Re: [ferret_users] Differences in mean computation CDO/Ferret?

To: Martin Schmidt <martin.schmidt@xxxxxxxxxxxxxxxxx>

Subject: Re: [ferret_users] Differences in mean computation CDO/Ferret?

From: Nicolas Freychet <nicolas.freychet@xxxxxxxxx>

Date: Mon, 23 May 2016 19:59:22 +0100

Cc: ferret_users <ferret_users@xxxxxxxx>

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Hi Martin,

Well the script is actually simple and follow this logic:

let threshold=35

repeat/range=1:365/name=it ( ;\
let composite = if tas[x=100e:120e@ave,y=20n:40n@ave,d=1,l=`it`] gt `threshold` then geop[d=2,l=`it`] ;\
if `composite` then (save/append/file=CompositeGeop.nc composite) )

So it saved only the day where the composite is defined.

In that case, the time-mean of the values in CompositeGeop.nc computed by Ferret will be different from the one computed by CDO.

Now I just removed the condition to save all the days even if they are masked:

let threshold=35
repeat/range=1:365/name=it ( ;\
let composite = if tas[x=100e:120e@ave,y=20n:40n@ave,d=1,l=`it`] gt `threshold` then geop[d=2,l=`it`] ;\
save/append/file=CompositeGeop.nc composite )

In this second case, the means computed by either CDO or Ferret are the same (because there is no gap in the calendar of CompositeGeop.nc).

Best,

Nicolas

2016-05-23 17:52 GMT+01:00 Martin Schmidt <martin.schmidt@xxxxxxxxxxxxxxxxx>:

Nicolas,

it would help to see your scripts, may be only the most important lines.

Best,
Martin

Am 23.05.2016 um 12:13 schrieb Nicolas Freychet:

Dear Ferret users,

I computed composites of the geopotential by selecting specific days only (using Ferret condition IF (....) then .... ), and created a file with only these days (thus it's continuous serie but the calendar is not continous are there are gaps in the days)

I then computed the time-mean from the composites with CDO (cdo -timmean .....) and Ferret ( L=@ave ) and I can notice strong differences between the 2 results (attached, left is CDO and right is Ferret). It's not just the magnitude, also the patterns differ (though the main signal is quite close), which is really strange.

I dont know if it's a Ferret or CDO problem, or if it comes from the gaps in the calendar of the composites... But even so, the patterns should at least look similar.

Best,
Nicolas

----------------------------------------------------

Nicolas Freychet

PDRA, School of Geosciences
University of Edinburgh
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