NASA Scatterometer Observes the Extratropical Transition of Pacific Typhoons

monitored the evolution of the twin typhoons Violet and Tom. They moved north from the western tropical Pacific, acquiring the features of mid-latitude storms, with developing frontal structures, increasing asymmetry, and introduction of dry air into the core of the typhoons. While Violet hit Japan, causing destruction and death (Fig. 1), Tom merged with a mid-latitude trough and evolved into a large extratropical storm with gale-force winds (Fig. 2). We understand relatively little of the extratropical transition of tropical cyclones because of the complex thermodynamics involved [e.g., Sinclair, 1993]. Since the transition usually occurs over the ocean, there has been very little measurements of it. Yet the mid-latitude storms resulting from tropical cyclones usually have strong winds and heavy precipitation. The transition is a fascinating science problem, but it also has important economic consequences. The transition occurs over the busiest trams-ocean shipping lane, and when the resulting storms hlt land, they usuall y cause devastation to populated areas. NSCAT was successfully launched into a near-polar, sun-synchronous orbit on the Japanese Advanced Earth Observing Satellite (ADEOS) in August 1996 from Tanegashima Space Center in Japan. The six antennas of NSCAT send microwave pulses at a frequency of 14 GHz to the Earth's surface and measure the backscatter. The antennas scan two 600-km bands of the ocean which are separated by a 330-km data gap. From NSCAT observations, surface wind vectors can be derived at 25-km spatial resolution, covering 7'7% of the ice-free ocean in one day and 97% of the ocean in two days, under both clear and cloudy conditions. Together with the precipitable water (vertically integrated water vapor) derived from the Special Sensor Microwave / Imager (SSM/1) on the operational 2. spacecraft of the Defense Meteorological Space Program (DMSP), the wind observations provide a good opportunity to monitor and understand the evolution of Violet and Tom. The precipkable water from SSMII has been evaluated by Liu et al. [1992] and others. The detailed structure of the wind-field in Fig. 1 illustmtes the high spatial resolution of NSCAT. The abrupt change in wind direction northeast of the me is likely to be have been caused by the development of frontal structure. The overlay of wind on precipitable water dramatically visualizes not only the structure of this evolving system, but also the relation between the dynamics and the hydrologic balance in the mesoscale. The best-track analyses from typhoon centers and the surface …