Abstract
The surface-atmosphere coupling is a crucial factor in representing the stable boundary layer in numerical weather prediction models. Different aspects of the surface-atmosphere coupling are therefore often subject to model tuning. Here we investigate the effect of such adjustments to the operational weather prediction models AROME-Arctic and MEPS. Currently, an upper limit on the Richardson number is imposed on the turbulent exchange coefficients in the surface layer. In AROME-Arctic, stable stratification is prohibited from influencing the surface fluxes. In MEPS, a limited range of stability is allowed. We focus on the impacts of these model settings on the vertical structure and evolution of the stable boundary layer by analysing inversion strengths in the surface layer and in the lower atmosphere. In AROME-Arctic, the surface stays always coupled to the atmosphere, acting as a continuous heat sink that sustains atmospheric inversions. Conversely, in MEPS, the surface layer decouples from the atmosphere, resulting in pronounced surface layer inversions and a reduction in atmospheric inversion strength, reminiscent of the well-known run-away cooling problem. At the same time, MEPS shows colder surface and 2m-temperatures, reducing the warm bias compared to observations from Sodankylä, superficially providing justification for the tuning step. We argue that evaluating the performance of 2m-temperature alone is insufficient for assessing model performance in the stable boundary layer without a validation of the vertical structure in the atmosphere. Additionally, we emphasize that transparent communication and detailed documentation of model tuning are essential for precise model interpretation and lasting progress in improving complex forecasting systems.
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