The sound of rainfall at sea

Abstract

Knowledge of the distribution of oceanic rainfall is needed to more fully describe the marine ecosystem and to provide constraints for climate investigations and for process studies of air–sea interaction. Bubbles generated by raindrop splashes produce a loud and distinctive sound underwater. This sound is a signal that allows the detection and measurement of rainfall at sea. Different raindrop sizes have unique splash characteristics, producing distinctive acoustic signatures for at least four raindrop sizes. These signatures form a mathematical basis for the decomposition of the sound field to measure the drop size distribution in the rain. This information allows the acoustic identification of rainfall type and rainfall rate. The acoustic detection and measurement of rainfall at sea is demonstrated using over 100 buoy-months of data from ocean surface moorings in the tropical Pacific Ocean. This technology is now being transferred to sub-surface instrument platforms. By acoustically monitoring rainfall, and other surface processes including wind speed, from below the surface, many of the fouling, physical damage and vandalism problems that affect surface instruments will be avoided. This technology is passive, introducing no acoustic disturbance into the environment and thus poses no potential harm to marine mammals or other forms of life in the ocean.