docs: def spec_var(model, ph):
      """Compute variance of ``p`` from Fourier coefficients ``ph``.
  
      Parameters
      ----------
      model : pyqg.Model instance
          The model object from which `ph` originates
      ph : complex array
          The field on which to compute the variance
  
      Returns
      -------
      var_dens : float
          The variance of `ph`
      """
  
      var_dens = 2. * np.abs(ph)**2 / model.M**2
      # only half of coefs [0] and [nx/2+1] due to symmetry in real fft2
      var_dens[...,0] /= 2
      var_dens[...,-1] /= 2
      return var_dens.sum(axis=(-1,-2))