The sound produced by rainfall underwater can be used to quantitatively measure rainfall at sea. The Ionian Sea Rainfall Experiment (ISREX) and the operational POSEIDON project are unique data sets that combine deep underwater acoustic measurements with high resolution radar and can be used to evaluate the performance of the acoustic measurement. Acoustic rainfall rate accuracy will be estimated using disdrometer-calibrated and gauge adjusted high-resolution X-band radar (XPOL) data from the Ionian Sea Rainfall Experiment (ISREX) and subsequently evaluated using operational PAL/ground-radar/buoy-anemometer (POSEIDON buoy and Hellenic Meteorological weather radar networks - Aegean and Ionian Seas) and archived PAL/rain gauge data (ITCZ - Pacific Ocean). The acoustic classification of rainfall will be evaluated in comparison with radar estimated fields of rainfall classification. A model incorporating the influence of the spatial and temporal scale storm structure on the acoustic signal will be developed and tested using ISREX and Poseidon data. Finally, the performance of acoustic data in retrieving raindrop size distributions (rainfall DSDs) will be evaluated through comparisons with dual-polarization radar DSD retrievals from ISREX and the operational radar measurements over the POSEIDON buoy in the north Aegean Sea.
Understanding how to use passive ambient sound to monitor the marine environment is a fundamental tool that will contribute to our ability to monitor important climate processes globally. In particular, rainfall distribution is an important component of the global water cycle and has a strong acoustic signal on ocean surface constituting ~75% of global rainfall. Exploring the acoustic signal of rainfall is needed to use it for quantitative estimation of rain including presence, classification, accumulation and raindrop size distributions.
Improving our ability to measure rain over the oceans is critical to describing global water cycle and understanding potential trends in climate change, and for validating estimates by global coverage satellite sensor. Monitoring and predicting climate change has huge broader impacts on human society. Utilizing the underwater acoustic signal from rain will allow rainfall climatology to be measured in many remote or severe weather regions (Atlantic and Pacific ITCZ, North Atlantic, etc.). Rainfall makes only one of the acoustic signals that are part of the marine ambient sound budget. Other physical processes that can be measured are wind and sea state (bubbles) conditions. A more complete understanding of the acoustic signal will allow all of these processes (rain, wind and bubbles) to be measured from sub-surface platforms allowing all weather, all season data collection. These include many new ocean instrumentation platforms including sub-surface moorings, cabled networks, drifters, profilers, gliders, etc. Passive acoustic monitoring for marine mammals, especially whales, is critical to ecological studies of these animals. Finally, the impact of human-generated sound on the marine environment is of growing concern. A collateral result of this work will be to better monitor the marine sound budget to provide fundamental baseline data to allow informed decisions regarding management of sound-producing human activities in the ocean.
