Collaborative Experiments of Convection and Gravity Waves

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1. Balloon Observation of the troposphere and the stratosphereBy using daily radiosonde soundings from 1993 to 1996 in the equatorial Kalimantan, we have investigated long-term variability of atmospheric gravity waves and found their seasonal variation in the troposphere and a modulation by the Quasi-Biennial Oscillation in the stratosphere.We have conducted an observational campaign to launch radiosondes 4 times daily at three sites in each of Java, Sumatra, and Kalimantan Islands from November to December, 2000. Gravity waves were identified in the stratosphere, and their latitudinal dependence was revealed. Vertical wavenumber spectra showed tails with a -3 exponent down to the vertical wave length of 200m. Conventional theories have explained this feature in terms of saturation of gravity waves. Interestingly, however, it was shown that no saturation was seen in the observed lower stratosphere.2. Observation of gravity waves with all-sky imagersAn all-sky imager was installed in the suburb of Bandung, Indonesia to observe gravity waves projected onto OH airglow. We have successfully observed gravity waves at altitudes around 90km and studied horizontal structure and propagation. It was found that their propagation was mostly southward. The observation will be continued and is expected to provide data to reveal seasonal and year-to-year variability of the gravity waves.3. Observation of the mesosphere and lower thermosphere (MLT) by MF and meteor radarsA new antenna was added to the MF radar in Kalimantan in order to make it capable of measuring the momentum flux associated with waves in the MLT region. With the MF radar and a meteor radar in Java, long-term variation of gravity waves and long-period planetary-scale waves were investigated. Activities of gravity and Kelvin waves were found to be weak throughout two years from 1996. It was also found that waves with 30- to 60-day periods correlate well with tropospheric intra-seasonal oscillation.