[ncl-talk] Extracting temperature from specific grids where LU_INDEX = 2

[Imran Hosen]

Hi Guido and Barry,

Thanks again for your further advices.

Actually, in the study domain, it has 117 rows*117 columns (total 13689
grids). The grids have different land use index (LU_INDEX). I ran
simulations using the WRF model for simulating different climate variables.
Now, my goal is, I want to plot (XY plot) the hourly temperature difference
between the grid cells whose LU_INDEX=2 and LU_INDEX=1. Therefore, I want
to extract temperature seperately for the grid cells whose LU_INDEX=2. I'll
do same thing for the grid cells whose LU_INDEX=1. Then I will make XY
plot, although I never experinced NCL for making XY plot. Hope, now you
will understand my goal.

If I use mask function as the script I wrote, then T2_masked = T2 (only for
the grid cells where LU_INDEX is not equal to 2), right??? So, I made a
mistake in my script, and I should follow your previous suggestion, right??

When I use mask or where function, it is showing both real and FillValue.
Really, I want to extract only real values for the grid cells where
LU_INDEX=2 and want to escape the FillValues.

Okay, I'll try to implement your advice and let you know the update.

Wish your uninterrupted sound health.

Kind regards,
Imran

On Tue, Mar 14, 2017 at 8:24 PM, Guido Cioni <guidocioni at gmail.com> wrote:

> Can you tell me what is the final goal? Otherwise it is really difficult
> to give you advices :)
>
> Are you *sure* that your script is working correctly? As far as I
> understand the mask function acts only on the values where the mask array
> condition is true, so in your case T2_masked should be untouched, i.e.
> T2_masked=T2. Have you tried printing or plotting the difference
> T2-T2_masked?You should instead mask all the points that *DO NOT* have
> LU_INDEX=2, as I previously suggested. Furthermore, you should set the
> FillValue BEFORE calling mask, otherwise the True, False conditions will
> not apply correctly. If you are more confident with the where syntax use
> something like this
>
> where( LU_INDEX=2, T2(time_i,:,:), T2 at _FilllValue)
>
> However you would need to do that for every time instant with a loop...
>
> Now to the other question. *Are you really sure* that you want to SUBSET
> your data given a certain condition? If you just need to extract statistics
> from those data having the original array masked is the best way to go as
> NCL will deal with Missing Values without problems. If you instead try to
> subset your data you will end up with a weird shaped array, as the
> LU_INDEX=2 condition, I believe, is not spatially coherent like say a box
> or a circle. One way to go would be to convert the temperature array to a
> 1-D one with ndtooned (https://www.ncl.ucar.edu/
> Document/Functions/Built-in/ndtooned.shtml), apply the ind function and
> then convert back to a multidimensional array (Example 3 at this page is
> useful http://www.ncl.ucar.edu/Document/Functions/Built-in/minind.shtml).
> But really, it is cumbersome and useless depending on what is your final
> goal.
>
> I think there are a lot of examples on the online guide.
>
> Good Luck
>
> Guido Cioni
> http://guidocioni.altervista.org
>
> On 14 Mar 2017, at 10:06, Imran Hosen <hosen.imran09 at gmail.com> wrote:
>
> Dear Guido and Barry,
>
> Thank you so much for your heartfelt and prompt response. Yes, your
> suggestions are working fine for my script.
>
> Guido, your assumption is absolutely right. According your suggestion I
> wrote the script as follows:
>
> T2 = f_1->T2(:,:,:)
> LU_INDEX = f_1->LU_INDEX(0,:,:)       ; (LU_INDEX = landuse index)
> T2_masked = mask(T2,(LU_INDEX.eq.2), True)
> T2_masked at _FillValue = -9999
> print(T2_masked)
>
> The script works fine and it is extracting both real values and FillValues
> (missing values). *But, I want to extract only real values*. I have seen
> that "*ind*" function can be used for extracting real values for *one-dimensional
> array*. Could you please suggest me what function I can use to extract
> the real values for *two-dimensional array*.
>
> Your further suggestion will be highly appreciated.
>
> Kind regards,
> Imran
>
>
> On Tue, Mar 14, 2017 at 6:55 PM, Guido Cioni <guidocioni at gmail.com> wrote:
>
>> Hi Imran,
>> unfortunately I’m not really an expert of WRF, so I’m going to give you a
>> general hint which may work. You should be more precise next time as
>> generally people do not know what is LU_INDEX and what is its structure ;)
>>
>> I assume that you want to extract temperature, which is 3-D (time, lat,
>> lon) using a mask array LU_INDEX which is 2-D (lat, lon). Again, if this
>> assumption is wrong please be more specific. You have many choices, but I
>> believe that for the sake of clarity you may want to go for an array which
>> has the same dimensions but missing values where LU_INDEX is not 2. So you
>> may want to do something like this:
>>
>> T2_masked=mask(T2, LU_INDEX.ne.2, False).
>>
>> You need to have a Missing Value attribute set for the T2 variable of
>> course. As a result you will get an array with the same dimensions, so that
>> it’s easier to compare with the original one, but with Missing Values where
>> your condition is False, so that for every computation these points will
>> not be considered. You can achieve something similar with the where
>> function. Please have a look here (http://www.ncl.ucar.edu/Docum
>> ent/Functions/Built-in/mask.shtml).
>>
>> Cheers
>>
>>
>> Il giorno 14 mar 2017, alle ore 04:38, Imran Hosen <
>> hosen.imran09 at gmail.com> ha scritto:
>>
>> Dear NCL users,
>>
>> Good day!
>>
>> I'm a new user of NCL. I want to extract temperature (T2) from wrfout
>> file for the specific grid points where the LU_INDEX = 2.
>>
>> For example,
>> f = addfile("./wrfout.nc)
>> T2 = f->T2(:,:,:)
>>
>> It will extract temperature for whole domain for all time steps. In my
>> case, I only want to extract temperature for all time steps but for the
>> specific grids which grids represent LU_INDEX = 2.
>>
>> Could you please suggest how to do it. I would grateful to you if anyone
>> please share a script.
>>
>> Thanks in advance.
>>
>> Kind regards,
>> Imran
>>
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>
>
> --
> *Md. Imran Hosen*
> PhD Student
> Victoria University
> College of Engineering and Science
> Footscray Park Campus
> Victoria, Australia
> Mob. +61470371562 <+61%20470%20371%20562>
>
>
>
>


-- 
*Md. Imran Hosen*
PhD Student
Victoria University
College of Engineering and Science
Footscray Park Campus
Victoria, Australia
Mob. +61470371562
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