Subtract two variables with different dimensions

Added by Catarina Guerreiro over 7 years ago

Hi,

I am trying to do the following calculus with cdo: anomaly=var(time,lon,lat)-var_spatialmean(time).

I estimated the var_spatialmean, which is the spatial mean for each time step:

1) # cdo fldmean inputfile outputfile

The variable dimensions are time, lon,lat, with lon and lat = 1.
And this becomes a problem when I do:

2) # cdo expr,'var_anomaly=var-var_spatialmean' inputfile outputfile

I would like to eliminate lon and lat dimensions of the second variable but I couldn't find any cdo command for the purpose.
I appreciate your help.

Catarina

Replies (3)

RE: Subtract two variables with different dimensions - Added by Uwe Schulzweida over 7 years ago

Hi Catarina,

You can remove lon/lat=1 with the CDO option --reduce_dim:

cdo --reduce_dim fldmean inputfile outputfile

But I think this will not help you much for the calculation of the anomaly. Here are two examples to calculate the anomaly.
1. With expr for specific variables:

cdo expr,"var_anomaly=var-fldmean(var)" inputfile outputfile

2. With combination of CDO operators for all variables:

cdo sub inputfile -enlarge,inputfile -fldmean inputfile outputfile

We will remove the necessity of the "-enlarge,inputfile" operator in the above example in CDO version 1.8.1.

Cheers,
Uwe

RE: Subtract two variables with different dimensions - Added by Catarina Guerreiro over 7 years ago

Hi Uwe,

Thank you very much for your suggestions!
I had to update my cdo version to use the operators you suggested.

I have still one question regarding the variable properties in the outputfile. I lose part of my variable information after applying the cdo operators.
Is there a way to recover or to keep this information in the output file?

In the inputfile:

short msl(time, latitude, longitude) ;
        msl:standard_name = "air_pressure_at_sea_level" ;
        msl:long_name = "Mean sea level pressure" ;
        msl:units = "Pa" ;
        msl:add_offset = 99552.1862391848 ;
        msl:scale_factor = 0.252521630323654 ;
        msl:_FillValue = -32767s ;
        msl:missing_value = -32767s ;

After applying # cdo expr,"var_anomaly=var-fldmean(var)" inputfile outputfile:

float msl_anomaly(time, latitude, longitude) ;
        msl_anomaly:_FillValue = -32767.f ;
        msl_anomaly:missing_value = -32767.f ;

After applying # cdo sub inputfile -enlarge,inputfile -fldmean inputfile outputfile:

double msl(time, latitude, longitude) ;
        msl:standard_name = "air_pressure_at_sea_level" ;
        msl:long_name = "Mean sea level pressure" ;
        msl:units = "Pa" ;
        msl:_FillValue = -32767. ;
        msl:missing_value = -32767. ;

Thank you for your help.
Catarina

RE: Subtract two variables with different dimensions - Added by Uwe Schulzweida over 7 years ago

Hi Catarina,

Unfortunately all attributes will be lost when using the expression operator expr. There is no way to keep these attributes, sorry.

Cheers,
Uwe

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