Clear-Air Echoes Observed by Ka-band Polarimetric Cloud Radar: A Case Study on Insect Echoes in the Tokyo Metropolitan Area, Japan

In this study, the polarimetric variables of clear-air echoes (CAEs), that appeared on May 21, 2016 in the Tokyo metropolitan area, Japan, were investigated using the Ka-band (8.6-mm-wavelength) polarimetric cloud radar capable of detecting non-precipitating clouds. The objective was to establish the potential for distinguishing CAEs and hydrometeor echoes in the initial stage of cloud formation using a Ka-band polarimetric cloud radar. On the day being studied, CAEs showed evident diurnal variation. There were no CAEs before sunrise. The equivalent radar reflectivity (Ze) increased with time after sunrise, and horizontally widespread echoes (max. value > −15 dBZ) occurred within the radar observation range in the early afternoon. After sunset and into the early part of the night, Ze decreased rapidly. Range-height indicator observations showed that CAEs were restricted to heights of < 1.5 km. The differential reflectivity (ZDR) values of the CAEs were largely positive (1.8 dB) with a large standard deviation at 18:00 local time, i.e., considerably larger than those of cloud/weak precipitation echoes (0.4 dB) observed simultaneously. In comparison with cloud/precipitation echoes, the copolar correlation coefficient (ρhv) of the CAEs was smaller (< 0.9), whereas the variability of the total differential phase (ΨDP) in the range direction was larger. The upper limit of Ze and the distributions of ZDR and ρhv were inconsistent with the characteristics of the Bragg scattering observed by the S-band (10-cm-wavelength) radar in previous studies. However, the larger ZDR, smaller ρhv, and larger variability of ΨDP in the range direction, associated with the horizontally widespread echoes, were consistent with the characteristics of insect echoes. The depolarization ratio defined using ZDR and ρhv could be effective in distinguishing this type of CAE and hydrometeor echoes observed by Ka-band polarimetric cloud radar. The polarimetric variables obtained by Ka-band polarimetric cloud radar are useful in distinguishing between CAEs and hydrometeor echoes.