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