[gradsusr] correlation difference between GrADS and NCL

[Cheng, Rui]

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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