docs: def spec_sum(ph2):
      """Compute total spectral sum of the real spectral quantity``ph^2``.
  
      Parameters
      ----------
      model : pyqg.Model instance
          The model object from which `ph` originates
      ph2 : real array
          The field on which to compute the sum
  
      Returns
      -------
      var_dens : float
          The sum of `ph2`
      """
  
      ph2 = 2.*ph2
      ph2[...,0] = ph2[...,0]/2.
      ph2[...,-1] = ph2[...,-1]/2.
  
      return ph2.sum(axis=(-1,-2))