Relationship between Polarimetric Parameters Obtained By a Ka-Band Radar and Characteristics of Solid Hydrometeors in Snow Clouds

Nagoya University has installed a scanning Ka-band polarimetric Doppler radar whose observation range is 30 km since September 2014 and conducts a continuous observation using the radar. It can obtain horizontal radar reflectivity (Z_H), Doppler velocity, differential reflectivity (Z_DR), differential propagation phase (Φ_DP), correlation coefficient between horizontal and vertical polarization signals (ρ_HV), and specific differential phase (K_DP). To clarify the relationship between these polarimetric parameters and characteristics of solid hydrometeors, we installed the radar at Ishikawa Prefectural University in the winter season of 2016-2017. We also installed several instruments such as 2DVD, PARSIVEL, LPM, and Ground-based Particle Image and Mass Measurement System (G-PIMMS developed in Yamaguchi University) to observe characteristics of hydrometeors at Kanazawa University located within the observation range of the radar. These instruments can obtain the shape, size, number, and fall speed of hydrometeors. The radar observation was continued from December 19, 2016 to March 2, 2017.

During the observation period, three remarkable snowfall events were observed from January 14 to 15, from January 22 to 25, and from February 11 to 14. We analyze the snowfall event observed on January 24 when accumulated snowfall of 12 cm in a day is observed at Kanazawa Local Meteorological Observatory by a cold-air outbreak.

Periods of the graupel-dominant, the snowflake-dominant, the mixture of graupel and snowflake, and the crystal-dominant (less number of graupel particles and snowflakes) are evaluated using the size-fall speed plots obtained by the PARSIVEL every 10 minutes. For example, it is categorized as the period of the mixture of graupel and snowflake from 12:20 to 12:30 Japan Standard Time (JST) on January 24. During the period, large number of ice crystals (dendrites and needles) and graupel embryos with their diameter of 0.5 mm, spherical graupel particles with that of 1 - 2 mm, and snowflakes with that of 2-5 mm are confirmed by images obtained by the G-PIMMS. The observed ranges of Z_H and K_DP, are 10~25 dBZe and -0.2 - 0.6 deg./km, respectively. Greater Z_H is expected to the characteristics of snowflakes and spherical graupel particles, and positive K_DP should correspond to the existence of numerous ice crystals (dendrites and needles). It is categorized as the snowflake-dominant period from 12:50 to 13:00 JST. Few snowflakes and moderate number of ice crystals are confirmed by the G-PIMMS. The observed ranges of Z_H and K_DP, are 0 - 10 dBZe and 0.0 - 1.0 deg./km, respectively. Lesser Z_H is expected to the characteristics of lesser number of snowflakes without graupel particles. During the almost analyzed period, no significant Z_DR difference is confirmed and ρ_HV has greater than 0.98 (close to 1.0). High ρ_HV close to 1.0 corresponds to that obtained by X-band polarimetric radars in snow clouds, thus characteristics of particles are homogeneous.