Hurricane Irene: a case study for the use of underwater sound for the monitoring of mesoscale convective systems occurring in the ocean
John Proni, NOAA/AOML, Miami, FL; and J. Wilkerson
In October 1999 Hurricane Irene passed over south Florida with winds extending to include Andros Island in the Bahamas. The hurricane occurred while underwater sound recordings were being made at the US Navy Atlantic Undersea Test and Evaluation Center (AUTEC), adjacent to Andros Island, using a 12 hydrophone sound monitoring array located in area known as the Tongue of the Ocean (TOTO). The array was bottom-mounted at a depth of approximately 1.5 kilometers and recorded sound in the range extending from a few hundred cycles per second to fifty thousand cycles per second. Three air-sea interaction events engendered by three mesoscale convective systems (MCS) passing over the array were observed and underwater sound spectra recorded. Limited radar information on the systems was available from the NEXRAD radar facility located in Miami Florida, approximately 334 km from the observation range in AUTEC. Each of the hydrophones has a reception area for sound centered above the hydrophone such that approximately 95 % of the sound is received within a range of 7 km on the ocean's surface from the hydrophone. The MCS descriptions and evolution provided by Houze (Cloud Dynamics 1993) were used to help interpret the sound data obtained.
Three dominant underwater sound spectral shapes were observed: (1) the well-known shape associated with a steady wind field, (2) that associated with convective rainfall and (3) that associated with stratiform rainfall. Each MCS Passed over the axis of the array so that the level of development of the system and its speed of translation were discerned. The direction of translation of the systems appeared to be dominated by the (weak) wind field associated with the hurricane. The underwater sound spectra indicate that the more mature MCS affects the level of ambient sound production from wind after the passage of the rain fall component of the MCS, while the lesser developed systems do not as strongly affect the ambient sound field after passage. It will shown that two ambient sound spectra amplitudes are generally sufficient to evaluate the degree of evolution of the MCS and that the underwater sound from the MCS characterized by these two parameters forms a curve in the space defined by these two parameters and time. If the ambient wind field is steady before and after the passage of the MCS the curve is closed, if it is not, the curve is displaced by an amount proportional to the change in the wind field. Using the curve referred to above, a suggestion for a “boundary region “approach for the study and estimation of some MCS characteristics in the tropical ocean is suggested. This boundary approach contains a potential method of rainfall characterization utilizing observations along oceanic regional boundaries and would be complimentary to satellite-borne rain and wind sensor system observations.
Session 5B, Ocean Observations for Data Assimilation and Climate—II
Tuesday, 13 January 2009, 11:00 AM-12:00 PM, Room 131C
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