An Eddy Kinetic Energy View of Physical and Dynamical Processes in Distinct Forecast Scenarios for the Extratropical Transition of Two Tropical Cyclones*

Abstract

The extratropical transition (ET) of Hurricane Hanna (2008) and Typhoon Choi-Wan (2009) caused a variety of forecast scenarios in the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). The dominant development scenarios are extracted for two ensemble forecasts initialized prior to the ET of those tropical storms, using an EOF and fuzzy clustering analysis. The role of the transitioning tropical cyclone and its impact on the midlatitude flow in the distinct forecast scenarios is examined by conducting an analysis of the eddy kinetic energy budget in the framework of downstream baroclinic development. This budget highlights sources and sinks of eddy kinetic energy emanating from the transitioning tropical cyclone or adjacent upstream midlatitude flow features. By comparing the budget for several forecast scenarios for the ET of each of the two tropical cyclones, the role of the transitioning storms on the development in downstream regions is investigated. Distinct features during the interaction between the tropical cyclone and the midlatitude flow turned out to be important. In the case of Hurricane Hanna, the duration of baroclinic conversion from eddy available potential into eddy kinetic energy was important for the amplification of the midlatitude wave pattern and the subsequent reintensification of Hanna as an extratropical cyclone. In the case of Typhoon Choi-Wan, the phasing between the storm and the midlatitude flow was one of the most critical factors for the future development.

A Composite Perspective of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones

“…The dynamical connection between divergent outflow impinging upon the PV waveguide and Rossby wave amplification highlighted by this study is consistent with previous findings that the negative PV (or negative absolute vorticity) advection by the irrotational wind acts as a Rossby wave source (e.g., Sardeshmukh and Hoskins 1988;Riemer et al 2008;Riemer and Jones 2010;Hodyss and Hendricks 2010). The PV framework used to diagnose Rossby wave amplification and dispersion in this study is complementary to the energetics framework used to diagnose downstream baroclinic development (Orlanski and Sheldon 1995) associated with Rossby wave dispersion following WNP extratropical cyclogenesis (e.g., Hakim 2003;Danielson et al 2006) and TC recurvature (e.g., Harr and Dea 2009;Cordeira and Bosart 2010;Keller et al 2014).…”

“…Given a strong, continuous meridional PV gradient (i.e., waveguide; Martius et al 2010), ridge amplification and jet streak intensification will excite or amplify a baroclinic Rossby wave train (RWT) that disperses eddy kinetic energy downstream while initiating surface cyclogenesis downstream (e.g., Riemer et al 2008;Harr and Dea 2009;Keller et al 2014). Thus, a recurving TC may indirectly reconfigure the extratropical flow pattern and influence the sensible weather thousands of kilometers downstream within a few days (e.g., Archambault et al 2013, their Fig.…”

“…1; Harr and Archambault (2014)], to a strengthened jet stream without substantial flow amplification [e.g., with TC Jangmi (2008); Grams et al (2013a,b)], to little change in the flow pattern [e.g., with TC Opal (1997); Harr and Elsberry (2000)]. Anticipating the extratropical flow response to TC recurvature is crucial given that flow amplification downstream of a recurving TC may induce extreme weather (e.g., Cordeira and Bosart 2010;Grams et al 2011;Chaboureau et al 2012;Pantillon et al 2014) and contribute to reduced midlatitude predictability (e.g., Harr et al 2008;Anwender et al 2008;Reynolds et al 2009;Keller et al 2011;Pantillon et al 2013Pantillon et al , 2014Harr and Archambault 2015).…”

“…The primary factors influencing the downstream flow response to TC recurvature are (i) the large-scale flow pattern into which the TC is moving (e.g., Harr and Dea 2009;Riemer et al 2008;Jones 2010, 2014), and (ii) the interaction between the TC and an extratropical disturbance such as a trough or jet streak (e.g., Klein et al 2002;Ritchie and Elsberry 2007;Jones 2010, 2014;Grams et al 2013b;Keller et al 2014). Characteristics of the recurving TC, such as size and intensity, are thought to be secondary factors (e.g., Harr and Dea 2009;Archambault et al 2013).…”

Comparison of Wave Packets Associated with Extratropical Transition and Winter Cyclones

“…By contrast, the notrough composite indicates little evidence of an upstream signal in either the meridional wind or PV fields (Figs. The larger-amplitude signal for cases having an upstream trough is not surprising since this flow configuration can lead to the baroclinic reintensification of the TC (e.g., Harr et al 2000) and is often associated with the generation of eddy kinetic energy, which acts to amplify the upper-tropospheric flow (e.g., Harr et al 2000;Harr and Dea 2009;Keller et al 2014). For both composites, the dominant signal is the ridge amplification associated with ET; however, the maximum 0-h meridional wind perturbation in the trough composite is 22 m s 21 , compared to 16 m s 21 in the no-trough composite.…”