What is "focus point" in WRF-plots grid box? (python code)

Question

has anyone used the python code from here (written by Geir Arne Waagbo) to plot skewT plot from WRF output?

I've got a question regarding some of the functions used in the code found in pywrfplotUtils.py - which is the getXY(lon,lat) function that gets called in getDimensions(nc) function.

Below are the codes:

def getDimensions(nc):
 Nx = nc.getncattr('WEST-EAST_GRID_DIMENSION')-1
 Ny = nc.getncattr('SOUTH-NORTH_GRID_DIMENTION')-1
 Nz = nc.getncattr('BOTTOM-TOP_GRID_DIMENSION')-1
 dx = nc.getncattr('DX')
 dy = nc.getncattr('DY')
 lons = nc.variables['XLAT'][0]
 lats = nc.variables['XLONG'][0]
 # find coordinates for focus point
 x,y = getXY(lons[Ny/2,:],lats[:,Nx/2])
 return Nx,Ny,Nz,lons,lats,dx,dy,x,y

def getXY(lon,lat):
 x_nr = 0
 y_nr = 0
 while (lon[x_nr] < lon_focuspoint):
  x_nr += 1
 while (lat[y_nr] < lat_focuspoint):
  y_nr += 1

 print "x_nr:" str(x_nr),"Lon:",str(lon[x_nr])
 print "y_nr:" str(y_nr),"Lat:",str(lon[x_nr])
 return x_nr,y_nr

The focus point is a variable set in the pywrfplotParams.py:

#This defines the grid box for which skewT-plots etc are based
lat_focuspoint = 60.2
lon_focuspoint = 11.08

Unfortunately, I don't know what focus point is referring to, so when I use the values set by the author, obviously I get an error, and that error is - IndexError: index out of bounds.

I have found this link useful in providing the definition for the attributes asked in that question, but I'm not entirely sure if I should also get the cell indices from latitude and longitude in the WRF model grids.

Your thoughts are very much appreciated!

Answer 1

A Skew-T/Log-p plot is a vertical profile from a point, and it appears in that code that the point you are plotting the skew-T for is 60.2 latitude, 11.08 longitude. The snippet of code:

def getXY(lon,lat):
 x_nr = 0
 y_nr = 0
 while (lon[x_nr] < lon_focuspoint):
  x_nr += 1
 while (lat[y_nr] < lat_focuspoint):
  y_nr += 1

finds the closest grid point west and south of your focus point by setting the x and y gridpoints to 0 and incrementing the index as long as the latitude/longitude values at that grid point are less than the focus point. This is what the focus point is for -- the lat/lon location you want the skew-t/log-p plot at.

The exception IndexError: index out of bounds. means that the above code tried to access a value lon[x_nr] or lat[y_nr] that was out of bounds. This means that the latitude/longitude coordinate of your focus point is not within your domain.

To run the code successfully, edit the focus point to be a point within your model domain, specifically the value of latitude and longitude that you want the skew-t profile for.