Asian Program

Japanese

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Former Fellows

Dissertation Abstracts

Indonesia

  The occurrence of deep convection in the tropics affects the global circulation, since it transports the heat from atmospheric boundary layer to the upper troposphere. Since the vertical distribution of diabatic heating depends on the vertical structure of the convective system, it is important to study the vertical structure of the precipitating clouds occurring in the tropics. UHF wind profiler radar can be used to study this vertical structure of precipitating cloud. These kinds of profilers are sensitive to both Bragg scattering from the radio refractive index of turbulence and Rayleigh scattering from distributed targets. To describe the characteristics of rain, we examined the raindrop size distribution (DSD) which describes the number and size of raindrops in precipitation. Vertically pointing VHF Doppler wind profiler radars are well suited to retrieve DSD because of their operating frequency and data collection methodology. The scope of the present work is concerned with the wind profiler radar measurements of tropical rainfall event. The aim of this thesis is to study the characteristics of precipitating clouds at Kototabang, West Sumatra, Indonesia.

  We have studied the temporal variations of precipitating clouds in equatorial Indonesia based on observations with 1357.5 MHz wind profilers at Kototabang (0.2^oS, 100.3^oE) in West Sumatra, and Serpong (6.4^oS, 106.7^oE) near Jakarta. The observed precipitating clouds are classified into four types, stratiform, mixed stratiform-convective, deep convective, and shallow convective clouds based on their vertical structure. We have found that diurnal variations of the occurrence of precipitating clouds at Kototabang and Serpong have the same characteristics, namely that the precipitating clouds mostly occur in the afternoon and the peak of the stratiform cloud comes after the peak of the deep convective cloud.The time delay between the peaks of stratiform and deep convective clouds corresponds to the life cycle of mesoscale convective system. The precipitating clouds which occur in the early morning are dominated by stratiform cloud. Concerning seasonal variations of the precipitating clouds, we have found that the occurrence of deep convective cloud is predominant in the dry season.

  The diurnal variability of DSD in precipitating clouds over Kototabang is studied using three types of Doppler radars, operated at VHF- (47 MHz), UHF- (1.3 GHz), and X-band (9.4 GHz) frequencies. Two precipitating events from 5 to 6 May 2004 in the first observation campaign of the Coupling Processes in the Equatorial Atmosphere (CPEA) project reveal the difference between precipitating clouds in the early afternoon and the nighttime. In the early afternoon the precipitating clouds are dominated by shallow convective types with high rainfall rate at the surface. In the nighttime precipitating clouds are dominated by stratiform types with small rainfall rate at the surface. A diurnal variation of horizontal wind is observed over this area. The westerly in the lower troposphere and the easterly in the middle troposphere begin to be enhanced in the afternoon (1400-1800 LT).

  DSD parameters are retrieved from a VHF-band Equatorial Atmosphere Radar (EAR). A modified gamma distribution is used to model DSD parameters. The shape parameter (µ) is larger during stratiform precipitation than during shallow convective precipitation events, which is consistent with previous studies in other area. During stratiform rain events on 5 May 2004 the median volume diameter (D₀) is around 1 mm, which is larger than D₀ during shallow convective rain events. Results presented in this study indicate that DSD has a diurnal cycle over the mountainous region of Sumatra.