Re: [ferret_users] Daily Averages from 6 hourly data

[Jaison Kurian <jaison@xxxxxxxxxxxxxxxxxx>]

Hi Ashley,
           You can use @AVE regridding on time axis. However, it 
should be noted that @AVE computes 'the average weighted by grid box 
size". Hence you should be carefull while defining the destination'
daily time axis. For more information, please see

User Manual 
             Ch3 Sec2.4.6.  @AVE-average
FAQ          
   http://www.ferret.noaa.gov/Ferret/FAQ/analysis/regridding_with_ave.html

Mail Archives (discussions starting with the following one)
   http://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2006/msg00679.html

A quick example is given below.

Hope this helps,

Regards,

Jaison

!----------example starts here---------------------------------
\cancel mode verify
!
! Description : Example for regridding 6-hourly data to daily time axis
!                 using @AVE.
!
! create some dummy data

    define axis/t="01-OCT-1998:00":"01-JUN-1999:18":6/units=hours\
            /calendar=GREGORIAN/T0="30-SEP-1998:00" t6hr

    let var = RANDN(t[gt=t6hr])+ SIN(t[gt=t6hr]/200) 

    plot var
 
! suppose we want to average data at 00, 06, 12 and 18 hours to get daily
!   value. In this case the time points on the daily time axis should
!   be centered at (00+18)/2 = 09 hours. Use exactly the same calendar
!   as that of 6-hourly time axis. If you are not sure, use return 
!   command : yes? say `my_6hr_var,return=calendar`.

    define axis/t="01-OCT-1998:09":"01-JUN-1999:09":1/units=days\
            /calendar=GREGORIAN/T0="30-SEP-1998:00" tdaily

    let var_daily = var[gt=tdaily@AVE]

! check it : if your 6hrly data does not start at 00 hours, the
!   following example may not work out....you have to choose proper
!   L-range.

    set list/precision=7
    list/l=1:4 var       ! (first days values at 00,06,12 & 18 hours)
    list var[l=1:4@AVE]  ! average it on L, not on T
    list/l=1 var_daily

    plot/ov var_daily

!-----------example ends here-----------------------------------------

On Wed, 20 Jun 2007, Ashley Watson wrote:

> Hi ferreters,
>           I am trying to get daily averages of a varaible which has values
> from 01-oct-1998 to 01-jun-1999 with timestep of 6hours. As it spreads
> over two years 1998 and 1999, I am getting problem in defining output
> timestep. Any hint or suggestion how to calculate this daily average.
> 
> 
> yes? show data
>      currently SET data sets:
>     1> ./latentheat.nc  (default)
>  name     title                             I         J           K
> L
>  LTH      Latent_Heat             1:150     1:130     ...       1:973
> 
> 
> Thanks in Advance
> Ashley
> 
> 

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