International coordinated observation of hectors at the tropical region in the northern Australia

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About 50-100 km offshore from Darwin, which is the central city on the Northern Territory, Australia, the two islands (Buthurst, Melville), called Tiwi Islands, are facing across a narrow strait. The very characteristic isolated type huge cumulonimbus cloud called Hector appears over these islands. We carried out the international common observation campaign (DAWEX: Darwin Area Wave Experiment) around Darwin in October to December, 2001, in order to study the generation of atmospheric waves by cumulonimbus clouds (Hector). In collaboration with the Australian Bureau of Meteorology (BOM), Monash university, and the university of Adelaide, we conducted radiosonde observation campaigns. In order to investigate the horizontal propagation characteristic of the atmospheric waves from the Hector, we selected 3 balloon launch sites; Garden Point on the Tiwi Islands, Darwin and Katherine, which are aligned on a nearly straight line with maximum distance of about 400km. We launches a total of 40 … More radiosondes every 3 hours from the 3 places for over 5 days in 3 periods; the comparatively calm dry season in the middle of October (10/13-18), and the active Hector periods in the second half of November (11/15-20) and mid-December (12/11-16). According to statistics of the balloon burst height, more than 3/4 reached the 25km, about 2/3 exceeded 30km.From a preliminary analysis of the observed profiles, we detected near the tropopause a persistent wave phenomenon with an oscillation period of about 84 hours. Since there is almost no phase difference among three observation points, this wave seems to have a large horizontal scale. Similar wave activities were also seen in November and December. We extracted the atmospheric gravity waves with a vertical scale smaller than about 3km, and estimated the wave energy from wind velocity and temperature fluctuations. The wave energy was enhanced at 15-20 km and 25-30 km, but, the gravity waves were considerably depressed at 20-25 km. The overall structure of the wave energy seems to correlate with the background zonal winds, though the underlying physical mechanism has not been clarified yet. Less