Abstract
Ensemble simulations are performed to quantify the internal variability of both regional atmospheric models and wave-atmosphere coupled model systems. Studies have shown that the internal variability in atmospheric models (e.g., wind or pressure fields) is increased during extreme events, such as storms. Comparing the magnitude of the internal variability of the atmospheric model with the internal variability of the coupled model system reveals that the internal variability can be reduced by coupling a wave model to the atmospheric model. While this effect is most evident during extreme events, it is still present in a general assessment of the mean internal variability during the whole study period. Furthermore, the role of this wave-atmosphere coupling can be distinguished from that of the internal variability of the atmospheric model since the impact of the wave-atmosphere interaction is larger than the internal variability. This is shown to be robust to different boundary conditions. One method to reduce the internal variability of the atmospheric model is to apply spectral nudging, the role of which in both the stand-alone atmospheric model and the coupled wave-atmosphere system is evaluated. Our analyses show that spectral nudging in both coupled and stand-alone ensemble simulations keeps the internal variability low, while the impact of the wave-atmosphere interaction remains approximately the same as in simulations without spectral nudging, especially for the wind speed and significant wave height. This study shows that in operational and climate research systems, the internal variability of the atmospheric model is reduced when the ocean waves and atmosphere are coupled. Clear influences of the wave-atmosphere interaction on both of these earth system components can be detected and differentiated from the internal model variability. Furthermore, the wave-atmosphere coupling has a positive effect on the agreement of the model results with both satellite and in situ observations.
Highlights
Air-sea interaction processes and the feedbacks of their interdependence must be better understood to further improve both the operational and the climate research capabilities of model systems
With regard to the impacts of the coupling on the mean sea level pressure (MSLP), the application of spectral nudging in the atmospheric model has a larger impact (Figure 9B; Supplementary Figure 2C)
From this study on assessing the uncertainties in ensemble simulations with a wave-atmosphere coupled model relative to the impacts of the coupling, it can be concluded that the ensembles of the coupled system and the reference model differ significantly from one another during the majority of the study period but especially during extreme events
Summary
Air-sea interaction processes and the feedbacks of their interdependence must be better understood to further improve both the operational and the climate research capabilities of model systems. Weisse et al (2000) and Weisse and Schneggenburger (2002) stated that the regional-scale effects of linking the wave model to the atmospheric model on the mean sea level pressure (MSLP) in the North Atlantic are not significant, indicating that the internal variability is large during events with large influences due to this coupling, and the impacts cannot be differentiated from the internal variability. In this study, a state-of-the-art high-resolution regional waveatmosphere coupled model system for the North and Baltic Sea in the framework of the Geesthacht COAstal model SysTem (GCOAST) is used to investigate the effects of the wave-atmosphere coupling relative to the internal variability of atmospheric models, especially during extreme events, by conducting ensemble simulations. A summary and conclusions along with a discussion of the results are given (section 6)
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