Loop to extract and specific lat and lon

[Alyce Sala Tenna]

Hello,

I am struggling with a loop to extract temperature at a specific lat and lon for every hours.

nl = wrf.extract_dim(f, 'Time')

len = nl-1

for i in range(0,len):

    temp_sub = temp[i,:,x_y[0],x_y[1]]

But it only returns the first level, and not all the hours in the files:

array([-273.15, -273.15, -273.15, -273.15, -273.15, -273.15, -273.15,

       -273.15, -273.15, -273.15, -273.15, -273.15, -273.15, -273.15,

       -273.15, -273.15, -273.15, -273.15, -273.15, -273.15, -273.15,

       -273.15, -273.15, -273.15, -273.15, -273.15, -273.15, -273.15,

       -273.15], dtype=float32)

Is there an easy solution for it? Ive been struggling for days!

Thanks

Alyce

[Michaela Sizemore]

Hi Alyce,

In my experience, if you are trying to use multiple time steps, you would have multiple wrfout files for each time step, however I don't think this is always the case. If this the case for you, this page may be of some assistance: https://www.reddit.com/r/learnpython/comments/57nn9v/python_loop_wrf_model_question/ . As that page says, you would want to first loop through to open/import each file of interest, then plot/print/manipulate the data from there. If you do not have multiple wrfout files, wrf-python has a function called 'extract_times' (https://wrf-python.readthedocs.io/en/latest/user_api/generated/wrf.extract_times.html) that may be able to help you. This function will return all of the time objects in the wrfout file which you may then use in a for loop to select the data "step" (date) and then use that step to feed into the for loop you have already been using. Give those two options a try and let me know if either works out for you!

Michaela

[Alyce Sala Tenna]

Hi Michaela,

I only have one wrfout file, so I could do the second option you suggested, but the problem is I actually don’t know how to write the loop in the first instance as I’ve only just started using Python. 

If I use wrf.extract_times how would I then write a loop to extract temperature at 2m at the specific grid lat and long?

Thanks 

Alyce 

[Michaela Sizemore]

Hi Alyce,

Would you mind attaching your wrfout file here? I haven't had much experience with extracting the time data of a single file, specifically one with multiple time steps, so I'd like to look at what is available in your file on my side and can further assist you from there. Overall, I believe it will be pretty similar to extracting the data from multiple wrfout files, but to be sure, I would like to experiment first before further offering advice. You can attach files using the paperclip symbol to the right of 'post message' below. Let me know if there are any difficulties with doing that (file size restrictions, file type restrictions, etc) and I will try to find a work around to that if it is the case.

Michaela 

[Xin Zhang]

Hi Alyce,

You can extract the variable at all times and use the x_y (calculated from lon_lat value) to get the grid from the 3D or 4D array.

Here's an example:

from netCDF4 import Dataset

from wrf import getvar, ll_to_xy, ALL_TIMES

ncfile = Dataset(filename)

var = getvar(ncfile, var_name, timeidx=ALL_TIMES)})

x_y = ll_to_xy(ncfile, lat, lon)

var_grid = var.sel(west_east=x_y[0], south_north=x_y[1])

Regards,

Xin

[Alyce Sala Tenna]

Hi Xin,

Thank you so much! That makes so much sense now. I did not realise you had to specify west_east and north_south.

You have saved me lots of time and lots of stress!

Regards,

Alyce

[Alyce Sala Tenna]

Hi Xi and Micheala,

I get 'nan' when trying to var_grid = var.sel(west_east=x_y[0], south_north=x_y[1]) for Temp at 2m and Temp 10m. 

I can however var.sel for temperature at 30m. Temp at 30m is interpolated from the same variable at temp interpolated at 10m.

Var.sel also works for wind speed and direction at 10m. 

Any thoughts? Is nothing in the actual data in the first instance?

Kind regards,

Alyce  

 

On Saturday, 10 October 2020 at 08:49:55 UTC+8 xinzha...@gmail.com wrote:

[Xin Zhang]

Hi Alyce,

You should check the original Temp2m variable in the wrfout file using ncview or python.

Here's the quick method of python:

Temp.isel(Time=0).plot()

If that's all 0 values, then you should check the setting in the namelist.input.

I remember Temp 2m is available for MYJ boundary option.

You can check it in the WRF USER GUIDE.

Regards,

Xin