Droplet Spectrum Analysis from Artificially Generated Rain Showers

Abstract

Severe droughts were experienced by many southerly states of the Indian subcontinent during the past decade. Farmer suicides were reported from the state of Andhra Pradesh year after year, especially when the monsoons failed. The government of India conducted expensive rain enhancement experiments through artificial cloud seeding; this did happen, but at enormous infrastructural costs and organizational endeavors. In this paper, the attempt is to find alternative shower generation mechanisms at the scale of an individual cropland. The application of a new rain simulator is explored. The crux of the problem lies in generating showers with drops that not only fall with approximately the same speeds as the natural rain showers but also have matching drop size distributions. Previous simulators have not looked at this aspect; none have attempted to recreate a drop size spectrum that is indistinguishable from natural rain showers. This is the paper’s mainstay; new calculations are shown for a uniquely fabricated rain simulator that allows tridirectional maneuverability with regard to the spraying geometry. Only when the nozzle placed at 2.5 m from the ground is tilted at an upward angle of 80° with respect to the boom’s horizontal axis, a resultant spectrum is obtained that closely matches the northeast monsoon shower spectra at optimal crosswind speeds. In contrast, when the spray nozzles point vertically downwards, as in sprinklers, the resultant droplet spectra do not match real rain shower spectra; the number concentrations of both large and small droplets are overpredicted with fall velocities grossly higher than their terminal velocities. The droplet energy flux obtained from a vertically downward tilted nozzle reach the highest value of