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Re: [ferret_users] Ensemble realization analysis



Hi -
I'm thinking that this has to do with the context of variables in Ferret, but I don't yet see what the answer is. 

Here's a script that does the same operations, but  runs faster, on local data:
use coads_climatology
save/clobber/file=airt.nc airt
save/clobber/file=sst.nc sst

cancel data/all
use airt.nc
use sst.nc

let k2 = zcat (airt[d=1], sst[d=2])

! This is all zero's
plot k2[k=1,x=@ave,y=@ave] - k2[k=2,x=@ave,y=@ave]

save/clobber/file=k2file.nc k2
can data/all; can var/all

use k2file.nc

! This has non-zero difference
plot k2[k=1,x=@ave,y=@ave] - k2[k=2,x=@ave,y=@ave]


On 4/26/2011 12:37 PM, Brockmann Patrick wrote:
Ansley Manke a écrit :
Hi -
I would agree; though the variables in the datsets are of type FLOAT, so Ferret's use of single precision for variables isn't causing a loss of accuracy in this case.
Hi,

If you plot


yes? use "http://dods.extra.cea.fr/cgi-bin/nph-dods/data/p86denv/IPSLCM5A/PROD/rcp45/v3.rcp45.1/ATM/Analyse/TS_MO_YE/v3.rcp45.1_20060101_21651231_1Y_t2m.nc"
yes? use "http://dods.extra.cea.fr/cgi-bin/nph-dods/data/p86denv/IPSLCM5A/PROD/rcp45/v3.rcp45.2/ATM/Analyse/TS_MO_YE/v3.rcp45.2_20060101_21001231_1Y_t2m.nc"
yes? plot/l=1:40 T2M[i=@ave,j=@ave,d=1]
yes? plot/l=1:40/over T2M[i=@ave,j=@ave,d=2]

to be compared to
let a=zcat(T2M[d=1],T2M[d=2])
plot/l=1:40 a[i=@ave,j=@ave,k=2], a[i=@ave,j=@ave,k=1]

You will see that this not an accuracy problem.
I would rather suspect the ZCAT function to be problematic.

Patrick

Ansley

On 4/26/2011 9:41 AM, Martin Schmidt wrote:
Hi Patrick,

I can reproduce your finding. I GUESS, the i/j-average of the difference between both
the experiments is pretty small, even if there are significant differences in some areas. The average runs over 10^4
points. The quantities are of the order 200, the difference is less than +/-10.

Hence it may be that you see simply the limited accuracy of ferret.

I do not know nothing about the experiments, but may it be that some procedure has been applied to
keep the average values fixed?

Greetings,
Martin




Brockmann Patrick wrote:
Hi all,

Sorry for the uncompleted previous message.

When you analyze ensemble realizations
(read http://cmip-pcmdi.llnl.gov/cmip5/docs/cmip5_data_reference_syntax.pdf
Ensemble menber paragraph), you would like to work with
a variable over its realizations seen as a nth dimension and then apply on it power of ferret transformations.

Unfortunatelly, there are only 4 dimensions in ferret.
But sometimes, you also may have the chance that the variable you analyze
has 3 dimensions and has a XZT shape.

I have imagined to use the ZCAT function to concat the XYT variable over the Z dimension
and then get a XYZT variable with the Z dimension representing a realization dimension.

It seems to work since for one time step I can shade a difference but when I plot
a time evolution I get the same plot from the 2 realizations. That is not correct.

Try the attached script if interested.

Any share of experience on this topic is welcome. The challenge of course is not
to physically save files.

Patrick






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