The HYDROP experiment: an empirical method for the determination of the continuum limit in rain

We describe the HYdrometeor Detection and Ranging using stereO-Photography (HYDROP) experiment, which is the first designed to measure the position and size of hydrometeors in a sufficiently large volume so that the continuum limit (large number of particles) can be examined directly. The experiment is instrumented by three medium-size cameras, two high-powered flash lamps and custom-made reflectors. The hydrometeors in approximately 500 scenes were photographed; six triplets of rain were digitized and analyzed in this study. Some details of the image processing and raindrop position and size reconstructions are outlined. We made the first determination of the sizes and positions of raindrops (diameters > 0.2 mm) in a significant volume (approximately 10 m(3)). Preliminary estimates of the errors involved in the reconstructions yield position uncertainties of 3 mm in the directions perpendicular to the field of view., 3 cm along the field of view, as well as 0.5-mm-diameter uncertainty. We perform some preliminary analysis of six three-dimensional reconstructions-with 11 000 - 22 000 raindrops each and with nominal rain rates in the range of 8-15 mm/h. Since the mean interparticle distance was 7-8 cm and the largest sphere of observation has a radius of the order 1 m, our range of scales is roughly a factor of 10. (C) 2001 Elsevier Science B.V. All rights reserved.