Middle atmosphere predictability in a numerical weather prediction model: Revisiting the inverse error cascade

The spectral predictability of the Met Office's Unified Model is examined using identical-twin experiments and the relative kinetic energy. In the troposphere, previous NWP results are recovered, namely the emergence of distinct regimes and the maximisation of the growth rate on synoptic scales at early times; in the middle atmosphere, the predictability decay is slower. This difference is attributed to the increase in the amplitude of gravity waves. The influence of small-scale motions is highlighted: improving their numerical representation by decreasing the timestep enhances predictability. © 2011