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:

../../documentation/users-guide/variables-xpressions/XPRESSIONS.html#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:

../../documentation/users-guide/variables-xpressions/XPRESSIONS.html#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 becomecompressed 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