  The predictability problem in the inverse energy cascade of two-dimensional
turbulence is addressed by means of direct numerical simulations. The growth
rate as a function of the error level is determined by means of a finite size
extension of the Lyapunov exponent. For error within the inertial range, the
linear growth of the error energy, predicted by dimensional argument, is
verified with great accuracy. Our numerical findings are in close agreement
with the result of TFM closure approximation.
