1 MEASUREMENTS OF SPATIALLY AVERAGED OCEAN SURFACE GEOPHYSICAL PARAMETERS USING UNDERWATER SOUND INTEGRATED WITH THE POSEIDON MONITORING SYSTEM M. N. Anagnostou a , J. Nystuen b , E. N. Anagnostou a,c , A. Papadopoulos a , D. Ballas a , K. Nittis a , P. Pagonis a , L. Perivoliotis a , and D. Georgopoulos a a Hellenic Center for Marine Research, 19013 Anavissos, Greece. b Applied Physics Laboratory, University of Washington, Seattle, WT, USA. c Corresponding Author: Hellenic Center for Marine Research, Institute of Inland Waters, 46.7 km Athinon Souniou Ave., P.O. Box 712, GR-19013, Anavissos Attikis, Greece, tel.: +30 22910 76399 – Fax: +30 22910 76323, email: managnostou@ath.hcmr.gr University of Connecticut, Department of Civil and Environmental, Storrs, CT, USA. Abstract: One of the most challenge and difficult atmospheric parameters to measure is rain, especially over the oceans, because of its spatial and temporal variability. Since, the last decades, many satellites have been lunched to observe globally the rain variability; however, they have large scale spatial and temporal resolution. Also, there are few surface validation sites over the oceans for calibrating/validating their retrievals. Even though, ocean plays a vital role in the hydrological cycle, till now air-sea feedback mechanisms are not properly resolved by numerical models. Therefore, it is significant to have geophysical (precipitation, wind speed, wind direction, etc.) observations over the oceans that can be used to initialize forecasting models, to improve their numerical parameterisation schemes, and validate satellite retrieval algorithms. Operational monitoring systems provide in real time this required information. However, rain and wind speed observations using raingauges and anemometers installed on buoys are point measurements and may introduce erroneous values in the measurements. A Passive Aquatic Listener (PAL) integrated for first time with an operational oceanographic monitoring (Poseidon) system, in the southwest Ionian Sea buoy. Acoustical measurements of rainfall and wind speed are reported from 500 meters depth, since November 2008. These spatial averaged measurements are compared with the buoy’s surface anemometer and the Hellenic National Meteorological Service C-band radar rainfall measurements. Keywords: Underwater acoustics, rainfall estimation, wind speed measurements, operational oceanography.