This paper compares the observed ambient sound levels at two very different sites, relating both to independent estimates of wind speed and rain rate. The spectra for wind-only conditions at the two sites show great differences, especially at low wind speed. The spectra associated with rain were sufficiently different from the wind-only spectra (either in terms of spectral slope or the intensity at 14.5 kHz) to support the development of a generic rather than site-specific rain detection algorithm

Birch, K.G.

Campbell, J.M.

Guymer, T.H.

Quartly, G.D.

Shannon, K.M.

English

NERC Open Research Archive

QUARTLY et al.1Intercomparison of ambient acoustic spectra in inland and coastal watersG. D. Quartlya, K. M. Shannona,b, T. H. Guymera, K. G. Bircha, J. M. CampbellaaSouthampton Oceanography Centre, Empress Dock, Southampton, Hants, UK,bAlso at University of Wales, Cardiff, UKAbstractThis paper compares the observed ambient sound levels at two very different sites, relating both toindependent estimates of wind speed and rain rate.  The spectra for wind-only conditions at the twosites show great differences, especially at low wind speed.  The spectra associated with rain weresufficiently different from the wind-only spectra (either in terms of spectral slope or the intensity at14.5 kHz) to support the development of a generic rather than site-specific rain detection algorithm.PACS — 43.30+m ; 43.30.Nb ; 43.50.Rq ; 43.60.Pt ; 92.40.Ea1. IntroductionRain at sea is important for the effect it has on thedamping of waves and the enhancement of air-seagas exchange.  However, it is the great concern aboutits effect on ocean circulation, through changes in thesalinity of surface waters, that has led to thewidespread interest in the global monitoring ofrainfall.  Tests in controlled environments haveshown rain to generate a distinctive spectrum, readilydistinguishable from that due to wind, andconsequently there have been plans to use acousticrain gauges (ARGs) to provide regular rainfallmonitoring in selected areas.  Jeff Nystuen hasperformed deep ocean tests of such equipment usingboth drifters1 and the TOGA-TAO moorings in thePacific2.Here we revert to tests close to land, where theavailability of a panoply of other sensors allows amore detailed validation.  The equipment used inbased on sensors and circuitry developed byMetocean Ltd of Nova Scotia, which, every 1.5minutes, records the acoustic intensities in 16channels spanning 500 Hz to 50 kHz.  We deployedthe equipment in two very different locations: LochEtive, a sheltered saltwater loch in southwestScotland, and secondly off the west Wales coast nearAberporth, where the buoys were exposed to swellfrom the Atlantic.  In both locations neighbouringmeteorological buoys provided regular wind speedmeasurements.  A number of rain sensors wereavailable; we use here data on a 5 km x 5 km gridderived from the Met Office radars, as theiravailability was common to both sites.  In section 2we compare the wind-only spectra observed at thetwo sites, and in section 3 we examine how well thespectra of rain can be distinguished from those ofwind.2. Comparison of wind-only spectraWe discuss first the acoustic intensities generated bywind, as its contribution is nearly always present.The meteorological records from the buoy site atAberporth were only available at hourly resolution,so, for both trials, we consider acoustic spectra inhourly ensembles.  Both  sets of  data have beenscreened for rain (using the rain radar data), and forLoch Etive we used only night-time data to minimisepossible anthropogenic noise.Figure 1 shows the mean acoustic intensities atchannels 4, 8 and 12 (2 kHz, 6 kHz and 20 kHz)plotted against wind speed.  Loch Etive had beenchosen for early trials as it was known to be anexceptionally quiet environment, with little waveactivity or noise from shipping.  The observations inFig. 1a show that at low wind speeds the observedintensities were typically 17 dB less than atAberporth for the same conditions.  The discrepancyis less for higher wind speeds and frequencies.  LochEtive was a sheltered location, such that wind speedsabove 8 ms-1 were rare.  On the other hand, suchwinds were more common at Aberporth, enabling themean intensity curve to be determined for higherwind speeds than in Loch Etive.Above approximately 6 kHz, there is no wind speeddependence below 2 ms-1, but such a dependenceexists at the lower frequencies.  However the moststriking observation from the intercomparison of thetwo datasets is the degree of scatter in the Loch Etivedata. All the plots show greater variability at lowwind speeds (not surprising given the use oflogarithmic units), but the variability at Loch Etiveremains high over a larger range of wind speeds.  Ofparticular note is the large apparent variability at lowwind speeds for the 20 kHz data in Loch Etive (Fig.1c).  It is probable that some other noise source isintermittently contributing to the high frequencysignal,  but  we  lack  the   ancillary  observations  toQUARTLY et al.2   2    6   10   253545556575   2    6   10   203040506070   2    6   10   102030405060a) b) c)Acoustic intensity (dB re 1µPa2 /Hz)Wind speed (ms-1) Wind speed (ms-1) Wind speed (ms-1)Figure 1.  Observed acoustic intensities as a function of wind speed at a) 2 kHz, b) 6 kHz and c) 20 kHz. (Note thescaling on the y-axes are different.)  The light circles (dark crosses) show individual hourly averages for Loch Etive(Aberporth), and the light (dark) line shows the average intensity for each 1ms-1 bin.  [ Note the wind speed datafrom Aberporth are quantized at 0.5 ms-1. ]confirm this.  In nearly all cases, the hourly averagesfor a given set of conditions (wind speed andlocation) show a greater degree of scatter than theindividual 1.5-minute records do within an hour.This indicates that in 'rain-free' conditions knowledgeof the wind speed alone is not sufficient to explainthe observed acoustic levels.  The acoustic intensitiesdue to wind may be varying due to wavedevelopment (fetch), atmospheric stability or thespasmodic existence of non-meteorological sources.3. The detection of rainThe underwater acoustic spectra associated withwind-only conditions are close to linear in log-logspace i.e. dB vs. log10(frequency).  We determinedthe spectral slopes by fitting a line over the span 0.5-10 kHz (Fig. 2a) and then examined the variation ofthis parameter with wind speed and theabsence/presence of rain.  Fig. 2b shows that at highwind speeds the spectral slope observed at both sitesis around -15 dB/decade, whereas at low wind speedsthe magnitude of the slope can be much greater.  Theresults for Aberporth are consistent with the windcontribution always having a slope of ~ -14 dB/decade, but that at low wind speeds some other'background' source is more prominent.The underwater acoustic spectrum in medium toheavy rain (more than, say, 3 mm h-1) is expected tohave a much flatter slope.  The circles on Fig. 2bshow the spectral slopes when the radar data indicaterain within the pixel over Loch Etive.  These points,including both light and heavy rain confirm that rainis associated with less steep spectral slopes, albeitthat some values lie within the variability associatedwith 'wind only'.  However, as there is a greatdisparity between the cell size of the rain radarproduct and the effective listening area of the ARG, itis probable that several of the spectra are incorrectlyclassified.  There were few observations of rain atAberporth, and all those corresponded to drizzle,which does not affect the spectral slope.The second distinctive feature of rain-affected spectrais the 13-20 kHz 'drizzle peak', associated with thesmall bubbles produced by light rain3.  Byextrapolating the line fitted for the range 0.5-10 kHz,we can determine how much the observed intensity at14.5 kHz exceeds that expected (see Fig. 2a).  Figure2c shows the size of the drizzle peak, for theobservations both with and without rain.  At lowwind speeds the enhancement by rain of the intensityat 14.5 kHz  can be more than 10 dB, but the effect ismuted for wind speeds above 6 ms-1.  This is becausewind reduces the efficacy of bubble formation bysmall raindrops4.4. DiscussionThere are considerable differences in the acousticintensities recorded at the two sites.  The shelteredlocation in Loch Etive led to typically lower windspeeds than Aberporth, but the sound levels werelower at Loch Etive for nominally the same windspeeds, with this effect being particularly pronouncedat low wind speeds (Fig. 1a).  Although the buoyshad undergone major modification between the twodeployments, this is not believed to be the cause.The difference in absolute sound levels at the twosites are probably due to i) the absence of waveactivity in Loch Etive, ii) a much shallower bottom atQUARTLY et al.3Aberporth, affording the possibility of additionallydetecting sound reflected from the bottom, and iii)strong stratification in Loch Etive due to fresh waterriverine input.However, despite all this, various parameters derivedfrom the spectra correspond quite clearly withindependent observations of rain.  The drizzle peak isthe most distinctive feature.  Under low windconditions, light rain causes an enhancement of the14.5 kHz signal by more than 10 dB; this effect ismuch diminished in high winds.  Medium and heavyrain rates reduce the spectral slope, but this effectappears less clear cut, on account of the greatvariability for wind-only conditions at Loch Etive.Thus, despite the very different wind-only'background' spectra, similar techniques can be usedfor detecting rain at the two sites.  Quantitativeestimates of rain from acoustical sensors often showlarge errors5, but much of the mismatch may be dueto errors in the ancillary data or a marked differencein the spatial or temporal resolution of the acousticand validation data.AcknowledgementsWe are grateful to James Gregory for help with theprocessing, to Simon Keogh for provision of rainradar data, and to the many people who have helpedwith construction, deployment and site managementfor the trials.References[1] J. A. Nystuen. and H. D. Selsor, 1997: Weatherclassification using passive acoustic drifters,.Journal of Atmospheric and OceanicTechnology, 14, 656-666.[2] J. A. Nystuen. and M. J. McPhaden, 2001: Thebeginnings of operational marine weatherobservations using underwater ambient sound,Proceedings of Acoustical Oceanography 2001,Southampton, UK, Inst. of Acoustics, pp. 135-141.[3] H. C. Pumphrey, L. A. Crum, and L. Bjørnø,1989: Underwater sound produced by individualdrop impacts and rainfall. Journal of theAcoustical Society of America, 85, 1518-1526.[4] H. Medwin, A. Kurgan, and J. A. Nystuen,1990: Impact and bubble sound from raindropsat normal and oblique incidence. Journal of theAcoustical Society of America, 88, 413-418.[5] G. D. Quartly, J. W. Gregory, T. H. Guymer, K.G. Birch, D. W. Jones, and S. J. Keogh, 2001:How reliable are acoustic rain sensors?Proceedings of Acoustical Oceanography 2001,Southampton, UK, Inst. of Acoustics, pp. 142-148.0 2 4 6 8 100 2 4 6 8 10-25-20-15-10-50-5051015b)c)Wind speed (ms-1)Spectral slope (dB/decade)δ 14,  Drizzle peak (dB)AberporthLoch EtiveK E Y0.5 1 2 4 8 16 3220304050a)Frequency (kHz)δ14Acoustic intensity (dB re 1µPa2 /Hz)Figure 2.  a) Definition of spectral slope (over 0.5-10kHz) and drizzle peak, δ 14, using an examplespectrum from Loch Etive. b) Spectral slope and c)drizzle peak as a function of wind speed and rainrate.   [ The lines indicate the approximate envelopeof observations for wind-only conditions, with thesolid lines delineating the mean binned in intervals of0.5 ms-1, and the dashed lines representing ±2 std.dev. either side.  The crosses and circles are forindividual instants classified as rain according to theradar data. ]

Intercomparison of ambient acoustic spectra in inland and coastal waters

Southampton (e-Prints Soton)

QUARTLY et al.1Intercomparison of ambient acoustic spectra in inland and coastal watersG. D. Quartlya, K. M. Shannona,b, T. H. Guymera, K. G. Bircha, J. M. CampbellaaSouthampton Oceanography Centre, Empress Dock, Southampton, Hants, UK,bAlso at University of Wales, Cardiff, UKAbstractThis paper compares the observed ambient sound levels at two very different sites, relating both toindependent estimates of wind speed and rain rate.  The spectra for wind-only conditions at the twosites show great differences, especially at low wind speed.  The spectra associated with rain weresufficiently different from the wind-only spectra (either in terms of spectral slope or the intensity at14.5 kHz) to support the development of a generic rather than site-specific rain detection algorithm.PACS — 43.30+m ; 43.30.Nb ; 43.50.Rq ; 43.60.Pt ; 92.40.Ea1. IntroductionRain at sea is important for the effect it has on thedamping of waves and the enhancement of air-seagas exchange.  However, it is the great concern aboutits effect on ocean circulation, through changes in thesalinity  of  surface  waters,  that  has  led  to  thewidespread  interest  in  the  global  monitoring  ofrainfall.    Tests  in  controlled  environments  haveshown rain to generate a distinctive spectrum, readilydistinguishable  from  that  due  to  wind,  andconsequently there have been plans to use acousticrain  gauges  (ARGs)  to  provide  regular  rainfallmonitoring  in  selected  areas.    Jeff  Nystuen  hasperformed deep ocean tests of such equipment usingboth drifters1 and the TOGA-TAO moorings in thePacific2.Here  we  revert  to  tests  close  to  land,  where  theavailability of a panoply of other sensors allows amore  detailed  validation.    The  equipment  used  inbased  on  sensors  and  circuitry  developed  byMetocean  Ltd  of  Nova  Scotia,  which,  every  1.5minutes,  records  the  acoustic  intensities  in  16channels spanning 500 Hz to 50 kHz.  We deployedthe equipment in two very different locations: LochEtive,  a  sheltered  saltwater  loch  in  southwestScotland, and secondly off the west Wales coast nearAberporth, where the buoys were exposed to swellfrom the Atlantic.  In both locations neighbouringmeteorological buoys provided regular wind speedmeasurements.    A  number  of  rain  sensors  wereavailable; we use here data on a 5 km x 5 km gridderived  from  the  Met  Office  radars,  as  theiravailability was common to both sites.  In section 2we compare the wind-only spectra observed at thetwo sites, and in section 3 we examine how well thespectra of rain can be distinguished from those ofwind.2. Comparison of wind-only spectraWe discuss first the acoustic intensities generated bywind, as its contribution is nearly always present.The  meteorological  records  from  the  buoy  site  atAberporth were only available at hourly resolution,so, for both trials, we consider acoustic spectra inhourly ensembles.  Both  sets of  data have beenscreened for rain (using the rain radar data), and forLoch Etive we used only night-time data to minimisepossible anthropogenic noise.Figure  1  shows  the  mean  acoustic  intensities  atchannels 4, 8 and 12 (2 kHz, 6 kHz and 20 kHz)plotted against wind speed.  Loch Etive had beenchosen  for  early  trials  as  it  was  known  to  be  anexceptionally  quiet  environment,  with  little  waveactivity or noise from shipping.  The observations inFig. 1a show that at low wind speeds the observedintensities  were  typically  17  dB  less  than  atAberporth for the same conditions.  The discrepancyis less for higher wind speeds and frequencies.  LochEtive was a sheltered location, such that wind speedsabove 8 ms-1 were rare.  On the other hand, suchwinds were more common at Aberporth, enabling themean  intensity  curve  to  be  determined  for  higherwind speeds than in Loch Etive.Above approximately 6 kHz, there is no wind speeddependence  below  2  ms-1,  but  such  a  dependenceexists at the lower frequencies.  However the moststriking observation from the intercomparison of thetwo datasets is the degree of scatter in the Loch Etivedata. All the plots show greater variability at lowwind  speeds  (not  surprising  given  the  use  oflogarithmic units), but the variability at Loch Etiveremains high over a larger range of wind speeds.  Ofparticular note is the large apparent variability at lowwind speeds for the 20 kHz data in Loch Etive (Fig.1c).  It is probable that some other noise source isintermittently  contributing  to  the  high  frequencysignal,  but  we  lack  the   ancillary  observations  toQUARTLY et al.2    2     6    10   253545556575    2     6    10   203040506070    2     6    10   102030405060 a) b) c)Acoustic intensity (dB re 1µPa2/Hz)Wind speed (ms-1) Wind speed (ms-1) Wind speed (ms-1)Figure 1.  Observed acoustic intensities as a function of wind speed at a) 2 kHz, b) 6 kHz and c) 20 kHz. (Note thescaling on the y-axes are different.)  The light circles (dark crosses) show individual hourly averages for Loch Etive(Aberporth), and the light (dark) line shows the average intensity for each 1ms-1 bin.  [ Note the wind speed datafrom Aberporth are quantized at 0.5 ms-1. ]confirm this.  In nearly all cases, the hourly averagesfor  a  given  set  of  conditions  (wind  speed  andlocation) show a greater degree of scatter than theindividual  1.5-minute  records  do  within  an  hour.This indicates that in 'rain-free' conditions knowledgeof the wind speed alone is not sufficient to explainthe observed acoustic levels.  The acoustic intensitiesdue  to  wind  may  be  varying  due  to  wavedevelopment  (fetch),  atmospheric  stability  or  thespasmodic existence of non-meteorological sources.3. The detection of rainThe  underwater  acoustic  spectra  associated  withwind-only conditions are close to linear in log-logspace i.e. dB vs. log10(frequency).  We determinedthe spectral slopes by fitting a line over the span 0.5-10 kHz (Fig. 2a) and then examined the variation ofthis  parameter  with  wind  speed  and  theabsence/presence of rain.  Fig. 2b shows that at highwind speeds the spectral slope observed at both sitesis around -15 dB/decade, whereas at low wind speedsthe magnitude of the slope can be much greater.  Theresults for Aberporth are consistent with the windcontribution  always  having  a  slope  of  ~ -14 dB/decade, but that at low wind speeds some other'background' source is more prominent.The  underwater  acoustic  spectrum  in  medium  toheavy rain (more than, say, 3 mm h-1) is expected tohave a much flatter slope.  The circles on Fig. 2bshow the spectral slopes when the radar data indicaterain within the pixel over Loch Etive.  These points,including both light and heavy rain confirm that rainis associated with less steep spectral slopes, albeitthat some values lie within the variability associatedwith  'wind  only'.    However,  as  there  is  a  greatdisparity  between  the  cell  size  of  the  rain  radarproduct and the effective listening area of the ARG, itis probable that several of the spectra are incorrectlyclassified.  There were few observations of rain atAberporth,  and  all  those  corresponded  to  drizzle,which does not affect the spectral slope.The second distinctive feature of rain-affected spectrais the 13-20 kHz 'drizzle peak', associated with thesmall  bubbles  produced  by  light  rain3.   B yextrapolating the line fitted for the range 0.5-10 kHz,we can determine how much the observed intensity at14.5 kHz exceeds that expected (see Fig. 2a).  Figure2c  shows  the  size  of  the  drizzle  peak,  for  theobservations  both  with  and  without  rain.    At  lowwind speeds the enhancement by rain of the intensityat 14.5 kHz  can be more than 10 dB, but the effect ismuted for wind speeds above 6 ms-1.  This is becausewind reduces the efficacy of bubble formation bysmall raindrops4.4. DiscussionThere are considerable differences in the acousticintensities recorded at the two sites.  The shelteredlocation in Loch Etive led to typically lower windspeeds than Aberporth, but the sound levels werelower at Loch Etive for nominally the same windspeeds, with this effect being particularly pronouncedat low wind speeds (Fig. 1a).  Although the buoyshad undergone major modification between the twodeployments, this is not believed to be the cause.The difference in absolute sound levels at the twosites  are  probably  due  to  i)  the  absence  of  waveactivity in Loch Etive, ii) a much shallower bottom atQUARTLY et al.3Aberporth, affording the possibility of additionallydetecting sound reflected from the bottom, and iii)strong stratification in Loch Etive due to fresh waterriverine input.However, despite all this, various parameters derivedfrom  the  spectra  correspond  quite  clearly  withindependent observations of rain.  The drizzle peak isthe  most  distinctive  feature.    Under  low  windconditions, light rain causes an enhancement of the14.5 kHz signal by more than 10 dB; this effect ismuch diminished in high winds.  Medium and heavyrain rates reduce the spectral slope, but this effectappears  less  clear  cut,  on  account  of  the  greatvariability for wind-only conditions at Loch Etive.Thus,  despite  the  very  different  wind-only'background' spectra, similar techniques can be usedfor detecting  rain  at  the  two  sites.    Quantitativeestimates of rain from acoustical sensors often showlarge errors5, but much of the mismatch may be dueto errors in the ancillary data or a marked differencein the spatial or temporal resolution of the acousticand validation data.AcknowledgementsWe are grateful to James Gregory for help with theprocessing, to Simon Keogh for provision of rainradar data, and to the many people who have helpedwith construction, deployment and site managementfor the trials.References[1] J. A. Nystuen. and H. D. Selsor, 1997: Weatherclassification  using  passive  acoustic  drifters,.Journal  of  Atmospheric  and  OceanicTechnology, 14, 656-666.[2] J. A. Nystuen. and M. J. McPhaden, 2001: Thebeginnings  of  operational  marine  weatherobservations using underwater ambient sound,Proceedings of Acoustical Oceanography 2001,Southampton, UK, Inst. of Acoustics, pp. 135-141.[3] H. C. Pumphrey, L. A. Crum, and L. Bjørnø,1989: Underwater sound produced by individualdrop  impacts  and  rainfall.  Journal  of  theAcoustical Society of America, 85, 1518-1526.[4] H.  Medwin,  A.  Kurgan,  and  J.  A.  Nystuen,1990: Impact and bubble sound from raindropsat normal and oblique incidence. Journal of theAcoustical Society of America, 88, 413-418.[5] G. D. Quartly, J. W. Gregory, T. H. Guymer, K.G. Birch, D. W. Jones, and S. J. Keogh, 2001:How  reliable  are  acoustic  rain  sensors?Proceedings of Acoustical Oceanography 2001,Southampton, UK, Inst. of Acoustics, pp. 142-148.0 2 4 6 8 100 2 4 6 8 10-25-20-15-10-50-5051015b)c)Wind speed (ms-1)Spectral slope (dB/decade)δ14,  Drizzle peak (dB)AberporthLoch EtiveKEY0.5 1 2 4 8 16 3220304050a)Frequency (kHz)δ14Acoustic intensity (dB re 1µPa2/Hz)Figure 2.  a) Definition of spectral slope (over 0.5-10kHz)  and  drizzle  peak,  δ14,  using  an  examplespectrum from Loch Etive. b) Spectral slope and c)drizzle peak as a function of wind speed and rainrate.   [ The lines indicate the approximate envelopeof observations for wind-only conditions, with thesolid lines delineating the mean binned in intervals of0.5 ms-1, and the dashed lines representing ±2 std.dev. either side.  The crosses and circles are forindividual instants classified as rain according to theradar data. ]

How reliable are acoustic rain sensors?

Impact and bubble sound from raindrops at normal and oblique incidence.

The beginnings of operational marine weather observations using underwater ambient sound,

Underwater sound produced by individual drop impacts and rainfall.

Weather classification using passive acoustic drifters,.

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Intercomparison of ambient acoustic spectra in inland and coastal waters

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