Typhoon Sinlaku during T-PARC: Sensivity of the re-intensification and downstream development to the track following recurvature

An accurate forecast of many tropical and extratropical factors associated with the extratropical transition (ET) of tropical cyclones remains a challenging problem and was a principal focus area for the THORPEX-Pacific Asian Regional Campaign (T-PARC). In September 2008, Typhoon Sinlaku became an intense tropical cyclone that recurved over Taiwan and moved poleward to pass just south of the primary islands of Japan. Typhoon Sinlaku presented an excellent opportunity to examine predictability issues associated with ET, as there was a large uncertainty in the track at recurvature and there was large variability in predicted downstream development following the ET. The ET of Typhoon Sinlaku was observed by multiple satellite and airborne reconnaissance platforms in support of the T-PARC field program, which provides for a unique opportunity to compare and contrast forecast, analyzed, and observed conditions associated with the character of ET and impacts on the midlatitude circulation into which the decaying tropical cyclone is moving.

Following recurvature over the northern tip of Taiwan, deep convection in TY Sinlaku was severely reduced under the influence of strong vertical wind shear due to the midlatitude westerly winds. However, significant convection developed downshear of the low-level center of the decaying tropical cyclone. The resurgence of deep convection contributed to the re-intensification of TY Sinlaku such that it regained typhoon intensity south of Japan. The re-intensification altered the structure of the typhoon, enabled the onset of ET, and contributed to reduced predictability of the impact of the tropical cyclone on downstream midlatitude circulations.

The post-recurvature fluctuation in intensity and structure of TY Sinlaku prior to ET was not accurately forecast in any of the operational global models that were used during T-PARC (i.e., GFS, NOGAPS, UKMO, ECMWF, JMA), and we hypothesize that this error was directly related to errant track and re-intensification forecasts. In all cases, the reduced predictability was in the sense of over development of significant downstream high-amplitude circulations that propagated to western North America. To more thoroughly understand the mechanism for the deepening, it is necessary to examine the relative role(s) of the typhoon track following recurvature and its relation to the re-intensification stage as the storm passed south of Japan. In several forecast sequences when overdevelopment of downstream development was severe, the forecast track was north of the analyzed track. This track shift influenced the interaction between the decaying typhoon and the midlatitude circulation into which it is moving. The northward shift in the forecast track is examined relative to the forecast storm structure and to factors related to downstream development.