Monday, 11 August 2003
Development of a new C-band bistatic polarimetric radar and observation of typhoon events
Communications Research Laboratory (CRL) has developed a new C-band (5340MHz) multi-parameter Doppler radar system with a bistatic Doppler radar network to establish the next-generation technology of rain observation for meteorological and hydrological applications such as weather forecasts and run-off analysis in predicting floods. This new radar is named COBRA (CRL Okinawa Bistatic polarimetric Radar). The weather targets of this system are typhoons, Baiu-frontal rainfall, meso-scale precipitation in subtropical zones, and clear air turbulence. Two transmitter (klystron) units are used for the polarization observation. The transmission polarization for each pulse is selected from six possible polarizations, namely, horizontal polarization, vertical polarization, +45- and -45-degree tilt linear polarizations, and right- and left-handed circular polarizations. The return signal is measured simultaneously by two receivers, one for horizontal polarization and one for vertical polarization. The pulse width can be changed to 0.5, 1.0, or 2.0 us. This system is controlled by GPS signal of 10MHz clock, and fully synchronous employment of the bistatic Doppler network is also carried out using this GPS signal. Doppler observation is selected from two modes, namely, the pulse pair observation for the measurement of the Doppler velocity and spectral width and the FFT observation for the measurement of the Doppler spectrum. To correct fot the Doppler velocity folding, settings can be made to switch the double PRF between pulses. The antenna has a diameter of 4.5 m, a beam width of 0.9 deg., and a radome diameter of 8 m. The antenna scan speed can be adjusted in increments of 0.1 rpm between 0.5 - 10 rpm in the horizontal direction, and between 0.1 - 3.6 rpm in the vertical direction. The maximum observation range is approximately 300 km in radius, although it depends on the pulse repetition frequency and transmitted pulse width. The spatial resolution is 37.5 °V 600 m, depending on the pulse width and over-sample rate. COBRA was used to observe typhoon events, Typhoon Ramasun and Typhoon Shinraku. A synchronous observation with TRMM/PR was performed in the case of Typhoon Ramusun, and detailed rain structures, such as the eye of the typhoon and a rain region of the shape of a swirl of the circumference, were observed in the case of Typhoon Shinraku. Examples of polarization parameters (ZDR, LDR, rHV(0), KDP, fDP, etc.) will also be presented.