Equatorwards-breaking Rossby waves in the North Atlantic storm track : forecast quality, predictability and dynamics

摘要

Upper-level disturbances (ULDs) penetrating from the extratropics into low lati- tudes are a frequent feature of the synoptic-scale circulation over the North Atlantic and can be involved in significant weather events such as heavy precipitation over the western and northern parts of Africa, or Saharan dust outbreaks. The present project is the first to systematically explore the huge dynamical information content of long-term data from the European Centre for Medium-Range Weather Forecasts operational ensemble prediction system (EPS) and from the recently established THORPEX Interactive Grand Global Ensemble (TIGGE) for that particular at- mospheric feature. It investigates the dynamics, forecast quality and predictability of this atmospheric phenomenon exploring a wide range of verification and analysis tools based on potential vorticity (PV). The main conclusions from this work are: (i) The EPSs shows a generally underdispersive behaviour in the ULD region. (ii) Forecast errors are reduced by about 50% if each TIGGE centre's own analysis is used instead of the ECMWF analysis, particularly in regions of large PV gradients. (iii) There is a tendency of too weak Rossby wave breaking and therefore a north- ward shift of the PV streamers in the forecasts. (iv) Ensemble-mean multi-model forecasts of a four-day accumulated precipitation event appear accurate enough for a successful severe-weather warning in contrast to some single-model EPSs. (v) Diabatic processes upstream early in the forecasts appear to be more impor- tant than blocking downstream for strengthening the PV streamers in the medium range. The results and the analysis tools developed in this project have improved the understanding of such ULDs and their impacts and has advanced the verifica- tion of EPSs, which can both contribute to an improvement of numerical weather forecasting in the long term.