You can use CDO (https://code.zmaw.de/projects/cdo/) to regrid the data sets on same grid then you can subtract them. Please see the link below

https://code.zmaw.de/boards/1/topics/533

Also you can use the script that comes with RegCM4.2 to find the bias. Look for it in the RegCM directories.

On Thu, Feb 28, 2013 at 12:50 AM, mahmoud elkarim <mgaber_elkarim@hotmail.com> wrote:

Dear Griaziano Giuliani and regcm users
i have run regcm4.2 using LAMCON projection and i want to find precipitation bias (observed - simulated) using ncl script but a found mismatch between cru ,gpcp and regcm data due to curve linear and rectilinear data can you help me how subtract observed data from regcm data
regards

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