The structure of singular vectors associated with extratropical transition of tropical cyclones

Extratropical transition of tropical cyclones is associated with a reduction of predictability due to the high sensitivity of the forecast to the evolution of both the upper-level midlatitude flow and the tropical cyclone. Further sources of uncertainty are moist processes and convection. Strong error growth can occur in regions of the atmosphere that are unstable to small-amplitude perturbations. Singular Vectors can be used to identify such regions and indicate the structure of the fastest growing perturbations over a given time interval. For that reason the structure of singular vectors associated with the extratropical transition of a tropical cyclone could provide valuable information on the important dynamical and physical processes during such an event. In the ECMWF ensemble prediction system (EPS) singular vectors targeted on tropical cyclones are operationally calculated with linearized diabatic physics and an optimization interval of 48 hours. The calculations are performed with respect to a total energy norm.

In this study the ECMWF singular vectors associated with extratropical transition of tropical cyclones are investigated. Operationally calculated total energy singular vectors targeted on tropical cyclones are analyzed for a number of cases in 2005 – 2007. Characteristic patterns of singular vector evolution are identified. Furthermore, the evolution of singular vectors during their optimization interval and their sensitivity to spatial resolution is studied by performing new singular vector calculations for selected cases. Finally, operationally calculated singular vectors are compared to singular vectors calculated with a new representation of physical processes.