[ncl-talk] plot lead/lag correlation with statistical significance

[Dennis Shea]

My opinion:

[1] You should remove the climatological annual cycle from each dataset.

*http://www.ncl.ucar.edu/Document/Functions/Contributed/clmMonTLL.shtml*
<http://www.ncl.ucar.edu/Document/Functions/Contributed/clmMonTLL.shtml>
*http://www.ncl.ucar.edu/Document/Functions/Contributed/calcMonAnomTLL.shtml*
<http://www.ncl.ucar.edu/Document/Functions/Contributed/calcMonAnomTLL.shtml>


[2] Typically, the 'swe' file contains a _FillValue (-999.0) when there is
no snow cover. I suggest setting this  to 0.0.

   sn= in1->swe(iStrt:iLast,{50:70},{20:140})  ;only specific region
averaged over total Eurasia
   sneu r=* wgt_areaave_Wrap*(sn,1.0,1.0,0);;;create area averaged snow
over Eurasia
   sneur = *where*(*ismissing*(sneur), 0.0, sneur)
   *printVarSummary*(sneur)
   p*rintMinMax(*sneur,0)
   print("---")
   ymd    := *cd_calendar*(sneur&time, -2)  ; yyyymmdd
   print(ymd+"  "+sneur)
   print("---")

[3] You *know* the climatology. You want to know the feedback of sst/sneur
anomalies on each other.
Use the anomalies in the correlation calculations.

[4] There are statistical issues. EG: the number of independent values to
be used in the testing.

Typically, successive monthly values of SST or SNE are not independent. If
(say) a January SNEUR is very large, then the February SNE will likely be
large also. So estimating the number of independent values is an issue.

 *
http://www.ncl.ucar.edu/Document/Functions/Built-in/equiv_sample_size.shtml
<http://www.ncl.ucar.edu/Document/Functions/Built-in/equiv_sample_size.shtml>*

Estimate for the SNEUR/SST anomalies separately. Use the smaller value.

[5] I am sure the IITM has staff members who know more about statistical
testing/inference than ncl-talk. I suggest talking with them.

Good luck

On Tue, Nov 20, 2018 at 6:13 AM Sujata Mandke <amin at tropmet.res.in> wrote:

> Dear NCL community,
> Greetings!
>  I have ploted lag/lead correlation (cc) between two area-averaged
> (Eurasian snow and East Pacific SST) monthly time series as xy plot
> (line).
> Further, I want to indicate which part of cc is statistically
> significant on this line using line markers.
>
> I had tested statistical significance of cc using “rtest”.
>  In the ncl script(attached),  I have plotted  line for
>  lead/lag correlation. How to mark part of this cc (line),
> which is statistically significant by using line markers.
>
> My questions are:
> (i) Is my statistical significance testing correct?
>
> (ii) how to indicate those points (by marker or any other way),
>  that are statistically significant on the   lead/lag cc line plot.
>
> I had extensively searched NCL-talk archives but
> did not find solution to my problem. My guess is that,
>  i had to use “where” function and then gsn_add_polymareker,
>  but do not know how to implement this in the ncl script.
> I am using NCL version 6.4.0 on linux machine.
>
> Any suggestion would be of great help.
> Many thanks in advance.
> With best regards
> Dr. Sujata Mandke
> Scientist, Indian Institute of Tropical Meteorology
> PUNE, INDIA_______________________________________________
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