Project title and acronym

Underwater Sound and Radon Measurements of Rainfall and Wind at Sea

Host facility

Western 1 – Mediterranean Moored Multi-sensor Array (W1 – M3A)

Modality of Access

MoA2 – Partially remote (the presence of the user is required at some stage, e.g. for installing and uninstalling an instrument)


Understanding the distribution and change of oceanic wind fields and rainfall patterns is a major component of global/regional water cycle and climate change. In addition to rainfall rate a detailed knowledge of type and drop size distribution (DSD) is needed for particular scientific applications and this characterization is extremely important in area subjected to flood hazards. Satellite-based measurements provide global coverage of wind and rainfall distribution, but these measurements need to be verified by surface measurements.

The most common instruments used to measure rainfall are rain gauges, which however represent a point measurement. On the other hand,  a  more  advanced  technique  for quantifying rainfall in larger spatial and temporal scales is the meteorological radar. Radar observations can support hydro-meteorological and flood forecasting modelling due to the distributed rainfall estimation. Typically, both instruments are used on land and limited are the types of rain gauges used to measure precipitation over the oceans. At sea, they are usually installed on moored buoys and even though extended research has focused on removing errors caused by the movement of the platform due to the sea state conditions, they are still not reliable when installed on small and discuss-shaped buoys.

Underwater acoustics can contribute to improve our capability of measuring rain over the oceans since rain falling onto water and the break of the surface waves (strongly correlated to wind blowing over the sea) are two of the loudest sources of underwater sound. They produce sound underwater by their impacts onto the ocean surface and, more importantly, by sound radiation from any bubbles trapped underwater during their splashes. In addition, because different raindrop sizes produce distinctive sounds, the underwater sound can be inverted to quantitatively measure drop size distribution in the rain.

Consequently, underwater acoustic measurements can be used to detect and classify rainfall type and subsequently quantify the wind speed, rainfall rate and its drop size distributions. Other physical processes that can be measured are sea state (bubbles) conditions as well as sounds generated from biological and human activities. Developing and verifying an advanced real-time system for recording and interpreting the underwater acoustic signal will allow all of these  processes  (rain,  wind  and  bubbles)  to  be  measured  from  sub-surface  platforms, facilitating all weather and all season data collection. A collateral result of this project 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. On the other hand, radon progenies in the atmosphere are transported to the sea surface by the scavenging effects of rainfall. Radon can be detected via its daughters (214Bi and 214Pb) which are gamma-ray emitters. The continuous monitoring of gamma radiation in the marine environment provides significant information on various environmental processes where radon and thoron can be applied as a tracers. 222Rn (half life 3.825 d) is a noble gas and is found in aerosol particles in accumulation-mode. It has been observed qualitatively after rainfall from the short-lived radon daughters (214Bi and 214Pb). However, the variation of radon activity is not constant mainly due to rainfall intensity, rainfall type and humidity. It has been measured (using the lab-based method) that the volumetric activity of radon decay products in rainwater amounts up to 105 Bq/l.

This phenomenon causes the environmental gamma ray intensity at the sea surface to increase significantly, anywhere from several to tens of percentage points of intensity compared to dry conditions. The study of radon progenies is necessary in order to correctly assess rates of precipitation. Furthermore, the radon progenies in rainwater are useful when studying the atmospheric scavenging of harmful substances and aerosols because these progenies behave as tracers that reveal the dynamics of the process. The vision of the proposed technology is to apply effectively autonomous, robust, low power consumption and cost-effective in-situ sensors for real-time measurements of rainfall tracers and to correlate the results with wind speed monitoring data over ocean.