Re: [ferret_users] Re: Average with ferret vs nco

[Muyin Wang <muyin.wang@xxxxxxxx>]

Or to get rid of the area weight on lat/lon grid,
use let xm=xin[x=@sum,y=@sum]/xin[x=@ngd,y=@ngd]

this will do simple averages.
Muyin

Ansley Manke wrote:

> Hi Tony,
> Yes, if the grid is defined with units of degrees longitude and 
> latitude, then Ferret does compute with weighted averages.  A way to 
> compare with the nco computation is to set the units of the X and Y 
> axis to something else
>
> yes? set axis/units=" "  `temperature,return=xaxis`
> yes? set axis/units=" "  `temperature,return=yaxis`
>
> And then compute the average - this will treat the units of the axes 
> as just numbers, and might compare with the nco computation.
>
> Ansley
>
> Tony Jolibois wrote:
>
> >     Hi Ansley,
> >
> > You're right for the correction, nco do not compute weighted average, 
> > ferret yes, I think this is the difference (I tested average on 
> > longitude only I have the same result with nco and ferret).
> > Thanks for your reponse,
> > Tony
> >
> > Ansley Manke wrote:
> >
> > > Hi Tony,
> > > Could you send us the grid information, that is the output of:
> > >
> > >   ncdump -h mercatorPsy3v1R1v_med_levitus_1998_M01.nc
> > >
> > > One thing that could make a difference is whether the averaging 
> > > computes the area for each grid cell based on the latitude 
> > > correction for the location of each cell.  Also, whether the grid is 
> > > irregularly-spaced might have some impact. 
> > > Ansley
> > >
> > > Tony Jolibois wrote:
> > >
> > > >        Hi all,
> > > >
> > > > I have to compute averages on longitude and latitude axes to have a 
> > > > one value result, data are ocean model outputs (depth=0).
> > > > When I use ferret and nco to compute these averages, I don't find 
> > > > the same result, I don't understand why. With nco I find a 
> > > > temperature_averaged = 14.06548 and with ferret I find 14.38.
> > > >
> > > > Source file : mercatorPsy3v1R1v_med_levitus_1998_M01.nc (levitus 
> > > > climatology over Mediterranean sea)
> > > >
> > > >     * With nco :
> > > >
> > > > ncwa -a latitude,longitude -d depth,0,0 
> > > > mercatorPsy3v1R1v_med_levitus_1998_M01.nc 
> > > > mercatorPsy3v1R1v_med_levitus_1998_M01_averaged.nc
> > > > result : temperature_averaged = 14.06548
> > > >
> > > >     * With ferret :
> > > >
> > > > yes? use mercatorPsy3v1R1v_med_levitus_1998_M01.nc
> > > > yes? define variable/unit="degC"/title="Temperature (degC)" 
> > > > temperature_averaged = temperature[x=@AVE,y=@AVE,z=0]
> > > > yes? list temperature_averaged
> > > >              VARIABLE : Temperature (degC) (degC)
> > > >              DATA SET : MERCATOR PSY3V1R1 VITRINE CLIM (with bathy 
> > > > mask)
> > > >              FILENAME : mercatorPsy3v1R1v_med_levitus_1998_M01.nc
> > > >              LONGITUDE: 6.1W(-6.1) to 45.1E(45.1)
> > > >              LATITUDE : 30N to 48N
> > > >              DEPTH (m): 0
> > > >           14.38
> > > > yes?
> > > >
> > > > Can someone help me ?
> > > > Tony
> > > >
> > > > --
> > > > Tony Jolibois
> > > >
> > > > CLS - Direction Océanographie Spatiale
> > > > 8-10 rue hermès - 31520 Ramonville Saint-Agne
> > > > tjolibois@cls.fr
> > > >
> > > > Tél : +33 (0) 561 393 797
> > > > Fax : +33 (0) 561 393 782
> > > >  
> > >
> > > Click here 
> > > <https://www.mailcontrol.com/sr/kLAXQ2OAfY+sg8rlqvO39eQW5+zPrcG%21+EgUFuWX7Qd21ga+VaqQIGy%21cYV+Gz6EGOSEfbdsqI84clXU5dBDqudA%21wHWYCkn2B7LV20df%215fKeR8dJPQzxXwarnKoyYCTTb8lrBLkxS%2104YlmMYgQgf1u3gfn8tFXuBGI263SbjrcpSnPdvffsPPhvPXK%2199DghS21ev4dbn5ZxYJVKoj2FRgKP94rKf> 
> > > to report this email as spam.
> > --
> > Tony Jolibois
> >
> > CLS - Direction Océanographie Spatiale
> > 8-10 rue hermès - 31520 Ramonville Saint-Agne
> > tjolibois@cls.fr
> >
> > Tél : +33 (0) 561 393 797
> > Fax : +33 (0) 561 393 782
> >