docs: def _calc_eddy_time(self):
          """ estimate the eddy turn-over time in days """
          ens = 0.
          for j in range(self.nz):
              ens = .5*self.Hi[j] * self.spec_var(self.wv2*self.ph[j])
  
          return 2.*pi*np.sqrt( self.H / ens.sum() ) / 86400