Intrinsic predictability from the troposphere to the mesosphere/lower thermosphere (MLT)

Abstract

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.