Re: [ferret_users] grid compressed after subtract area average

["Ansley C. Manke" <ansley.b.manke@xxxxxxxx>]

Hi Xiaoyu,

First, what you have done with TEMP-TEMP[X=@AVE,Y=@AVE] is to remove the mean over longitude and latitude.  That will not be less noisy.  If you want to work with a less-noisy version of TEMP, you might use one of the smoothing transformations such as the boxcar smoother, @SBX.  Here is a list of all the transformations:

https://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/variables-xpressions/XPRESSIONS#Chapter3-Transformations, and further down that page are detailed descriptions of the different transformations.

Now, it looks as if you've discovered an error in the SHOW GRID output.  It should not be listing the x and y axes as "compressed".  So we'll look into that.

    yes? use coads_climatology
      yes? let sst_demean =  sst - sst[x=@ave,y=@ave]
      
      yes? show grid sst_demean
          GRID GSQ1
       name       axis              # pts   start               
      end                 subset
       COADSX    LONGITUDE          180mr   21E                 
      19E(379)            compressed
       COADSY    LATITUDE            90 r   89S                 
      89N                 compressed
       normal    Z
       TIME      TIME                12mr   16-JAN 06:00         16-DEC
      01:20        full

However sst_demean does still vary in longitude and latitude.  See how STAT shows 180 points in X and 90 points in Y, and a plot at one time and latitude varies in X.

      yes? stat/brief sst_demean
       
       Total # of data points: 194400 (180*90*1*12*1*1)
       # flagged as bad  data: 89622
       Minimum value: -23.622
       Maximum value: 12.722
       Mean    value: -2.1127 (unweighted average)
    
    yes? plot/L=1/y=0 sst_demean

First, what you have done with TEMP-TEMP[X=@AVE,Y=@AVE] is merely to remove the mean.  If you want a less-noisy version of TEMP, you might look at the smoothing transformations. Here is a list of all the transformations:

https://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/variables-xpressions/XPRESSIONS#Chapter3-Transformations

On 12/3/2019 4:38 PM, Xiaoyu Bai wrote:

Dear ferreters,

I noticed that when I subtract area average from a variable, it will make the variable's Lat and Lon become compressed instead of full. For example:

use levi_climatology

let test=TEMP-TEMP[X=@AVE,Y=@AVE]

sh grid TEMP,test
    GRID GMS1
 name       axis              # pts   start                end                 subset
 XAXLEVITR LONGITUDE          360mr   20.5E                19.5E(379.5)        full
 YAXLEVITR LATITUDE           180 r   89.5S                89.5N               full
 ZAXLEVITR DEPTH (m)           20 i-  0                    5000                full
 normal    T
    GRID GMS1
 name       axis              # pts   start                end                 subset
 XAXLEVITR LONGITUDE          360mr   20.5E                19.5E(379.5)        compressed
 YAXLEVITR LATITUDE           180 r   89.5S                89.5N               compressed
 ZAXLEVITR DEPTH (m)           20 i-  0                    5000                full
 normal    T

What I want is to get a less noisy variable, then apply @DIN to it. But this compressed grid was blocking me to use @DIN.

So I am wondering how to get my coordinate to full again.

Thank you in advance.

 

--

Xiaoyu