Rainfall Microphysical Observations Using the High-Speed Optical Disdrometer
Rainfall microphysical observations are conducted using the High-Speed Optical Disdrometer (HOD). The main objective of this study is to observe microphysical properties of raindrops such as drop shape, fall velocity/ acceleration, drop size distribution, oscillation, and collision using the HOD. The HOD is a new optical disdrometer that consists of a state of the art camera with high-speed video recording capability, an LED light, and a sensor unit. The High-speed camera points at the LED light and captures the backlit silhouettes of the particles. The sensor unit sends a digital signal to the camera when a drop passes through the sensor unit. The digital signal of the sensor unit triggers the camera and camera capture a pre-defined number of frames of the drop. The raindrop images are then digitally processed using an image processing software built in-house. The captured raindrop from the sequential images provides a wide range of raindrop microphysical information such as drop shape, oscillation, collision, and fall speed. The HOD was laboratory tested with high-precision spherical lenses of different sizes to ascertain the accuracy of the image-processing algorithm. In addition, water drops were generated in the laboratory using needles of different sizes attached at the bottom of a constant head tank. The drops were then freely fallen through the measurement volume of the HOD, which confirms the proficiency of the sensor system around the focal plane of the high-speed camera. The HOD was field tested at different geographic locations, and its measurement capabilities were evaluated by comparisons with measurements from a collocated commercial disdrometer and rain gauges. iv These evaluations confirmed the research readiness of this new optical disdrometer. Using the HOD, binary raindrop collisions were observed during the field experiments for the first time. However, no spontaneous breakup was observed. These observations of binary raindrop collisions provide a strong support for the postulated governing role of raindrop collisions in defining DSD shape evolution.