Re: [ferret_users] question on yearly average

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

Hi Paul,
What you're computing is the average of each 12 months, centered at 
months 1,13,...  The first data returned is at the first L you asked 
for, L=1. Then the next value is a 12-month average centered at L=13, 
L=25, etc.  You probably wanted to ask for the average of 1993, 1994, 
1995, ...  You can verify that with a LIST command.  The data will be 
shown as January 1993, Jan 1994, and so on. So what was computed is the 
data averaged from July-June of each year.

In Ferret we always want to be thinking about data defined over grid 
cells, and whether commands are directing Ferret to work with the grid 
cell coordinates or the grid cell edges. By default Ferret commands 
refer to coordinates at the center of the grid cell. Here, you could get 
at least much closer to what you intended with this:

   list var[L=6:234:12@ave]

but because there are an even number of months in the year, there's an 
issue with whether you should use L= 6 or 7 to split the year at the end 
of June/beginning of July, right?

To be sure of what you're doing, the best way is to define a time axis 
which captures the result you really want and use a regridding 
transformation:

   yes? define axis/EDGES/t=1-jan-1993:31-dec-2012:1/unit=year tyear
   yes? list var[gt=tyear@ave]

There is a good discussion of transformations and regridding, here
http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/variables-xpressions/XPRESSIONS#GeneralInfo

and in particular time-averages are discussed here - it shows another 
perhaps better way to define the yearly axis:
http://ferret.pmel.noaa.gov/Ferret/faq/averaging-to-regrid-a-time-series

and further good information is here,
http://ferret.pmel.noaa.gov/Ferret/faq/regridding-a-time-series-with-ave






On 8/14/2013 9:01 AM, Paul Goddard wrote:
> Hi,
>
> I have a basic question but it is made me wonder for quite some time.  
> Let's say I have 20 years of monthly data, Jan 1993 -Dec 2012, and I 
> want to list out the yearly average of those months.  I usually compute,
> list = var[L=1:240:12@AVE]
>
> The output is 20 data points, however, the values are not the same as 
> if I extract the 1:240 data points into excel and calculate the 
> average for each year "by hand."
>
> Why the discrepancy?  and what should I do if I want to calculate 
> yearly means.
>
> Thanks in advance,
>
> Paul