Role of resolving gravity waves for estimating the intrinsic predictability of the middle atmosphere

The atmosphere's flow becomes unpredictable beyond a certain time due to the inherent growth of small initial-state errors. While much research has focused on tropospheric predictability, predictability of the middle atmosphere remains less studied. This work contrasts the intrinsic predictability of different layers, with a focus on the mesosphere/lower thermosphere (MLT, ~50–120 km altitude). Ensemble simulations with the UA-ICON model for an austral winter/spring season are conducted with a gravity-wave permitting horizontal resolution of 20 km, and are contrasted to coarser resolution simulations. Initially small perturbations grow fastest in the MLT, reaching 10% of saturation after 5–6 days, compared to 10 days in the troposphere and two weeks in the stratosphere. However, perturbation energy in the MLT reaches 50% saturation only after about two weeks, similar to the troposphere. Those saturation times are overestimated  by up to a factor of two when using a coarser resolution (grid size 160km),  highlighting the need for gravity wave-resolving models. Predictability in the MLT depends on horizontal scales. Motions on scales of hundreds of kilometers are predictable for less than five days, while larger scales (thousands of kilometers) remain predictable for up to 20 days. This scale-dependent progression of predictability cannot be explained by simple scaling for upscale error growth. Vertical wave propagation plays a significant role, with gravity waves transmitting perturbations upward at early lead times and planetary waves enhancing long-term predictability. In summary, the study shows that MLT predictability is scale-dependent and highlights the necessity of high-resolution models to capture fast-growing perturbations and assess intrinsic predictability limits accurately.