Re: [ferret_users] grid problem y axis (latitude)

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

Hi Hella -
Well, here's what it is.  By default, Ferret renames variables and axes to their upper-case spelling.

Your file uses lowercase names. If you use ncdump -c to look at the coordinates after appending the new variable to it, there are two Y axes: the original one called lat, with coordinates that are reversed relative to the default ordering,

    lat = 87.8638, 85.09653, 82.31291, ...

and an upper-cased version called LAT, which Ferret wrote out on the APPEND command, which is not reversed.

    LAT = -87.8638000488281, -85.0965270996094, -82.3129119873047, ...

Then when you open up that file, the variables have different versions of the axis, one marked reversed and the other not.  This is the first time I'm aware of that the up-casing of names really gets us into trouble.

For your files, you will want to put in this command, at the start of your script

CANCEL MODE upcase_output

It would probably be a good idea to have this in your .ferret startup file, so that it is always issued.  With that in place

> cp orig.nc test.nc
> ferret
        NOAA/PMEL TMAP
        FERRET v6.64 
        Linux(gfortran) 2.6.9-89.0.20.ELsmp - 09/20/10
        26-Oct-10 15:38    

yes? use test.nc
 *** NOTE: If no missing_value or _FillValue attribute on variables, will use NaN
yes? CANCEL MODE upcase_output

yes? let testvar = q[L=1:2]
yes? save/append/file=test.nc testvar
 LISTing to file test.nc
 **TMAP ERR: Requested data range is outside of data set limits
             Expected: testvar[J=           1 :          64 ] Found: testvar[J=          64 :         127 ]

Now, this is an outright error message, not a warning. It will force you to first write to a file, and then append to that.  The output of Ferret will always be a non-reversed axis.  Ferret don't have code to write reversed axes and reversed data to netCDF files.

Ansley

On 10/26/2010 3:08 PM, Hella Riede wrote:

Hi Ansley,

alright - sounds good to me. I am curious about what's going on and there is a whole lot of people working with those reverse-order files. For sure EMAC and ECHAM people, maybe even ECMWF, as this is where ECHAM was derived from.

Best wishes,
Hella



------------------------------------------------------------------------
*Subject:* Re: [ferret_users] grid problem y axis (latitude)
*Sent:* 2010-10-26 11:13:14 -0700
*From:* Ansley Manke <Ansley.B.Manke@xxxxxxxx>
*To:* Hella Riede <hella.riede@xxxxxxx>

Hi,
Thank you for the example file.  I'll look into this and we can report back to the List when we understand what's happening.

Ansley


On 10/26/2010 10:58 AM, Hella Riede wrote:

Hi Ansley,

When Ferret detects an axis with reversed coordinates, it checks to see if they're regularly spaced, and if so reverses them.

There is probably more to it as latitude in our case is NOT regularly spaced (Gaussian grid) and is still reversed.

Note that if Ferret writes a reverse-ordered variable to a new netCDF file (with the SAVE command), the coordinates and data in the output file will be in monotonically increasing coordinate order-reversed from the input file.

This is true. If I create a new file as you suggest (saving a part of the original file to new.nc and then appending to new.nc), the file looks fine and consistent with a latitude axis in ascending order. But I would like to append something to the original (reversed-order) file with all its variables.

NOTE that when I try to create an example where I append to the input file which has a reversed axis, I get an error message and cannot do the append.

This is exactly what I would expect: 1) issue an error message or 2) take the reverse order also into account when appending so the file is still consistent. I do NOT get any error message. Instead, ferret quietly appends and I end up with a file mirrored in latitude. Try the following with the attached file (orig.nc):

> cp orig.nc test.nc
> ferret
yes? use test.nc !(i,j,k,l; including specific humidity q)
yes? let testvar = q[l=1:2]
yes? save/append/file="test.nc" testvar
yes? quit

=> There is no error appending to the open file with reversed axis!

> ferret
yes? use orig.nc
yes? shade/i=1/l=1 q
yes? use test.nc
yes? set win 2
yes? shade/i=1/l=1 q
yes? set win 3
yes? shade/i=1/l=1 testvar

=> The newly appended variable "testvar" looks fine - exactly like "q" in orig.nc. But the variable "q" in test.nc now contains the mirror image of "q" in orig.nc!

By now I have updated to ferret version 6.64 and still get no note or error message when loading a file with or appending to the reverse-order axis. Any help on this behavior is appreciated.

Thanks a lot in advance,
HellA

On 10/25/2010 6:30 AM, Hella Riede wrote:

Hi Ansley,

Hi helle,I'm working off-line and can't see your image, but I know the issue. What are the units of the latitude axis?

Units of latitude axis from ncdump (original file):
float lat(lat) ;
        lat:long_name = "latitude" ;
        lat:units = "degrees_north" ;

What is the range of the y coordinate values?

From "show grid" (original file):
LAT       LATITUDE            64 i   87.863S              87.863N

As mentioned in my first email, this is already strange as the values inside start start +87.863 (which should be north) and end at -87.863 (which should be south). Here are the start and end values of an "ncdump -v lat":

lat = 87.8638, 85.09653, 82.31291, 79.5256, 76.7369,...
    -76.7369, -79.5256, -82.31291, -85.09653, -87.8638 ;

Is this a netCDF file?  If so then looking at the y-axis information from a ncdump -c. would tell us if there's more that ferret could do to issue a note or something.

Yes, it is a netCDF file and the header information and values of "lat" are already provided above. Is there anything else you need from the "ncdump -c"?

Kind regards,
Hella

To use the dataset with reversed coordinates, the USE/ORDER qualifier let's you tell ferret to reverse the Y axis.
Ansley

-----Original message-----
From: Hella Riede <hella.riede@xxxxxxx>
To: ferret <oar.pmel.ferret_users@xxxxxxxx>
Sent: Thu, Oct 21, 2010 14:47:37 GMT+00:00
Subject: [ferret_users] grid problem y axis (latitude)

Dear ferreters,

in our default model output, the y axis (latitude) is unconventionallydefined from north to south. Working with the unchanged file in ferretseems to work fine. But since ferret does not recognize thisunconventional axis/grid correctly, it produces a mirror image of thedata with respect to latitude when for example appending a variable tothe file. This "mirroring" happens without error notice or warning,which can of course lead to misunderstandings interpreting the data.

I prepared an example, see attachment.

Thank you in advance for taking notice.

Best wishes,
Hella