20050815: IDV - Default units and values for cloud top temperature in NAM model?

>From: "Russ Rew" <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200508151933.j7FJXQ1r008799 IDV defaults

Hi Russ-

>Institution: Unidata
>Package Version: 1.2
>Operating System: Solaris
>Hardware Information: irrelevant
(it may be) ;-)

>I just downloaded the new IDV 1.2 release and tried to bring up a
>display of data from the latest NAM nodel run, specifically the 2D field
>"Temperature at cloud tops" in the T_cltp variable using a Contour Plan
>View. The display came up, but with very closely spaced contours and a
>default color table that went from a minimum of -686.5K to a maximum
>of 467.4K. 

Yikes, those are pretty odd numbers! 0 K should be absolute 0, so you
shouldn't have negative numbers. I looked at a dump of the netCDF file
and there are negative numbers in there. I also looked at the raw GRIB
data through the IDV and the values are the same, so something's screwy.
I'll check with our developer who works on that code and get back to
you.

In the mean time, I'll explain a little about what you are seeing.

If default values for contour information and color table range are
not specified in the system defaults, the IDV makes a guess at
what to use.  Currently it is:

- Color Table Range:
   - the max/min range for the color table is the range of the data values.
- Contour values:
   -use the max/min range of the data and come up with a reasonable number of
    contours for that range.  It's not the best algorithm and we have
    on our TODO list to use the same algorithm that GEMPAK, which is probably 
better.

You can set a default color table and color table range for a particular
field through the Edit->Resources->Parameter Defaults menu item. Or, you
can associate a particular variable name with a predefined alias for 
which defaults already exist using the Edit->Resources->Parameter Aliases
menu item.  You can find more information on using these resource editors
in the IDV User's Guide from the Help->User's Guide menu (under the Tools
section) or on line at:

http://my.unidata.ucar.edu/content/software/IDV/docs/userguide/tools/ParameterDefaultsEditor.html

and

http://my.unidata.ucar.edu/content/software/IDV/docs/userguide/tools/AliasEditor.html

>I tried changing the contour configuration with the Contour
>Properties Editor to something more reasonable, but it didn't change the
>values shown at the ends of the Color Table bar and seems to have had
>no effect on the display either. 

What values did you use?  I tried this and changed the base contour to
0 and the contour interval to 100 and it did affect my display.   I also
set the minimum contour to 0.

>Maybe this is just a case of bad data,
>but it seems like the labels on the Color table bar should change and
>the actual displayed contours should change after I modify the contour
>interval, minimum, and maximum ...

The color bar max/min is independent from the contour max/min by design.
The contour properties editor only affects the contour values, not the
color table.  The idea is that you can create a color table with breakpoints 
at certain values.  If you change contour information, you would still want to 
have
a particular breakpoint at the same color.  Altering the range of the
color table will break that relationship.  You can change the
max/min of the color table by clicking on the color table name
("default" in this case) and selecting "Change Range..." from the
pop-up menu.

However, I agree that changing the contour intervals should have changed
the display.  Let me know what values you used for the contours.

Don Murray
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