Re: modifying WRF geo_em.d01.nc file

On Dec 6, 2011, at 5:57 PM, Jinwoong Yoo wrote:

> Hello, NCL users.
>
> I'd like to get some tips how to modify an geo_em.d0X.nc file to modify topography of the WRF model domain.
> I know somebody had similar question in the ncl-talk previously and I followed their ideas but I'm still struggling to modify my geo_em.d0X.nc file.
> First, I used NCL method, "Change netCDF file" as described in http://www.ncl.ucar.edu/Applications/change_netCDF.shtml.
>
> I collected HGT_M values from "ncdump -v HGT_M geo_em.d01.nc" for my 198 by 261 grid domain and pasted the HGT_M values
>
>
> begin
> in = addfile("./geo_em.d01.nc","w")
> HGT_M = (/30,40,89,78,45,234,98,23,456,294,12,17,40,56,17,32,85,93,72,\
> ... ... \
> 20,46,24,90,89,25/)
>
> in->HGT_M=(/HGT_M/)
>
> end
>
> where I put my modified elevation values in the HGT_M = (/.../).
> However, this failed due to the dimension mismatch about which I could not find any help or explanation on the web.
> Because the domain is 198 by 261 array, there should be some (NCL) way to specify the HGT_M = (/.../) instead of just putting numbers separated by comma as I did.
> If you know how, please let me know.

Dear Jinwoong,

You're right, you shouldn't have to list the values by hand with commas between each value. The example on the web was just to show you how you can directly change a variable on a file, but you don't have to do it exactly this way.

How are you getting the new values that you need to replace the old values with?
If these values are in an array that you read in from another file, say, you can just use this variable.

For example, let's say the new HGT_M values are on another NetCDF file called "geo_hgt.nc", and are stored in a variable called "HGT". Then you could do something like this:

  hid = addfile("geo_hgt.nc","r")
  in->HGT_M = (/hid->HGT/)

Note that in order to do this, both in->HGT_M and hid->HGT must be exactly the same dimensionality. Also, hid->HGT must be coercible to the type of in->HGT_M.
That is, if in->HGT_M is of type "float", and hid->HGT is of type "double", then this won't work. You would have to do:

  in->HGT_M = tofloat(/hid->HGT/)

Finally, you need to be careful that both variables have the same units. If they don't, then you can simply fix the units:

   in->HGT_M@units = "..."

Where "..." contains the new units.

I will let Dave answer the question about PyNIO.

--Mary

>
> Second, I tried PyNIO to modify the geo_em.d01.nc file.
> Actually, I could get array values with it and could modify the values on the memory.
> However, it seems that I cannot modify the original geo_em.d01.nc even when I used the "w" mode although you can create a new nc file.
> I found this line from PyNGL and PyNIO webpage: "Note that PyNIO never overwrites an existing file with an entirely new file."
> If I'm wrong and an existing nc file can be overwritten with new values, please let me know how to.
> Personally, I'd like to use PyNIO to modify the nc file if possible.
> Maybe PyNIO developers can change the code to let users overwrite their nc file directly in the future if it has not been done yet.
>
> Thank you in advance.
>
> Best,
>
>
> Jinwoong Yoo, Ph.D.
> University of South Florida
>
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Received on Wed Dec 7 08:01:12 2011