Comparison of Polarimetric Parameters Obtained by a Ka-band Radar with those by C- and X-band Radars in Relation to Characteristics of Hydrometeors

At present, there are only several scanning Ka-band polarimetric radars in the world, thus there are plenty of possibilities for new findings. To research the characteristics of polarimetric parameters of Ka-band, a field observation using polarimetric radars with different wavelength, Ka-, X-, and C-band, was carried out in Okinawa, Japan in June 2016. The Ka-band radar whose observation range is 30 km, is located at Sesoko Research Facility, University of Ryukyu, 127.866E, 26.738N. The X-band radar whose observation range is 60 km, is located at Chihara Campus, University of Ryukyu, 127.768E, 26.250N. The C-band radar; CRL Okinawa Bistatic Polarimetric Radar (COBRA), whose observation range is 150 km, is located at Nago Precipitation Radar Facility, National Institute of Information and Communications Technology (NICT), 128.064E, 26.584N. Balloon-based hydrometeor videosonde (HYVIS) observations are conducted at NICT Okinawa Electromagnetic Wave Technology Center located within the observation range of these radars. Each radar can obtain the polarimetric parameters, such as horizontal radar reflectivity (Z_H), differential reflectivity (Z_DR), correlation coefficient between horizontal and vertical polarization signals (ρ_HV), differential propagation phase (Φ_DP), and specific differential phase (K_DP). HYVIS is a direct observation instrument for recording images of cloud particles as movie data by a microscope. During the HYVIS observations, the Ka- and C-band radars operated the continuous RHI observation directed to the azimuth angle of the HYVIS every 30 seconds, and the X-band radar operated the volume scanning observation with 14 elevation angles every 6 minutes. On 3 June 2016, a precipitation system with a leading stratiform region passed over the observation area. We launched three HYVISs into the stratiform region. HYVIS-A launched at 1116 JST, HYVIS-B launched at 1512 JST, and HYVIS-C launched at 1610 JST. The Z_H value of C-band at the HYVIS launch point is the weakest in HYVIS-A and the strongest in HYVIS-C.

We analyze the polarimetric parameters of the Ka- and C-band radars at a height of HYVIS location. As for K_DP, there are characteristic differences between the liquid phase and the solid phase region. Below the bottom of the melting layer, almost 4km, the K_DP values obtained by three radars are almost zero. The K_DP value with near 0 deg./km suggests the existence of small and spherical droplets. However, for HYVIS-C the Z_DR value of C-band is positive around 0.5 – 1.0 dB below the melting level. The HYVIS observation showed that the large droplets with the diameter of 1mm only existed for HYVIS-C in addition to a lot of small droplets. Therefore, the radar observation consistent with the HYVIS observation in that a small number of large diameter droplets exist. Above the melting level, for HYVIS-A and HYVIS-C the K_DP value of Ka-band is larger than that of C-band. The maximum difference is around 2.5 deg./km. For HYVIS-B the K_DP value of Ka-band is relatively small, so difference is also small. The K_DP difference between wavelengths cloud be contributed by the wavelength dependence of K_DP. In addition to this, however, the K_DP difference could be contributed by the existence of small and high aspect ratio ice particle (e.g. plate, needle, columnar crystal) to which only Ka-band has sensitivity. In the profile of Z_H, there is a height where the value takes a local maximum for Ka-band, but for C-band this characteristic is not observed. This means that a large amount of small particles which are no sensitive to C-band exist at this height. From the HYVIS observation, for HYVIS-A and HYVIS-C a large number of small ice particles with the diameter of several tens of micrometer or less were observed at this height, whereas for HYVIS-B small ice particles weren’t observed as much as HYVIS-A and HYVIS-C.