Re: [ferret_users] Different results between ferret and other softwares

[Steve Hankin <steven.c.hankin@xxxxxxxx>]

Hi Markus,

This is a huge difference in value (285 versus 275) -- not something attributable to round off errors.  It's as if the calculations are using data from a different vertical level?  Or averaging missing value points in as zero values?  Or something of that nature.

One difference that seems apparent:  this matlab calculation

tas          = input_nc{'tas'}(:) ;
tas          = squeeze(tas(1,:,:));
mean2(tas)

is a simple arithmetic mean of the non-missing values, whereas the Ferret calculation of

tas[i=@ave,j=@ave,l=1]

is an area-weighted average.

    - Steve

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On 7/28/2011 2:41 AM, markus.mingel@xxxxxxxxx wrote:

Dear Users,

I got different results between ferret and matlab/idl/ncl when I compute the 
global mean temperature from the CMIP5 models.

Namely, taking the Canadian atmospheric model this is the result of the dump 
for the temperature:

	float tas(time, lat, lon) ;
		tas:standard_name = "air_temperature" ;
		tas:long_name = "Near-Surface Air Temperature" ;
		tas:units = "K" ;
		tas:original_name = "ST" ;
		tas:cell_methods = "time: mean (interval: 15 minutes)" ;
		tas:cell_measures = "area: areacella" ;
		tas:history = "2011-03-16T18:49:59Z altered by CMOR: Treated scalar 
dimension: \'height\'. 2011-03-16T18:49:59Z altered by CMOR: replaced missing 
value flag (1e+38) with standard missing value (1e+20)." ;
		tas:coordinates = "height" ;
		tas:missing_value = 1.e+20f ;
		tas:_FillValue = 1.e+20f ;
		tas:associated_files = "baseURL: http://cmip-pcmdi.llnl.
gov/CMIP5/dataLocation gridspecFile: 
gridspec_atmos_fx_CanESM2_historical_r0i0p0.nc areacella: 
areacella_fx_CanESM2_historical_r0i0p0.nc" ;


To compute the mean global temperature of the first month of data with ferret 
I use the following command:

list tas[i=@ave,j=@ave,l=1]
             VARIABLE : Near-Surface Air Temperature (K)
             DATA SET : CanESM2 model output prepared for CMIP5 historical
             FILENAME : tas_Amon_CanESM2_historical_r1i1p1_185001-200512.nc
             LONGITUDE: 1.4W(-1.4) to 1.4W(358.6) (XY ave)
             LATITUDE : 90S to 90N (XY ave)
             TIME     : 16-JAN-1850 12:00 NOLEAP
             284.8


Now I expect having the same value (284.8) in matlab/IDL/NCL; indeed with all 
these other 3 softwares I get the same value (275.13). Why???

Am I wrong reading the data with matlab/IDL/NCL? Since there aren't any 
scale_factor or add_offset values to read, in matlab/IDL/NCL I just read the 
value tas. As example, these are the matlab lines I used to compute the global 
mean:

input_nc = netcdf('tas_Amon_CanESM2_historical_r1i1p1_185001-200512.nc');
tas          = input_nc{'tas'}(:) ;
tas          = squeeze(tas(1,:,:));
mean2(tas)

ans =

  275.1371
 

Do anyone could explain me why I get this difference in the mean values?

Thanks into advance for help
Markus