# Re: annual average from monthly mean for precise 365-day years.

```Hi Wei,

I'm not sure what problem you are describing. Here is an example of averaging 24
months of data into 2 annual averages:

/export/home/ja3/hankin/tmp> ferret
NOAA/PMEL TMAP
Program FERRET
Version 5.22 - 07/27/00
17-Mar-01 14:03

yes? define axis/t=1-jan-1980:1-jan-1982/npoints=24/edges t24months
yes? let my_var = T[gt=t24months]
yes? define axis/t=1-jan-1980:1-jan-1982/npoints=2/edges t2years
yes? list my_var[gt=t2years@ave]
T[GT=T24MONTHS]
regrid: 8772 hour on T@AVE
JUL-1980 / 1:  696546.
JUL-1981 / 2:  705318.

Will this approach work for you?

- steve

P.S. Note that to simplify the example I have used equal-length months. See Ref
Sec4.2. DEFINE AXIS and the FAQ at
http://ferret.wrc.noaa.gov/Ferret/FAQ/analysis/annual_from_monthly.html for details
on creating exact calendar months and years.

==============

Wei Cheng wrote:

> > The best/right way to perform annual averaging is to define the desired
> > result axis so its cell boundaries exactly align with the cells of the source
> > axis. Your Method 2 looks like this approach. I have modified your Method 2
> > just slightly and plotted it using the V5.3 PLOT/STEP style to illustrate
> >
> >      define axis/edges/t=1:25:1/unit=months tax
> >      let newtaux = T[GT=tax]
> >      plot/step newtaux
> >      define axis/edges/t=1:25:12/UNITS=months tann
> >      let tauxx=newtaux[gt=tann]
> >      plot/step/over tauxx
>
> Hi, Steve,
>
> Thanks very much! Still, I have one last problem:
>
> In your above example, if number "25" is replaced by "24",
> and, if line
>
> "let tauxx=newtaux[gt=tann]"
>
> is replaced by
>
> "let tauxx=newtaux[gt=tann@ave]"
>
> By "list tauxx", we can see that the last point in tauxx is a bad value.
> This must be due to using @ave, plus the fact that now the total data
> point is an even number (24 instead of 25).
>
> How can I get the last point in tauxx back (it should still exist)?
> In climate studies, number of months is even for multiple whole
> year's data (how unlucky it is that each year has 12 months). But
> lucky, we have you!
>
> Thanks again.
>
> Wei

--

|  NOAA/PMEL               |  ph. (206) 526-6080
Steve Hankin    |  7600 Sand Point Way NE  |  FAX (206) 526-6744
|  Seattle, WA 98115-0070  |  hankin@pmel.noaa.gov

```