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The predictability of large-scale atmospheric motions during the onset period of a prominent atmospheric blocking event occurring over the Euro-Atlantic sector during 12-21 December 2007 is examined using Japanese 25-year Reanalysis (JRA25)/JMA Climate Data Assimilation System (JCDAS) dataset and one-week ensemble forecast dataset provided by the Japan Meteorological Agency (JMA). First, it is found that the predictability in the blocking region is temporarily reduced during the onset period of the blocking event. Second, a simple sensitivity analysis introduced by Enomoto et al. (2007) indicates that low-frequency variations associated with a quasi-stationary Rossby wave train significantly affect the prediction of the blocking onset in comparison with high-frequency variations. This is also confirmed by the time evolution of the spread among ensemble members based on 300-hPa height field. Finally, the predicted vorticity flux divergence by the low-frequency variations, rather than the high-frequency variations, is also found to significantly correlate with the predicted blocking strength, which stresses the importance of the low-frequency variations for the prediction of the onset of this blocking event.

Predictability during the Onset Period of a Euro-Atlantic Blocking Event during 12-21 December 2007