Observational study on cirriform clouds in the tropical upper troposphere

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Cirriform clouds (cirrus, cirrostratus, cirrocumulus) existing in the upper part of the troposphere consist almost entirely of ice particles, and are known to play a significant role in regulating the radiation balance of the earth-atmosphere system. However, mechanisms which control generation, maintenance, and dissipation of cirriform clouds in the tropics are not well understood due to a scarcity of observations in the region. In this research project, an observation campaign of cirriform clouds using VHF wind profiler to observe vertical wind, W-band Doppler radar to observe falling velocity of ice particles in clouds, lidar to observe small-sized ice particles, weather radar to observe precipitating particles, and radiosonde to observe vertical profiles of temperature and humidity has been carried out at Kototabang, West Sumatra, Indonesia (at latitude 0.2 degrees south and longitude 100.32 degrees east).From detailed data analysis using data derived from the observation campaign, time-altitude variations of vertical wind in and around tropical cirriform clouds has been shown for the first time. Further, effects of vertical wind on particle falling velocity in cirriform clouds are studied in detail. Using vertical wind data observed by a VHF wind profiler installed at Kototabang (the Equatorial Atmosphere radar), vertical wind changes in the middle and upper troposphere associated with eastward-moving synoptic-scale cumulus activity (Yamamoto et al., in press), fine time-altitude variations of vertical wind in the precipitating stratiform region (Nishi et al., 2007) and vertical motion of cirriform clouds in the middle troposphere (Seto et al., 2006) have been studied and published. This research project has demonstrated that a combination of VHF wind profiler and other instruments to observe inside of cirriform clouds is useful to observe processes related to such clouds.