SIXTH INTERNATIONAL WORKSHOP on TROPICAL CYCLONES Topic 2.5 : Physical Processes and Downstream Impacts of Extratropical Transition Rapporteur:

The importance of the ET in influencing the dynamics of the atmosphere has become more evident in recent years. The occurrence of ET events over the North Pacific has been observed to coincide with periods of reduced forecast model skill (Jones et al. 2003). Harr et al. (2004) examined downstream propagation of increased ensemble standard deviation in mid-tropospheric heights from several operational global model ensemble prediction systems during the ET of typhoon Maemi (2003) that also coincided with reduced forecast skill over the Northern Hemisphere. The complex physical and dynamical processes during ET are extremely sensitive to sources and impacts of initial condition errors and forecast model uncertainty. Therefore, factors that impact forecast model error growth downstream of an ET event must be identified. For example, predictability during an ET event may exhibit large variations due to the phasing between the decaying tropical cyclone and the midlatitude circulation (Klein et al. 2002). McTaggart-Cowan et al. (2006a) have shown that the ET of Hurricane Katrina (2005), though lacking in reintensification, had an important impact on the middle-latitude flow and reduced medium-range predictability.