Using TIGGE data to diagnose initial perturbations and their growth for tropical cyclone ensemble forecasts

The characteristics of initial ensemble perturbations around Typhoon Sinlaku (2008) produced by ECMWF, NCEP and JMA ensembles are compared using the THORPEX Interactive Grand Global Ensemble (TIGGE). The dynamical mechanisms of the growth of the advection flow are then compared between the ECMWF and NCEP ensembles. In the comparison, it is found that the vertical and horizontal distributions of initial perturbations as well as the amplitude are quite different among the three NWP centers prior to, during and after the recurvature of Sinlaku. In addition, it turns out that those differences cause a difference in the advection flow not only at the initial time but also during the subsequent forecast period. ECMWF ensembles have shown a relatively large growth of the advection flow, which results from 1) the baroclinic instability in a vortex, 2) the baroclinic instability associated with the mid-latitude waves and 3) the barotropic instability in a vortex. Those features are less distinctive in NCEP ensembles. A statistical verification shows that the ensemble spread of TC track predictions in NCEP (ECMWF) is larger than ECMWF (NCEP) for one (three) day forecasts on average. It can be inferred that while ECMWF starts from relatively small amplitudes of initial perturbations, the growth of the perturbations help to amplify the ensemble spread of tracks. On the other hand, the relatively large amplitudes of initial perturbations seem to play a role in producing the ensemble spread of tracks in NCEP.