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[Support #VMQ-273039]: Difference between gridded data sets

Hi Lee-

> I am a new IDV user trying to do something that I think should be simple, but 
> I have not yet figured out the trick to it.  I have several netCDF files of 
> gridded climate data--the files have the lat and long positions of each cell 
> and a single climate variable (e.g. temperature).  I would like to be able to 
> calculate the difference in the value of the climate variable in each cell 
> between two files.  For example, one file has an average of current observed 
> temperatures and a second file has an average of future projected 
> temperatures, and I want to know that absolute increase in temperature for 
> each cell.
> I am no programming whiz and hoped to be able to accomplish this using the 
> readily available formulas.  I thought that the Simple Difference formula 
> would do the job, but I have not been able to apply it and get satisfactory 
> results as the resulting values range from -270 to 270 rather than a few 
> degrees.  I assume I have misunderstood the Simple Difference formula...do 
> you have any recommendations as to how I might better approach this problem?

It's a units issue.  The IDV doesn't know about delta degrees.  In your files, 
Tavg has units of "C" which is interpreted as Celsius.  In reality, "C" is
Coulombs in the international standard, but we account for this.  If you are 
generating these files, it would be better to use "Celsius" or "degC" which are 
udunits compatible names.

Nevertheless, when subtracting two temperature fields, the units are converted 
to Kelvin since Celsius and Fahrenheit are offset units.  Since the IDV doesn't 
know that in "delta degrees" 1K=1Celsius, when you display the result in 
Celsius, it does the conversion as degrees (Celsius = K-273.15).  Thus, you get 
values in the -270 range.

If you change the display unit to Kelvin from the Edit->Change Display Unit.. 
menu of the display control, you will see the values in K which will 
essentially be delta C units.


Ticket Details
Ticket ID: VMQ-273039
Department: Support IDV
Priority: Normal
Status: Open

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