Development of a New Balloon-Borne Sensor Attached to a 400-MHz Radiosonde for Precipitation Particle Electric Charge Measurement

It is well known that precipitation particles in clouds, especially solid particles, play important role for the electrification. Takahashi (1978, 1984) proposed riming electrification as a charge separation mechanism. At about -10ºC, negatively charged ice crystals are carried aloft in an updraft and combine with negatively charged graupel falling from above, which associated with increasing precipitation (Takahashi et al. 1999). A balloon-borne instrument “videosonde” was developed by Takahashi (1990) for the better understandings of the precipitation mechanisms and the electric charge distribution in clouds. The system has not only a stroboscopic illumination that provides information on particle size and shape but also an induction ring to measure the electric charge of the particles. In the previous studies, several hundred videosondes launchings (Takahashi et al. 1999; Takahashi and Suzuki 2010; Takahashi et al. 2017, etc.) provided valuable results to contribute to the lightning study. However, the electric charge measurement by the videosonde has some disadvantages. Because the videosonde could transmit only the video signal by a 1680 MHz carrier wave, the charge sign and magnitude of particles were displayed on a series of LEDs inside the field of view of the CCD camera. Therefore, it takes a lot of time for the analysis. In addition, the videosonde was too big and expensive to use in the field experiment easily.

In-situ electric charge measurement, which is real-time and user-friendly monitoring system, is essential for the understanding of electrification in clouds and the validation of the numerical models including the lightning processes. In this study, a new balloon-borne EC sensor attached to the 400-MHz radiosonde for the measurement of the precipitation particle electric charge is being developed. The EC sensor has an induction ring inside. The induction ring leaves the same performance as the videosonde that was developed by Takahashi (1990) in 1980’s, and the signal control unit is improved from the cloud particle sensor (CPS; Fujiwara et al. 2016). A charged particle falls through the inlet, and then an induction ring detects a pulse. By the wave pattern processing algorithm, information of electric charge (positive/negative) for first three particles in one second and integrated electric charge (positive/negative) for one second measured by EC sensor is transmitted every second by the 400-MHz radiosonde, and then is monitored in real-time.

On June 2, 2019, test flights were carried out in Okinawa during the Baiu rainy season. The new sensors were launched into precipitating clouds with active lightning. The results showed that data processing and transmitting well worked. However, below the freezing level, excessive count of charged particle was suggested. Moreover, the distribution in the upper layer was inclined toward “negative” due to the expected mirror image effect associated with the sensor housing problem. These problems will be improved in the next prototype. On the next development stage, simultaneous flights of new EC sensor and Takahashi’s videosonde will be carried out in the near future. By the end of 2020, we will succeed in commercial product for practical use.