SHORT-RANGE PREDICTABILITY OF THE ATMOSPHERE - MECHANISMS FOR SUPEREXPONENTIAL ERROR GROWTH

Recently it has been established, using simple mathematical models of chaos, low-order models, and large numerical models of the atmosphere, that small errors grow in the mean in a superexponential manner. In this paper the mechanisms behind this behaviour are examined with special emphasis on the non-orthogonality of the eigenvectors of the linearized evolution operator and the variability of the local Lyapunov exponents on the attractor. The study reveals a picture that is far more complex and system-dependent than what has been advanced so far in the literature. The general ideas are illustrated by Lorenz's low-order atmospheric model for which a simple phenomenological model of error growth is developed and tested successfully against the simulations.