Development of ambient noise measurement technique and its applications on Argo floats over the past two decades

The distinctive underwater sound generated by raindrops on the ocean surface has been used to detect and quantify rainfall. Knowledge of the intensity and spatial–temporal distribution of rainfall over the ocean is critical in understanding the global hydrological cycle. However, rainfall is difficult to measure accurately over the ocean due to its spatial and temporal variability. To reduce these problems, satellite-based rain-monitoring instruments are used but they do not capture the full in situ temporal and spatial variability. The Passive Aquatic Listener (PAL) was developed at the University of Washington’s Applied Physics Laboratory (Nystuen, J. Acoust. Soc. 79, 92–98). PAL has been incorporated into Argo floats and deployed over global oceans. PAL-Argos are capable of telemetering back estimated rain rate and wind speed with a temporal resolution of 2–8 min, representing a circular surface footprint of a few kilometer radius (Yang et al., Oceanography 28, 124–133). In this work, the major operations of PAL-Argos in NASA’s Aquaris Satellite Mission, SPURS field efforts related to the Tropical Rainfall Measuring Mission, NOAA’s Tropical Pacific Observing System Initiative, and NSF’s Measurements and Modelling of the Indonesian Throughflow are chronicled with selective field data presented. [Work supported by NASA, NOAA, and NSF.]