Ground-based W-band FMCW CPR for coordinated observation with spaceborne CPRs

CPR (Cloud Profiling Radar) is a radar system designed to monitor cloud vertical structure with millimeter wavelengths, normally at Ka band (35 GHz/8.5 mm) or W band (78 GHz or 95 GHz/3.8 mm or 3.2 mm). Although clouds and aerosols are one of the important components of radiative forcing of the earth for better understanding of global warming, estimate of the contributions from cloud and aerosols is still a long-standing problem in atmospheric science. One of the difficulties of estimation is caused by lack of understanding of cloud-climate feedback models, which needs to be validated and improved by better cloud parameterization scheme, using cloud properties such as cloud type, location, liquid and ice water content, height, particle size and shape, or lifetimes observed by CPR. Since the concept of CPR for detecting non-precipitating cloud at W-band was proposed in America in 1981, several CPRs have been developed mainly in America, U.K., Germany and Japan. Although many CPRs operate in pulsed mode, CPRs with FMCW (Frequency Modulated Continous Wave) mode has been developed recently. Advantages of FMCW mode are: (1) higher range resolution by wide FM bandwidth and (2) higher S/N ratio by longer observation term. Although range resolution at pulsed mode, usually about hundreds of meters, is determined by pulse length which needs certain length, wide FM bandwidth makes range resolution of FMCW CPR much higher from a few meters. Although usual PRF (Pulse Repetition Frequency) at pulsed mode for CPR is about thousands of Hz which means a few profiles every 1 ms, FMCW mode keeps digiting signals at higher than 10 MHz of sampling rate, which leads to higher S/N ratio by averaging effect of FFT (Fast Fourier Transferom). Because this higher S/N ratio allows the system to use less power, a solid-state transmitters are used to transmit signals inspite of high power transmitters like such vacuum emitters, which is safer and more moderate cost. In Japan, Chiba University has developed a solid-state ground-based FMCW CPR at 94.79 GHz named FALCON-I (FMCW Radar for Cloud Observation since 1999, which succeeded the first demonstration of cloud observations in 2002. FALCON-I not only provides more detailed cloud microphysical properties, but also contributes validation of spacebourne CPRs such as CloudSat/CPR and EarthCARE/CPR. Combining ground-based and spaceborne observation provides us new point of views of cloud microphysics. Spaceborne CPR such as CloudSat/CPR constantly observes global cloud vertical distribution. On the other hand, ground-based FMCW CPR observes very detail cloud vertical structure of targeted cloud with high temporal and range resolution. Combining the both scheme of macro and micro observation leads to better understanding of cloud process and its lifetime to study influences of clouds on the Earth's water cycle and energy budget. Coordinated observation of ground-based and spaceborne CPRs is also significant for validation of calibration. Main calibrations of spaceborne CPR are sea-surface calibration, internal calibration and external calibration using ground-based calibration equipment such as ARC (Active Radar Calibrator). These schemes calibrate hardware parameters such as transmitting power, antenna patterns, gain of the receiving system, and etc. The coordinated observation of CPRs is for evaluating the effect of attenuation, multi-scattering and minimum detectable sensitivity. The coordinated observation of FALCON-I and CloudSat was done for 3 months in 2008, in Okinawa, Japan. As CloudSat passed 5 times over FALCON-I within 5 km during the observation term, we got 4 cases of cloud observation. We found that effect of gaseous attenuation is the largest factor of the each profile differences, ground-based CPR tends to underestimate ice cloud because of attenuation, and homogeneous clouds show high correlation between the both CPRs. We are aiming to establish the validation and analysis scheme from the coordinated observation of FALCON-I and CloudSat/CPR, which will be useful for EarthCARE/CPR launching in 2015. EarthCARE/CPR is the world's first W-band doppler spaceborne CPR developed by JAXA and NICT. Success of spaceborne doppler measurement has a huge meaning on cloud observation.We expect FALCON-I's doppler measurement function for validation of EarthCARE/CPR.