MAY 2014 Q U I N T I N G E T A L . 1945 Structural Characteristics of T-PARC Typhoon Sinlaku during Its Extratropical Transition JULIAN F. QUINTING Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany MICHAEL M. BELL* AND PATRICK A. HARR Department of Meteorology, Naval Postgraduate School, Monterey, California 1 SARAH C. JONES Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany (Manuscript received 28 September 2013, in final form 27 December 2013) ABSTRACT The structure and the environment of Typhoon Sinlaku (2008) were investigated during its life cycle in The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC). On 20 September 2008, during the transformation stage of Sinlaku’s extratropical transition (ET), research aircraft equipped with dual-Doppler radar and dropsondes documented the structure of the con- vection surrounding Sinlaku and low-level frontogenetical processes. The observational data obtained were assimilated with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft In- strumentation (SAMURAI) software tool. The resulting analysis provides detailed insight into the ET system and allows specific features of the system to be identified, including deep convection, a stratiform pre- cipitation region, warm- and cold-frontal structures, and a dry intrusion. The analysis offers valuable information about the interaction of the features identified within the transitioning tropical cyclone. The existence of dry midlatitude air above warm-moist tropical air led to strong potential instability. Quasigeo- strophic diagnostics suggest that forced ascent during warm frontogenesis triggered the deep convective development in this potentially unstable environment. The deep convection itself produced a positive po- tential vorticity anomaly at midlevels that modified the environmental flow. A comparison of the operational ECMWF analysis and the observation-based SAMURAI analysis exhibits important differences. In partic- ular, the ECMWF analysis does not capture the deep convection adequately. The nonexistence of the deep convection has considerable implications on the potential vorticity structure of the remnants of the typhoon at midlevels. An inaccurate representation of the thermodynamic structure of the dry intrusion has considerable implications on the frontogenesis and the quasigeostrophic forcing. 1. Introduction 50% of these storms reintensifying as extratropical cy- clones in the North Atlantic basin (Hart and Evans About 45% of tropical cyclones (TCs) interact with 2001). The ET process is determined by structural the midlatitude flow after recurvature (Jones et al. 2003) changes of a TC and can be considered generally as and undergo extratropical transition (ET), with over a transformation from a symmetric warm-core TC to an asymmetric cold-core cyclone. Evans et al. (2006) showed that operational global numerical models do not * Current affiliation: Department of Meteorology, University of properly simulate these structural changes. However, Hawaii at Manoa, Honolulu, Hawaii. 1 the dynamical processes that are associated with these Current affiliation: Deutscher Wetterdienst, Offenbach, Germany. structural changes perturb the midlatitude flow. In ide- alized simulations Riemer et al. (2008) showed that these complex perturbations to the midlatitude flow may Corresponding author address: Julian F. Quinting, Institute for Meteorology and Climate Research (IMK-TRO), Karlsruhe In- result in a baroclinic downstream development. Real stitute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany. case ensemble forecasts indicate that initially small E-mail: [email protected] perturbations may lead to plumes of forecast uncertainty DOI: 10.1175/MWR-D-13-00306.1 Ó 2014 American Meteorological Society 1946 MONTHLY WEATHER REVIEW VOLUME 142 in downstream regions (Anwender et al. 2008; Reynolds The first aircraft measurements during ET were ac- et al. 2009). Therefore, ET does not only affect those re- complished in 2000 by the Meteorological Service of gions in the vicinity of the ET event but also regions far Canada and the Canadian National Research Council downstream (Agusti-Panareda et al. 2004; Archambault inside Hurricanes Michael and Karen (Wolde et al. et al. 2007; Cordeira and Bosart 2010; Grams et al. 2011). 2001; Abraham et al. 2004). The data from these re- Only a small number of ET events have been in- search flights identified asymmetries in moisture and vestigated by aircraft-based measurements. Therefore, winds associated with the ET systems. A further ob- most studies on ET were limited to model data or to serving study involving research flights into the ET satellite-based observations. In an examination of 30 ET of Tropical Storm Ophelia was accomplished in the cases Klein et al. (2000) divided the ET process into Hurricane Field Program of 2005 (Fogarty 2006; Rogers a transformation stage and a reintensification stage. et al. 2006). As part of the Intensity Forecasting Ex- Based on infrared satellite imagery, they showed that the periment (IFEX) the structure of Ophelia was in- transformation stage is characterized by the interaction of vestigated from genesis to ET by research aircraft and a TC and the midlatitude baroclinic zone. Vancas (2006) aerosondes. During the ET of Ophelia, two National utilized temperature retrievals from the Advanced Mi- Oceanic and Atmospheric Administration (NOAA) P-3 crowave Sounding Unit to investigate the thermal char- flights sampled for the first time the core dynamical acteristics of the atmosphere in the vicinity of one ET structure and structural changes of an ET system using event. Based on these retrievals, Vancas (2006) identified airborne Doppler radar and dropsondes. The observa- cold air to the west and warm air to the east during the tions showed that the upper portion of Ophelia’s circu- interaction of the TC and the midlatitude baroclinic zone. lation was sheared away by strong upper-level winds. This interaction involves lower-tropospheric temper- Findings from the research flights also indicated the in- ature advection, the deformation of the TC warm core, trusion of dry, midlatitude air. However, high-resolution lower-tropospheric frontogenesis, and slantwise ascent observations of the structural changes of a TC during ET (descent) to the east (west) of the TC in a baroclinic and its environment are still rare and have not been environment (Harr and Elsberry 2000; Harr et al. 2000). investigated yet in detail. Harr and Elsberry (2000) showed that the modification One of the aims of The Observing System Research of the equivalent potential temperature gradient during and Predictability Experiment (THORPEX) Pacific the transformation stage results in warm frontogenesis Asian Regional Campaign (T-PARC) in 2008 was to in the northeastern quadrant of the transitioning TC. extend the knowledge of the various physical processes Furthermore, they found that cold frontogenesis to the involved in ET using aircraft observations. During this west of the TC is often suppressed as a result of a ther- field campaign one of the major typhoons in the mally direct circulation including the descent of cold air western North Pacific Ocean in 2008, Typhoon (TY) from upstream of the cyclone. In an investigation of the Sinlaku, was investigated from tropical cyclogenesis ET of Hurricane Floyd (1999), Atallah and Bosart until ET. In total 28 research flights were carried out (2003) showed that the dynamics of ET can be repre- by four research aircraft during the life cycle of TY sented in a quasigeostrophic framework. In their study, Sinlaku. the Sutcliffe approximation of the quasigeostrophic TY Sinlaku developed in the western North Pacific, omega equation provided an accurate physical expla- strengthened quickly to typhoon intensity, and recurved nation of the evolution of the precipitation distribution to the north of Taiwan on 14 September. Moving east- during ET. Agusti-Panareda et al. (2004) derived a po- northeastward, the cyclone weakened to a tropical storm tential vorticity (PV)-based conceptual model of the on 16 September. A very asymmetric structure indicated processes involved in ET. They suggested that the that Sinlaku had entered the transformation stage of ET. moisture that is collocated with the TC and that is ad- However, instead of completing transformation, Sinlaku vected by its circulation is spread over the warm sector reintensified (Sanabia 2010) and regained typhoon in- of the cyclone, leading to a short-lived convective de- tensity on 19 September (Foerster et al. 2013, manuscript velopment with associated diabatically generated PV. submitted to Mon. Wea. Rev.). At that time, Sinlaku Agusti-Panareda et al. (2004) stated that the convective exhibited a partial eyewall around the low-level circu- burst might be crucial for maintaining the diabatically lation maximum that is here defined as the low-level produced PV tower associated with the TC. They sug- center. The focus of the current study is 20 September gested that when the burst of deep convection weakens, when Sinlaku approached the primary midlatitude baro- the diabatically produced PV tower is not supported any clinic zone to enter the final stage of ET. Sinlaku decayed longer as latent heat release stops maintaining the pos- in the Pacific over several days with continued convective itive PV anomaly at low levels. activity. MAY