The Eddy Kinetic Energy Budget for Several Forecast Scenarios of the Extratropical Transition of Typhoon Choi-Wan

The extratropical transition (ET) of a tropical cyclone may strongly influence the mid-latitude circulation pattern by amplifying or even triggering a Rossby wave. This modification of the the mid-latitude flow impacts even regions far downstream of the ET event, as the amplification of the mid-latitude wave pattern coincides with the potential for strong cyclogenesis. The interaction between the tropical cyclone and the mid-latitude circulation involves complex interactions of physical processes, whose mis-representation in NWP often leads to a reduction of predictability around and downstream of an ET event. Hence, a deeper understanding of the physical and dynamical aspects of the interaction between the tropical cyclone and the mid-latitude flow is crucial to improve numerical forecasts during ET events. A possibility to examine the processes during the downstream impact of ET events is provided by an analysis of the eddy kinetic energy budget. Thereby, the kinetic energy distribution of the actual atmospheric circulation is examined with respect to a monthly mean distribution. In this framework of downstream baroclinic development, a downstream dispersion of eddy kinetic energy, emanating from the transitioning storm may support the amplification of the downstream wave pattern.

To examine the role of a transitioning tropical cyclone in different forecast scenarios, the analysis of the eddy kinetic energy budget in the context of downstream baroclinic development is employed on ensemble forecasts for the extratropical transition of Typhoon Choi-Wan (2009) for the first time. Using an EOF- and subsequent cluster analysis the dominant possible development scenarios for the ET of TY Choi-Wan are extracted from an ECMWF ensemble forecast. The eddy kinetic energy budget is then computed for representative ensemble members of interesting and opponent forecast scenarios for the ET event. The budget terms, involving the baroclinic conversion of eddy available potential into eddy kinetic energy and the advection and dispersion of eddy kinetic energy enable to identify distinct physical processes during the ET. The investigated forecast scenarios range from a strong reintensification of the transitioning tropical cyclone, together with a strong amplification of the mid-latitude wave pattern, to a nearly decaying cyclone during its transition. Hence, the influence of the TC itself as well as the role of the distinct mid-latitude flow pattern that surrounded the transitioning storm can be examined in detail. In this presentation the important processes during the ET of TY Choi-Wan causing the distinct forecast scenarios will be highlighted and discussed, after a brief introduction of the underlying analysis method.