INTRODUCTION
Currently, a weak link in atmospheric and oceanic modeling are the coupled air-sea models.  Part of the problem is identifying appropriate parameterizations of the crucial air-sea fluxes of heat, fresh water and momentum which dynamically drive the models.  In order to develop and verify coupled atmosphere-ocean models, long-term observations of these fluxes are needed.  This need has been clearly identified, and is one of the program priorities for PACS and EPIC (Eastern Pacific Investigation of Climate).  The ambient sound field offers a means to make these measurements as the processes associated with precipitation and wave breaking are the principal sources and modifiers of underwater sound in the frequency band from 500-50,000 Hz.  Specifically, the ambient sound field provides quantitative measurements of rainfall, rainfall type (heavy convective rain versus stratiform drizzle), wind speed (± 1 m/s) and ambient bubble populations (near-surface void fraction).  The ambient sound measurement can be made from ocean moorings and will compliment measurements made from more traditional sensors on such moorings.

Of particular interest is the measurement of precipitation, including its detection, the identification of rainfall type and quantification. The hydrological cycle of the upper ocean layer is an important part of mixing, both lateral and vertical. The hydrological cycle in oceanic regions is particularly poorly sampled because of the difficulty of obtaining salinity and precipitation measurements. Rainfall is also a major component of the latent heat exchange between the atmosphere and the oceans.  The distribution, type and intensity of rainfall is part of the engine that helps to drive atmospheric circulation.  The type of rainfall, convective or stratiform, is important because different types of rain have different heating profiles in the atmosphere.  Thus, careful measurements of oceanic rainfall, including rainfall type, are needed to help understand the evolution of atmospheric convection.

Rainfall is also responsible for a unique underwater acoustic signal easily distinguished from other common sound sources (breaking waves, biology, etc.) and, furthermore, the sound levels produced by rain are much louder, by orders of magnitude, than these other sources.  This allows detection and measurement of rainfall at sea.  Different types of rainfall (convective heavy rainfall versus stratiform drizzles) are comprised of different distributions of raindrop sizes. Laboratory studies of the physics of individual realistic raindrops have identified unique sound generation mechanisms associated with different raindrop sizes.  The measured underwater sound field can be decomposed into these unique sounds, allowing quantitative estimations of the drop size distributions within the rain.  From this information different types of rainfall can be identified and rainfall rate can be quantified.  When it is not raining, the underwater ambient sound field can be used to measure wind speed.

GOAL:
1) Obtain long-term measurements of air-sea fluxes needed for input into coupled air-sea models, especially precipitation.

2) Use these data to establish rainfall statistics at the mooring locations, and to provide flux data for process studies involving precipitation.

METHODOLOGY:
To take advantage of the underwater acoustic signal, Acoustic Rain Gauges (ARGs) have been designed and built at the Applied Physics Laboratory.  These instruments are designed to be attached to oceanic moorings.  They record the sound field every few minutes, and change sampling rates if rainfall is detected.  The ARGs are designed to operate unattended for up to one year.

The eastern tropical Pacific has been identified as the focus region for the PACS and EPIC measurement programs.  Instrumented ocean moorings are part of this phase of the PACS program.  In particular, NOAA PMEL has augmented the Tropical Atmosphere Ocean (TAO) array along 95°W with additional sensors, and Anderson/Weller (WHOI) are planning to place a long-term ocean/atmosphere (IMET) mooring in the stratus deck region of the eastern Pacific south of the Equator (at 18°S 85°W, west of Peru and Chile).  These are regions of very different precipitation climatologies.  North of the Equator, the ITCZ migrates seasonally from roughly 5-12°N, while south of the Equator persistent stratus cloud decks are present west of Peru and Chile.  ARGs are part of these mooring deployments.

RESULTS and ACCOMPLISHMENTS:
1)  Ten new ARGs were built at the Applied Physics Laboratory.

2) Three ARGs were deployed on TAO moorings at 8°, 10° and 12° N, 95° W  in December 1999 and were recovered in April 2000.  New ARGs were set on the TAO  moorings at 5°, 8°, 10° and 12° N, 95° W and will be recovered and replaced during servicing of the moorings.  Data processing has just begun.

3) An ARG has been sent to WHOI to be part of the mooring planned for 18°S, 85°W starting in October 2000.  Data will be recovered in October 2001.

4) A paper reporting precipitation measurements collected by ARGs on an ocean mooring has been prepared and accepted for publication.  This paper compares the acoustic rainfall measurements collected during the South China Sea Monsoon Experiment (SCSMEX) using a TAO-style ATLAS mooring.  The methodology reported therein is similar to the data expected to be collected during this project.

PUBLICATIONS RESULTING FROM THIS WORK:
2000 Nystuen, J.A., M.J. McPhaden and H.P. Freitag,  “Surface Measurements of Precipitation from an Ocean Mooring:  The Underwater Acoustic Log from the South China Sea”, accepted by J. Applied Meteor. (in press).

RELATED PUBLICATIONS:
2000 Nystuen, J.A., “Listening to Raindrops from Underwater:  An Acoustic Disdrometer”, submitted to J. Atmos. and Oceanic Tech. July 2000.

1999 Nystuen, J.A., “Listening to Raindrops”, Solstice 10, Number 2,  http://www.csfnet.org/image/solstice/win99/janystuen/listen.html

CONTACTS:
Dr. Jeffrey A. Nystuen
Applied Physics Laboratory
College of Ocean and Fisheries Sciences
University of Washington
Seattle, Washington  98105
email:  nystuen@apl.washington.edu

Return to CLIVAR PACS Home Page