Secondary circulation within a tropical cyclone observed with L-band wind profilers

Abstract. In association with the passage of a Tropical Cyclone (TC) around Japan, the secondary circulation in the region from the outer side to the center was investigated in detail by two separately located L-band wind profilers and the rawinsonde observations from 1 to 2 October 2002, for the first time. As the wind profilers can observe wind fields not only within rainbands but also in between, the mesoscale wind circulation including the vertical wind component in wide areas from the lower layer to the upper layer was investigated.

While the TC center approached the profiler stations, several rainbands associated with the TC subsequently passed. Relatively warm, moist inflow with a cyclonic rotation was observed in the lower-troposphere while the TC center approached. The inflow reached the inside of the main rainband where the updraft was observed. Above 5-km height (with temperature below 0°C), outflow and weak downdraft corresponding to falling frozen particles were observed. It is considered that the frozen particles formed precipitating clouds mainly in the outer rainband region. The continuous wind circulation transported water vapor from the lower troposphere to the upper troposphere via the vicinity of TC center.

On the other hand, after the passage of the TC center, the developed rainband passed, which was located in the south and southwest quadrant of the TC. It is suggested with the profilers' data that the rainband was intensified mainly by warm and moist outflow below 3-km height.