Re: [ferret_users] ave vs. sum and ngd

[Stephen Guimond <sguimond@xxxxxxx>]

Also, what I have done to get the unweighted mean is to remove the section of data and then use @ave...

let section = temp[l=16:18]
let mean = section[l=@ave]

Steve

----- Original Message -----
From: Ansley Manke <ansley.b.manke@xxxxxxxx>
Date: Monday, June 22, 2009 9:59 am
Subject: Re: [ferret_users] ave vs. sum and ngd
Cc: ferret <oar.pmel.ferret_users@xxxxxxxx>

> Hi all,
> There's an FAQ related to this discussion, about the averaging 
> weights 
> that Ferret uses. It shows a way to see the weights that are being 
> used.
> http://ferret.pmel.noaa.gov/FERRET_17sep07/FAQ/analysis/regridding_with_ave.html
> 
> David Wang wrote:
> > Peter,
> >
> > if you do
> >
> > show grid/l=16:18 temp
> >
> > are three numbers under TBOX are the same? this is the weight 
> that 
> > Ferret would take.
> >
> > David
> >
> > On Mon, Jun 22, 2009 at 10:16 AM, Peter Szabo 
> <szabpet83@xxxxxxxxx 
> > <mailto:szabpet83@xxxxxxxxx>> wrote:
> >
> >     Martin,
> >     as i am dealing with climatology, all the months are equally
> >     important, so i should use the @sum/3 for a spring average.
> >     Thanks, Peter
> >
> >     ---------- Forwarded message ----------
> >     From: *Martin Schmidt* <martin.schmidt@xxxxxxxxxxxxxxxxx
> >     <mailto:martin.schmidt@xxxxxxxxxxxxxxxxx>>
> >     Date: Mon, Jun 22, 2009 at 12:28 PM
> >     Subject: Re: [ferret_users] ave vs. sum and ngd
> >     To: Peter Szabo <szabpet83@xxxxxxxxx 
> <mailto:szabpet83@xxxxxxxxx>>>
> >
> >     Peter, the answer is most probably the following example:
> >
> >     yes? list (1.74*31/2+8.74*30+15.34*31/2)/61
> >                VARIABLE : (1.74*31/2+8.74*30+15.34*31/2)/61
> >             8.638
> >
> >     The values at the edges are counted with half weights. 
> However, I
> >     am not sure about the remaining difference.
> >
> >     Greetings,
> >     Martin
> >
> >
> >     Peter Szabo wrote:
> >
> >         Dear Users,
> >         i know there is a difference between @ave and @sum, @ngd. It
> >         is not really a
> >         question, but still want to post it.
> >
> >         I list 3 timesteps:
> >         yes? list/nohead temp[l=16:18]
> >          16-MAR-1962 / 16:   1.74
> >          16-APR-1962 / 17:   8.74
> >          16-MAY-1962 / 18:  15.34
> >
> >         Here is the real average for it. The mean i guess.
> >         yes? list/nohead temp[l=16:18@sum]/3
> >                  8.605
> >
> >         The (weighted) "average" is slightly different. Though if
> >         there is no
> >         missing value, they should be the same.
> >         yes? list/nohead temp[l=16:18@ave]
> >                  8.643
> >
> >         Why is that weightening in @ave? The 3 timesteps are
> >         equivalent for me. I
> >         should just use the sum and dividing then?
> >         If i use @var later on (which is also a weighted variance),
> >         should i use
> >         @ave then?
> >
> >         Thanks, Peter
> >
> >
> >
> >          
> >
> >
> >
> >
> >
> >
> >
> >
> > -- 
> > turn and live.
> 
> 
> 

=======================================================
Stephen R. Guimond
Graduate Research Assistant
Florida State University
Center for Ocean-Atmospheric Prediction Studies (COAPS)
=======================================================