Particle imaging auto-measurement system for microphysical characteristics of raindrops in natural rain

In order to explore microphysical properties of raindrops, light field and imaging system were designed based on the particle imaging measurement technology. Meanwhile, the software for image identification, extraction and calculation were investigated, and then a raindrop measurement system combined with a charge coupled device sensor was developed. The repeatability and accuracy of steel ball estimation were checked in diameter and fall velocity and yielded low standard deviations and small relative errors. The results demonstrate that the system provides a reliable detection of raindrop size and fall velocity. Additionally, the designed system was operated for in situ measurement of natural rain. Raindrop diameter and fall velocity were, on average, 1.02 mm and 3.45 m s(-1), respectively, and small-sized particles were the predominant contributor to the natural rainfall. Notably, raindrop fall velocities were slightly higher than the corresponding terminal velocities calculated by classical models under the standard sea-level conditions. In order to identify completely the features and variations of raindrops, it is essential to implement long-term measurements and take into account the driving factors such as air motions, raindrop breakup or coalescence. This work will be pursued soon in coming future.