Structural transition of Typhoon 0416 observed by weather radars, radiosondes and wind profilers

Tropical cyclone (TC; Typhoon 0416) passed over the Japanese Archipelago during 29--31 August, 2004 and transformed into extratropical cyclone in the northern side of Japan. Based on structural change of precipitating clouds obtained by weather radars, the transition process of TC was classified into four stages as follows: First stage: eyewall surrounds TC eye and spiral rain bands are obvious; Second stage: TC eye disappears but spiral rain bands still exist; Third stage: spiral rain bands become vague and strong precipitating clouds exists only in the front side of TC; Fourth stage: strong precipitating clouds in the front side become rather weak.

Radiosonde observations were conducted every 6 hours in four sites which are close to TC trajectory. Remarkable warm core in the TC center was observed from first to third stages. In the fourth stage, weak warm core was still observed though precipitating clouds considerably changed from the early stages. In the third stage, which corresponds to rapid weakening stage of TC, dry (moist) layer was observed at lower (upper) level in front side and upper (lower) level in rear side of TC. In spite of the intrusion of dry air at upper level of rear side, re-intensification of TC was not encouraged since stable layer continuously existed above the boundary layer to the upper troposphere.

Our group has studied TC based on wind profiler observations (Teshiba et al., 2001; Shibagaki et al., 2003). They showed detailed wind behavior in the vicinity of TC which cannot be observed by other meteorological instruments. We examined transition of wind behavior in the vicinity of TC center using observation data by Wind profiler Network and Data Acquisition System (WINDAS) operated by Japan Meteorological Agency (JMA). WINDAS is composed of 31 L-band wind profilers (Kato et al., 2003; Hashiguchi et al., 2004), but four wind profilers near TC trajectory were used in this study. To examine the wind variation associated with TC, mean transitional velocity of TC center is subtracted from the observed wind at each site. Strong peak of cyclonic wind corresponding to eyewall and spiral rain bands were observed in the first and second stages, respectively. In the third stage, the peak became vague with extent and moved away from TC center. In the fourth stage, though it was before extratropical transition, the peak of cyclonic wind could not be seen and the wind structure differed from characteristics of TC. As mentioned above, the characteristics of TC were not seen in the precipitating clouds and wind structures in the fourth stage, though thermal structure still had characteristics of TC.