Re: How to calculate the annual average

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Ming,
When the time series is done in terms of index value, you caon compute 
the annual averages like this:

 define axis/t=6:4800:12  yearaxis
 let sst_year = sst[gt=yearaxis@ave]

The output axis is in the same units (which is just the index value, but 
you can think of it as months).  I start with 6 so that the averages are 
taken over the first 12 points, then 13 through 24, and so on.  To test 
what this is doing, here is an example where I make up some data, where 
the variable has the value of the index.

  yes? def axis/t=1:4800:1 tmonth
  yes? def axis/t=6:4800:12 tyear

  yes? let tt =  t[gt=tmonth]
  yes? let ttyear = tt[gt=tyear@ave]

  yes? list/l=1:5 ttyear
            VARIABLE : TT[GT=TYEAR@AVE]
            SUBSET   : 5 points (T)
   6    / 1:   6.26
   18   / 2:  18.00
   30   / 3:  30.00
   42   / 4:  42.00
   54   / 5:  54.00

The first value is different from the expected 6 because the first 
averaging interval in T (t=1 to 12)  is taken from the start of the time 
axis, 0.5,  to 12, so the weighting factor is 11.5. The start of the 
axis is t=0.5 not 0. The second interval (t=12 to 24) takes the average 
from 12 to 24, so the weighting is 12.

We can also see this if we compute unweighted average computed by taking 
the transformation @SUM and dividing by the number of good data, @NGD:
 
  yes? let ttsum = tt[gt=tyear@sum]
  yes? let ttcount = tt[gt=tyear@ngd]
  yes? let ttyear = ttsum/ttcount
  yes? list/l=1:5 ttsum, ttcount, ttyear
               T: 0 to 60
   Column  1: TTSUM is TT[GT=TYEAR@SUM]
   Column  2: TTCOUNT is TT[GT=TYEAR@NGD]
   Column  3: TTYEAR is TTSUM/TTCOUNT
            TTSUM TTCOUNT TTYEAR
  6    / 1:   72.0   11.50   6.26
  18   / 2:  216.0   12.00  18.00
  30   / 3:  360.0   12.00  30.00
  42   / 4:  504.0   12.00  42.00
  54   / 5:  648.0   12.00  54.00



Ming Yang wrote:

> Hi, Ansley
>   After I reading the FAQ, I still cannot figure out how to compute the
> annual average. It is probably because my data set is different from the
> example listed in the FAQ. Here is the basical information of my dataset
> yes? use cresum001710.sst.nc
> yes? show data
>     currently SET data sets:
>    1> ./cresum001710.sst.nc  (default)
> name     title                       I         J         K      L
> SST      Sea surface temperature    1:144     1:72      ...   1:4800
>
>   I think the difference is that in my dataset the grid is defines as
> IJKL instead of XYZT. And also the L axis is not based on any calendars.
> Basically what I want to do is to calculate the average values for every
> 12 Ls so that to the transformed data has 400 points in L axis. Do you
> know what is the best way for me to do so? Thank you for your help.
>
>
> Ming
>
>