Representation of the extratropical circulation in the MiKlip decadal prediction system - impacts of resolution and initialization

This study analyzes the representation of the extratropical circulation over the North Atlantic (NA) region using the German decadal prediction system (MiKlip) of two different spatial resolutions. Four quantities are assessed, i.e. the storm track, blocking frequencies, cyclone frequencies, and windstorm frequencies. We investigate the effect of model initialization for the representation of the circulation in a lower resolution (LR, atm: T63L47, ocean: 1.5° L40) and higher resolution version (HR, atm: T127L95, ocean: 0.4° L40) of the decadal prediction system.

While LR shows common deficits in the climatological representation in both the initialized prediction system and the uninitialized historical projection, e.g. an overly zonal extratropical storm track and a deficit in blocking frequencies over the North Atlantic and Europe, the higher resolution version counteracts these biases. The initialized LR prediction system largely overestimates NA cyclone frequency, which is not the case for the uninitialized LR counterpart. This positive bias is mainly due to weak and short lived systems and is an effect of the initialization in the LR prediction system. Similar biases cannot be identified in the windstorm frequency which implies that the short lived cyclones are low of impact with respect to wind speed.

The initialization effect leading to an overestimation of weak and short lived cyclones cannot be found in the HR version. The overall better representation of the extratropical circulation in the HR version leads to an increased decadal prediction skill, which is measured in terms of anomaly correlation, with the increase in resolution for all four quantities.