Validation of extratropical cyclone characteristics in sub-seasonal ECMWF forecasts

Extratropical cyclones strongly interact with the midlatitude waveguide and can thus actively influence onset, maintenance, and decay of large-scale weather regimes on sub-seasonal timescales (10 – 60 days). For instance, individual cyclones over the Pacific-North American region have shown to be able to trigger positive or negative phases of the North Atlantic Oscillation, depending on the type of Rossby wave breaking they are associated with. Likewise, strongly diabatically driven cyclones have shown to be relevant for ridge amplification and thus blocking onset and maintenance. Biases in cyclone activity and characteristics in sub-seasonal numerical weather prediction models might therefore hinder exploiting the potential large-scale predictability on these timescales. We thus, for the first time, identify and track extratropical cyclones in 21 winters (2000 – 2020) of sub-seasonal ensemble hindcasts from the European Centre for Medium-Range Weather Forecasts. This quasi-Lagrangian, object-oriented approach allows us to validate various cyclone life cycle characteristics such as the deepening rate, location of genesis, maximum intensity, and decay, propagation direction and speed, and size, age and lifetime. Overall, the hindcasts reproduce the climatology of cyclone activity and characteristics remarkably well up to 6 weeks lead time, both over the North Atlantic and the North Pacific. However, the hindcasts tend to underestimate the frequency of the strongly intensifying subset of North Atlantic cyclones. This underestimation is likely linked to an underestimation of cyclogenesis frequency along the southeastern U.S. coast. As strongly intensifying cyclones are particularly relevant for the reorganization of the large-scale flow downstream, this bias might influence sub-seasonal forecast skill and deserves further attention.