Microphysical Characteristics of Warm Convective Precipitation in Tokyo

It is generally considered that warm rain is less likely to occur in urban areas where the air is polluted. However, heavy precipitation from shallow convective clouds is occasionally reported in Tokyo. In this study, we observed microphysical characteristics of warm convective precipitation in Tokyo on 19-20 August 2019 using an X-band polarimetric radar, a Ka-band radar, a cloud droplet spectrometer and an optical disdrometer. The radar reflectivity and the specific differential phase from the X-band radar tended to increase in the lower layers, suggesting accretion growth of raindrops. On the other hand, the differential reflectivity decreased in the lower layers, suggesting the presence of low concentrations of large raindrops near the echo top. According to range height indicators, precipitation clouds were composed of streak-like echoes. The Z-R relationship on the ground was close to that of the Marshall-Palmer raindrop size distribution. Mean cloud number concentration (N_c) was 370 cm⁻³, which was larger than the average of low-level clouds in Tokyo (213 cm⁻³). Parcel model simulations suggested that warm rain could be initiated when N_c < 1200 cm⁻³, although the threshold of N_c depends on the cloud base temperature.