[gradsusr] correlation difference between GrADS and NCL

[Andrew Friedman]

Hi Rui,

As a more simple check than another program, can you compare your GrADS results with a simple correlation coefficient calculated using the standard formula with a handful of data points?
https://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient 

Andrew

On Jun 12, 2014, at 3:27 PM, Cheng, Rui <r5cheng at ucsd.edu> wrote:

> Hi all GrADS users,
> 
> I want to get the correlation between dcape and rainrate using GrADS. Meanwhile, NCL is used to test the result.
> 
> Assuming dcape is one time step prior to rainrate, so we can get the correlation (0.0266) using GrADS through,
> "set t 2 77; define dcapeave2=dcapeave(t-1); set t 1; d tcorr(dcapeave2,rainrateconvm,t=2,t=77)."
> 
> Also using NCL, we can get the correlation (-0.18815) through,
> "ccr2=esccr(dcapeave(1:76,0,0,0),rainrateconvm(1:76,0,0,0),1)."
> 
> I notice that the difference is small if there is no time lag between dcape and rainrate. Then we get the correlation "-0.1139" using GrADS and "-0.1101" using NCL.
> 
> Why the difference is so big when the time lag is present between two time series? 
> Would you like to help me?
> 
> Many thanks,
> 
> Rui
> 
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