Development of a New Balloon-Borne Particle Imaging Radiosonde and First Flight into a Convective Cloud

Special radiosondes that take magnified video images of precipitation particles had been developed in the late 1980s to understand cloud microphysics. They were able to downlink the images of precipitation particles in real-time by using 1680 MHz carrier radio wave to the ground station. Videosonde developed by Takahashi (1990) can capture precipitation particle images in the air without contact to the particles and can provide information on the size and shape of precipitation particles such as raindrops, snowflakes and graupel. The videosonde has been used as a strong tool for understanding cloud microphysics. In the 2000s, with the progress of remote sensing technology such as polarimetric radars and the progress of cloud-resolving models, these instruments were started to be used also as a reference instrument (Nakakita et al. 2009; Suzuki et al. 2012, etc.).
Conventional videosonde using the analog video transmission had a disadvantage of image degradation when the videosonde is far away from the receiving antenna. Therefore, using a new transmission system with a digital image compression technology, a new balloon-borne particle imaging radiosonde has been developed, which the carrier frequency of these instruments is changed from 1680 MHz band to 400 MHz band. This new instrument is no longer “video“ sonde, so we named it “Rainscope”. The development of the Rainscope has made it possible to acquire images that clearly recognize the outlines and irregularities of ice particles, as well as their state of aggregation and melting.
In addition, Rainscope was designed to measure the terminal velocity of a particle in the cloud by the equipped two infrared sensors measuring the passing time. Vertical profile measurements of the fall velocity of precipitation particles can contribute to understanding microphysical processes within clouds and provide validation data for cloud resolution models. For the ground test, measurements of rain and snow by a ground-based Rainscope showed a raindrop size-fall velocity distribution similar to that of Altas et al. (1977). They were also in good agreement with the distributions obtained by Parsivel placed next to the Rainscope. In the test flight of the Rainscope, raindrops were observed in the lower layers, and mostly melted particles, snowflakes in the process of melting, graupel, and snowflakes were observed above the freezing level. When multiple particles pass through the infrared sensor in succession, it is not possible to measure the fall velocity. However, even with a small number of samples, it was observed that the fall velocity varied depending on the type of precipitation particles.
The Rainscope was first deployed in the intensive observation campaign, which was conducted in Kyushu region in Japan during the rainy season in 2022. It was launched into convective clouds with active lightning and gust on 25 June 2022. It transmitted images of ice particle in the process of melting just below 0ºC, and frozen particles with semi-transparent and smooth outlines around 0ºC. And white and irregularly shaped graupel were observed across all layers up to -30ºC level. The clear particle images captured by the Rainscope enable us to get more detailed information of particle shapes, surface conditions, and contours, making it easier to evaluate their shapes quantitatively. The circularity of graupel was smaller in upper layer. It has a longer circumference and more irregular shapes, suggesting an active riming process originated from ice crystals. On the other hand, graupel in lower layer with larger circularity was suggested to be originated from a frozen particle. The different graupel formation processes was considered to exist in convective clouds.