A New Device for Continuous, Real-Time Acoustic Measurement of Rain Inclination

Driving rain or ‘wind-driven rain’ (WDR) arrives at the ground on an oblique trajectory, and drops may strike at a speed greater than their still-air terminal velocity. Oblique rain can affect a range of geomorphic processes including the splash dislodgment and transport of soil particles, and hydrological processes including overland flow, canopy interception and the generation of stemflow. The mean rain inclination angle at which WDR strikes the ground has been estimated from the catch of paired gauges, one with a conventional horizontal orifice, and one with a vertical orifice. Such data allow the resolution of rain vectors to find the rain inclination. This can only be carried out over periods sufficiently long for a measurable rain depth to be measured, and does not permit the real-time recording of rain inclination. Here, a new acoustic method for measuring rain inclination is introduced that provides an inexpensive tool for the continuous, real-time monitoring of WDR. Furthermore, the method also permits the simultaneous recording of rainfall duration and intermittency at high temporal resolution, with no additional apparatus. Data on rain inclinations collected during showers on a tropical coast exposed to strong trade-winds are presented to illustrate the operation of the acoustic measurement system. However, the focus of this paper is the presentation of the new method itself, and not on the climatology of WDR.