The THORPEX Pacific Asian Regional Campaign (T-PARC) objective on the extratropical transition of tropical cyclones: observed cases, their structure and downstream impacts

An objective of the THORPEX-Pacific Asian Regional Campaign (T-PARC) was to investigate the entire life cycle of a tropical cyclone plus address the influence of the shorter-range dynamics and forecast skill associated with high impact weather events (i.e., tropical cyclones) of one region (Eastern Asian and the western North Pacific) and their downstream impacts on the medium-range dynamics and forecast skill of another region (in particular, the eastern North Pacific and North America). To address this objective, a tropical-to-extratropical measurement strategy was designed. The primary focus of this strategy was the extratropical transition (ET) of tropical cyclones based on the poleward movement of a decaying tropical cyclone and the resulting intense cyclogenesis that results from its interaction with the midlatitude circulation. Because of the complex dynamical and thermodynamic environment associated with the extratropical transition process the observation strategy required the combination of multiple satellite and airborne reconnaissance platforms from a near global set of participants. This near global participation in the science objective associated with extratropical transition and downstream impacts is an indication that the scientific principles being examined with respect to downstream weather impacts by events upstream are applicable to many regions of the globe.

During the field phase of T-PARC, several tropical disturbances moved poleward to undergo a transition into the midlatitudes. The character of these disturbances included a weak circulation associated with widespread deep convection, a midget tropical cyclone, a typhoon, and a super typhoon. Corresponding to the variety of tropical disturbances was a wide range of forecast and actual structural changes and downstream developments.

In September 2008 two major tropical cyclones over the western North Pacific moved poleward to undergo a transition into the midlatitudes and were observed as part of the T-PARC objective on Extratropical Transition. Typhoon (TY) Sinlaku recurved over the northern portion of Taiwan as a category four tropical cyclone. Super typhoon (STY) Jangmi passed over central Taiwan as a category five storm before recurving over the Taiwan straits and moving north-northeast. Although both storms followed nearly identical tracks within ten days of each other, and interacted with similar midlatitude flow characteristics, their post-recurvature structural characteristics and their impact on the midlatitude flow varied greatly. In addition to these two major tropical cyclones, an unnamed midget tropical cyclone also moved poleward to interact with the midlatitude flow.

During the ET of Typhoon Sinlaku a primary T-PARC observational strategy of observing the structural changes in the cyclone core at the same time as the interaction of the tropical cyclone outflow with the midlatitude jet could be realized. Dropsondes, Doppler radar, rapid-scan satellite winds, ground based radar, and wind and water vapour lidar observations were made in the tropical cyclone environment and in the surrounding subtropical and midlatitude environment. Research aircraft missions were flown on 5 consecutive days during which Sinlaku first re-intensified as a tropical cyclone and then underwent ET. For the ET of STY Jangmi, the observations focused on the interaction of outflow from Jangmi with the midlatitude jet and on the structure of the decaying ET system. During the midget tropical cyclone and the convective disturbance observations were made to characterize the interaction of subtropical convection with the midlatitude flow. The observations collected with respect to each case are summarized. Furthermore, the spectrum of structural changes and their impacts is defined along with research priorities and goals associated with the cases.