[ncl-talk] Explicit levels (using WRF output)

[Mary Haley]

Hi Arianna,

NCL does its contouring by filling the contours based on what values fall
above or below a contour level, and not exactly equal to a contour level.
This causes confusion when your data is all integers.

If you specify contour levels of 1,2,3,4,5, then NCL is going to create a
contour plot with *6* colors, where the colors represent the following:

  color 1 - all values < 1
  color 2 - all values >= 1 and < 2
  color 3 - all values >= 2 and < 3
  color 4 - all values >= 3 and < 4
  color 5 - all values >= 4 and < 5
  color 6 - all values >= 5

This is why you see the extra "Label_1", because NCL only expects (n-1)
labels if you have n colors, because it's assuming you are labelling
*between* values.  When you tell it to center the labels, then suddenly you
are one label short, and hence NCL gives you the "Label_1" label.

Back to your contour levels:

In your code, you set the first contour level to 1, so this means that the
first color (darkgreen) is going to represent all values < 1, which means
any value equal to 0, since you only have integer values.

Then, then next color, "forestgreen" is going to represent all values >=1
and < 2, which, again, in your case, since you just have integers, is going
to be all colors equal to 1.

If you want the first color to represent all values equal to 1, and the
second color all values equal to 2, and you want twenty colors total, where
the last color represents values equal to 20, then you need to create 19
contour levels that start just slightly higher than 1, 2, 3, 4, etc:

This is one way of doing it, by adding 0.1 to the levels:

  res at cnLevels  = (/1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11,12,13,14,
15,16,17,18,19/)+0.1

I see you are using cnFillPalette, which is okay, but it's better to use
cnFillColors when you have a specific color for each contour level.  This
way you don't need to mess with gsnSpreadColors:

  res at cnFillColors     = colorscheme        ; distinct colors for categories
 ; res at gsnSpreadColors = False           ; comment this or remove it

If you are trying to plot this data over a map, you generally want to use
wrf_contour *and* wrf_map_overlays.  But, your XLAT and XLONG on the file
do not have a time dimension, which is going to cause problems for
wrf_map_overlays.

So, I modified your code to use gsn_csm_contour_map instead, but I used the
map projection defined on the file.

I've attached the modified script and a new PNG.


--Mary


On Thu, Jan 19, 2017 at 10:57 AM, Arianna Valmassoi <
arianna.valmassoi at gmail.com> wrote:

> Hi all,
>
> I'm working with WRF and it's landuse representation. In the plot output I
> have the following problems:
>
>    - label: there is one more tic in the label with the first color, and
>    has a name I did not put.
>    - assign color to discrete numbers: I had to impose to the landcat to
>    start with zero, not as one as it is, otherwise the first level will not
>    match
>    - Domain visualization: I can't use the model coordinates (even using
>    other plot option)
>
>
> I attach here both the code and the .nc file, in order to help the debug
> category 13 should be red and category 17 light blue.
>
> Thank a lot for any help and sorry if it's a duplicate problem!
>
> Cheers,
> Arianna Valmassoi
> --
> Arianna Valmassoi
>
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