A video precipitation sensor for imaging and velocimetry of hydrometeors

Abstract. A new method to determine the shape and fall velocity of hydrometeors by using a single CCD camera is proposed in this paper, and a prototype of a video precipitation sensor (VPS) is developed. The instrument consists of an optical unit (collimated light source with multi-mode fibre cluster), an imaging unit (planar array CCD sensor), an acquisition and control unit, and a data processing unit. The cylindrical space between the optical unit and imaging unit is sampling volume (300 mm × 40 mm × 30 mm). As the precipitation particles fall through the sampling volume, the CCD camera exposes twice in a single frame, which allows the double exposure of particles images to be obtained. The size and shape can be obtained by the images of particles; the fall velocity can be calculated by particle displacement in the double-exposure image and interval time; the drop size distribution and velocity distribution, precipitation intensity, and accumulated precipitation amount can be calculated by time integration. The innovation of VPS is that the shape, size, and velocity of precipitation particles can be measured by only one planar array CCD sensor, which can address the disadvantages of a linear scan CCD disdrometer and an impact disdrometer. Field measurements of rainfall demonstrate the VPS's capability to measure micro-physical properties of single particles and integral parameters of precipitation.