Atmospheric Blocking Representation in EC-EARTH Climate SPHINX Data with Stochastic Parameterization. ​

Atmospheric blocking is a mid-latitude weather pattern that strongly influences the climate of highly populated areas of the Northern Hemisphere. Blocking is a challenging phenomenon both for its complex and nonlinear dynamics and for its representation by numerical climate models. In recent years, the ability of simulating blocking has improved following increased atmospheric resolution, allowed by the growing computing capabilities of modern machines. A computationally cheaper alternative is provided by stochastic parameterizations, which introduce a gaussian noise representing subgrid scale processes. This thesis aims at investigating the impact of the implementation of stochastic parameterizations on atmospheric blocking representation by the EC-Earth Earth System Model. More specifically, data from the Climate SPHINX project - which includes three different model resolutions with multiple ensemble members, with or without stochastic parameterizations - has been analyzed. To this aim, a new atmospheric blocking Lagrangian tracking algorithm has been developed and validated making use of ECMWF ERA5 Reanalysis: this allowed us to estimate blocking frequency, size, duration and displacement. Hence, the algorithm is used to analyze Climate SPHINX data, in order to evaluate if stochastic parameterizations have an impact on specific features of atmospheric blocking (as frequency, duration, etc.). Significant differences are found in blocking frequency over the North Atlantic, where an anomaly dipole is present, showing enhanced/decreased blocking activity at low/mid latitudes for the stochastic run. It is therefore explored whether the systematic impact of the stochastic parameterization is caused by a different representation of the atmospheric blocking dynamics - assessed through the lagrangian tracking algorithm - or by substantial differences in the mean climate. This analysis suggested that the mean state differences are at the basis of the different blocking representation: further inspection of mean fields revealed that systematic Atlantic jet differences are caused by anomalous meridional thermal gradients likely driven by a different radiative balance between the two model configurations. This is caused by different cloud activity, especially in the Equatorial region, likely induced by the stochastic parametrizations. It is possible to conclude that the implementation of the stochastic parameterizations used in EC-Earth is unable to improve the atmospheric blocking representation. However potential benefits of the stochastic parameterizations may be offset by impact of the parametrizations itself on the tropical region: these findings can be therefore used as a starting point for a further development of the stochastic parameterization schemes.