Intrinsic Predictability From the Troposphere to the Mesosphere/Lower Thermosphere (MLT)

The atmosphere's flow becomes unpredictable beyond a certain time due to the inherent growth of small initial-state errors. While many research studies have 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. Initially small perturbations grow fastest in the MLT, reaching 10% of saturation after 5–6 days, compared to 10 days in the troposphere and 2 weeks in the stratosphere. A saturation level of 50% is reached only after about 2 weeks in the MLT, similar to the troposphere. Saturation times are overestimated in a coarser resolution model (grid size 160 km) by up to a factor of two, 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 5 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.