Scale Interactions during the Re-intensification of Typhoon Sinlaku prior to Extratropical Transition

The re-intensification of Typhoon Sinlaku as a warm-core tropical system in a highly sheared environment southwest of Japan in September 2008 altered the structure of the tropical cyclone (TC), enabled the onset of Extratropical Transition (ET), and influenced the impact of the poleward-moving TC on the midlatitude circulation into which it was moving. In this study, the sensitivity to the structural characteristics of the post-recurvature portion of the TY Sinlaku life cycle is investigated. Immediately following recurvature, strong westerly flow steered the decaying TC eastward. Under the influence of the westerly vertical wind shear, convection was restricted to be downshear of the visible low-level circulation. This overall condition existed for a 36-h period. After several episodes of deep convective development to the east of the TC, a major convective burst occurred in close proximity to the low-level center. After the development of this convection, the TC deepened rapidly and regained typhoon status.

To more thoroughly understand the mechanism for the deepening, the redevelopment of TY Sinlaku from a decaying system under (moderate-to-) high shear (0000 UTC 16 September 2008) to a warm-core typhoon (0000 UTC 18 September 2008) is examined using European Center for Medium Range Weather Forecasts (ECMWF) model analyses and observations from the THORPEX – Pacific Asian Regional Campaign (T-PARC) and Tropical Cyclone Structure – 2008 (TCS-08) field programs. Fortunately, the increase in convection that occurred close to the low-level TC center and was associated with the re-intensification to typhoon intensity occurred during aircraft missions conducted by the Naval Research Laboratory (NRL) P-3 and the USAF 53rd Weather Reconnaissance Squadron WC-130J. The Electra Doppler Radar (ELDORA) was operated on the NRL P-3 and provided high-resolution reflectivity, and winds from 1-17 km in the vertical. In addition to the identification of a severely tilted TC vortex, the ELDORA observations identify a series of low- and mid-level vortices associated with the convection along the eastern portion of the TC.

Through the combination of ELDORA observations, dropwindsonde data, and ECMWF model fields, the character of the interaction between the convection and the tilted TC vortex is examined. In particular, the role(s) of the vertical distribution of vorticity contained in the deep convection is examined in terms of re-organizing the tilted vortex to an upright circulation that intensified to typhoon strength. The overall combination of vertical wind shear, TC vortex, and deep convective activity provided for a unique interaction among synoptic-scale, storm-scale, and mesoscale factors. Finally, the impact of the structural changes of Sinlaku as a source of eddy kinetic energy for downstream development will be examined.