Demonstration of the Performance Characteristics of the Rain Imaging System (RIS)

Clemson University, Clemson SC

Investigators

Abstract

A recently developed instrument called the Rain Imaging System (RIS) offers promise of an improved capability of measuring the size, shape, and spatial concentration of raindrops. This information is critical for precipitation physics and certain applications in atmospheric remote sensing. Developed by NASA, RIS consists basically of a halogen light source and a CCD camera separated by a distance of 4 m. It records the forward-scattering image of individual drops as they fall through the scattering volume using a very short exposure time to "freeze" the drops in position and eliminate smearing of the image. Drops are counted and measured as they fall in the focal region of the camera. The size of the depth of field is crucial, because it determines the volume in which drops are measured. Drops within the focal region are identifiable as dark silhouettes with the light source appearing as a "hole" within them. Drops outside the focal region do not have the hole. Knowing the distance of the focal region from the camera enables determination of drop size from the image size. In a typical setup, the system has the camera lens focused at a distance of 2 m with an aperture of f/8. With these settings the camera images a region nominally 32 mm x 24 mm in cross section with a 175 mm depth of field. The purpose of this project is to evaluate and quantify the performance of RIS using laboratory-generated water drops of known size dropped through different parts of the focal volume. The experiments will determine (1) the smallest drop measurable by RIS and its dependence on position in the focal region; (2) the 3-D shape of the focal region and its possible dependence on drop size and position; (3) the accuracy with which RIS can measure drop size and shape. Answers to these questions will document the performance of RIS and enable the improvement of data processing algorithms. The work will be carried out at Clemson University in collaboration with Dr. Larry Bliven of NASA, who originally developed the instrument.