Structural characteristics of the core region of T-PARC Typhoon Sinlaku in a vertically sheared environment

The extratropical transition (ET) of a tropical cyclone (TC) goes along with significant structural changes. During the THORPEX-Pacific Asian Regional Campaign (T-PARC) in 2008 a unique observational data set for the entire life cycle of Typhoon Sinlaku has been collected. In this study we focus on the late tropical stage of Sinlaku when the TC weakens in an increasingly complex midlatitude environment. Research flights with the NRL P-3 and the USAF C-130 aircraft were conducted in the core region of Sinlaku. The P-3 aircraft carrying the Dual Doppler Radar ELDORA penetrated through the eyewall into the TC's center and thus allowed to capture a high resolution data set for the entire eyewall. These radar data, complemented by dropsonde and flight level data, were assimilated with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software at 1 km horizontal resolution. In both radar reflectivity and wind field the SAMURAI analysis shows marked asymmetries in the structure of the core region during the late tropical stage of Sinlaku. The maximum radar reflectivities are located to the north of the center. To the west and south pronounced gaps are apparent in the eyewall. The eastern part of the eyewall is covered by broad ascent and descent occurs mainly to the west of the center. Since for this case the vertical shear of the environmental flow is assumed to be the dominant environmental factor influencing the structure of the typhoon, the SAMURAI analysis domain is split up into quadrants according to the west-southwesterly shear vector. The characteristics of the quadrants are analyzed using Contoured Frequency by Altitude Diagrams (CFADs) and reveal that the main precipitation areas are located left of the shear vector and the main ascent region to the downshear side. These results are consistent with existing modeling and observational studies and highlight the importance of the vertical shear of the environmental flow on the dynamics and thus the structure of the core region of a tropical cyclone. This study enables an unprecedented, very detailed view of the eyewall structure of a typhoon near the end of its tropical stage.