Arthur
>
> On Wed, 2004-09-01 at 08:33, Jaison Kurian wrote:
>
> >
> > ! --------------example.jnl--------------------------------
> >    ! extract last three years data ie. abstract index L=25:60
> >
> >    let v_3to5yr = TSEQUENCE(v[L=25:60])
> >
> >    ! define monthly time axis from JAN-1997 to DEC-1999 & assign it
> >    !     T=0:`365.2425*3`/NPOINTS=36/UNITS=DAYS together will make it for
> >    !      3 years and  T0 will take care of actual year stamp.
> >
> >       define axis/T=0:`365.2425*3`/NPOINTS=36/EDGES/units=days/\
> >                   T0="01-JAN-1997:00:00" t3to5yr
> >
> >       let v_3to5yr_t = v_3to5yr[gt=t3to5yr@asn]
> >
> >    ! Define a monthly climatological axis
> >
> >       define axis/T=0:365.2425/EDGES/NPOINTS=12/T0=1-JAN-0001/\
> >                      UNITS=DAYS/MODULO tclim
> >
> >    ! Find monthly climatology by "MODULO" regridding
> >
> >       let v_clim_12mon = v_3to5yr_t[GT=tclim@MOD]
> >       plot v_clim_12mon
> >
> > ! If you want to find anomaly or want to plot the climatology
> > !   over the data then please refer :
> > !   http://ferret.pmel.noaa.gov/Ferret/Mail_Archives/fu_2003/msg01470.html
> >
> > !---------------------- end of example.jnl-------------
>
>
>


*********************
Elizabeth L. Dobbins
Research Scientist
JISAO/PMEL
phone: (206) 526-4581

"Various forms of jiggery-pokery were used to suit
horses to courses."  -- Michael Burleigh