The Sound of Tropical Cyclones

Abstract

Underwater ambient sound levels beneath tropical cyclones were measured using hydrophones onboard Lagrangian floats, which were air deployed in the paths of Hurricane Gustav (2008) and Typhoons Megi (2010) and Fanapi (2010. The sound levels at 40 Hz-50 kHz from 1-to 50-m depth were measured at wind speeds up to 45 m s 21 . The measurements reveal a complex dependence of the sound level on wind speed due to the competing effects of sound generation by breaking wind waves and sound attenuation by quiescent bubbles. Sound level increases monotonically with increasing wind speed only for low frequencies (,200 Hz). At higher frequencies (.200 Hz), sound level first increases and then decreases with increasing wind speed. There is a wind speed that produces a maximum sound level for each frequency; the wind speed of the maximum sound level decreases with frequency. Sound level at .20 kHz mostly decreases with wind speed over the wind range 15-45 m s 21 . The sound field is nearly uniform with depth in the upper 50 m with nearly all sound attenuation limited to the upper 2 m at all measured frequencies. A simple model of bubble trajectories based on the measured float trajectories finds that resonant bubbles at the high-frequency end of the observations (25 kHz) could easily be advected deeper than 2 m during tropical cyclones. Thus, bubble rise velocity alone cannot explain the lack of sound attenuation at these depths.

Autonomous and Lagrangian Ocean Observations for Atlantic Tropical Cyclone Studies and Forecasts

“…For example, an upper-ocean response analysis that used EM-APEX profiling floats deployed ahead of Hurricane Frances (2004) as part of the CBLAST program demonstrated the importance of using repeat ocean profilers to study the physics of the upper ocean's response to a storm (Sanford et al, 2011), confirming the theoretical model developed by Price (1981). Air-deployed Lagrangian floats equipped with ambient noise sensors measured wind speed and rainfall during the passage of Hurricane Gustav (2008) during CBLAST andTyphoons Megi (2010) andFanapi (2010) during ITOP, demonstrating the maturation of the Wind Observations Through Ambient Noise (WOTAN) technique (Zhao et al, 2014). Air-deployed surface drifters also played a critical role during these field programs by showing that drag coefficient behavior changes as it saturates at high wind speeds and may even decrease (Zedler et al, 2009).…”

The Saturation of Fluid Turbulence in Breaking Laboratory Waves and Implications for Whitecaps

“…Moreover, the few anecdotal measurements of the bubble Hinze scale available from the field are somewhat consistent with the saturation hypothesis, although it must be noted that the field observations of the Hinze scale do show more scatter than the laboratory data. Further indirect evidence of turbulence saturation can be found in Zhao et al (2014), who made measurements of the oceanic acoustic noise field in tropical cyclones at wind speeds between 10 and 50 m s 21 . While the overall power of the acoustic noise increased with increasing wind speed, the shape of the acoustic power spectrum (which is closely linked to the sizes and numbers of bubbles radiating sound during active wave breaking) remained relatively constant and was very similar to the acoustic power spectrum reported in Deane and Stokes (2002).…”

Seismic Remote Sensing of Super Typhoon Lupit (2009) with Seismological Array Observation in NE China

“…They carried out measurements of 10-50 Hz sound beneath Hurricane Gert in 1999 with a single hydrophone at 800 m depth, and suggested the sound at this frequency band was not affected by bubble attenuation and could be used to monitor the wind speed and quantify the destructive power of typhoons. Zhao et al [9] measured the underwater ambient sound levels at a frequency band of 40 Hz-50 kHz beneath three different typhoons using hydrophones onboard Lagrangian floats from 1 to 50 m depth, and analyzed the complex relationship between the sound level and the wind speed. However, the highlights of these underwater ambient sound studies were almost focused on the frequency band of several tens Hz and above, and the low-frequency (e.g., <1 Hz) noise generated by the pressure fluctuations due to ocean waves were often filtered out and removed.…”

“…Although the microseisms induced by typhoons have been measured and tried to track typhoons historically early in 1940s [31,32], the link between ocean storms and their generated ambient noise including microseisms has recently become a frontier topic in the sphere of seismology and ocean acoustics [9,11,33,34]. Recent examples of these types of studies include analysis on both surface and body wave microseisms generated by the 2005 Hurricane Katrina in the Gulf of Mexico using 150 Southern California stations [1]; tracking of western Pacific typhoons in 2006 using seismic records from OBSs and on-land stations [4]; seismological observations of ocean storms in the South China and East China Sea [6]; investigations of the microseisms induced by the Superstorm Sandy in 2012 as it approached the US east coast using the Earthscope Transportable Array (TA) stations [7]; investigation of the DF microseisms generated by the 2013 tropical cyclone Dumile on the seafloor with a large-scale network of 57 broadband OBSs in the southwestern Indian Ocean [8].…”