Ambient sound in the ocean induced by heavy precipitation and the subsequent predictability of rainfall rate

88 Pages·1 MB·en_US

by  McGlothin, Charles C.;Nystuen, Jeffrey A.

OL

NAVAL POSTGRADUATE SCHOOL
Monterey, California
THESIS
AMBIENT SOUND IN THE OCEAN INDUCED BY
HEAVY PRECIPITATION AND THE SUBSEQUENT
PREDICTABILITY OF RAINFALL RATE
by
Charles C. McGlothin, Jr.
June, 1991
Thesis Advisor: Jeffrey A. Nystuen
Approved for public release; distribution is unlimited
T25632

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NavalPostgraduateSchool (Ifapplicable) NavalPostgraduateSchool
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6c.ADDRESS(City,State,andZIPCode) 7b ADDRESS(City,State,andZIPCode)
Monterey,CA 93943-5000 Monterey,CA 93943-5000
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11 TITLE(IncludeSecurityClassification)
AmbientSoundintheOceanInducedbyHeavyPrecipitationandtheSubsequentPredictabilityofRainfallRate
12.PERSONALAUTHOR(S) McGlothin,CharlesC,Jr.
13a TYPEOFREPORT 13b TIMECOVERED 14 DATEOFREPORT(year,month,day) 15 PAGECOUNT
Master'sThesis From To June 1991 78
16 SUPPLEMENTARY NOTATION
TheviewsexpressedinthisthesisarethoseoftheauthoranddonotreflecttheofficialpolicyorpositionoftheDepartmentofDefenseortheU.S.
Government.
17 COSATICODES 18 SUBJECTTERMS(continueonreverseifnecessaryandidentifybyblocknumber)
FIELD GROUP SUBGROUP RainfallRate,UnderwaterSoundSpectrum,Algorithm,AmbientSound,HeavyPrecipitation
19 ABSTRACT(continueonreverseifnecessaryandidentifybyblocknumber)
AnexperimentbytheNavalPostgraduateSchoolandtheNationalDataBuoyCenterwaspreformedintheGulfofMexicotocharacterizethe
underwatersoundgeneratedbyheavy precipiationandtodetermineifrainfallratecanbemeasuredusingimderwatersound. During thisstage
oftheesperiment,twenty-twodatasetswererecordedwithrainfallratesupto340mm/hr. Foragivenrainfallrate,itisfoundthatsoundlevels
fromheavyconvectiveprecipitationarehigheratthebeginningofthestormandwhenrainfallrateisincreasingthanattheendofthestorm
eventorwhenrainfallratesaredecreasing. Thismaybeduetochangesinthedropsizedistributionduringthelifecycleofthestormorto
variationsinthetemperaturedifferencebetweentheraindropandtheoceansurface. Veryheavyrainfall(> 150mm/hr)generatesnearsurface
bubblelayersorbubblecloudswhichattenuatesoundenergyathigherfrequencies(> 15kHz). Thedistinctive15kHzpeakinthesound
spectrumforlightrainisabsentduringheavyrainsuggestingthatthesoundproductionmechanismpreviouslyidentifiedforsmalldrops(0.8-
1.1 mmindiameter)issuppressedbyheavyraineventhoughthosesmalldropsareundoubtedlypresentduringheavyrainfallrates. Thesedata
showaveryhighcorrelationbetweenunderwatersoundlevelandthelogarithmoftherainfallrateexceptwhenhighwindspeeds(> 10ni/s)and
highrainfallrates(> 150mm/hr)arepresent. Anempiricalrainfallratealgorithmforconvectiveprecipitationisproposedsuggestingthatsound
energyisdirectlyproportionaltorainfallrate,howeveranyempiricalalgorithmwhichdoesnotadjustforchangingstormcharacteristicsshould
beusedwithcaution.
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Approved for public release; distribution is unlimited.
Ambient Soimd in the Ocean Induced by
Heavy Precipitation and the Subsequent
Predictability of Rainfall Rate
by
Charles C. McGlothin, Jr.
Lieutenant, United States Navy
B.S., Mississippi State University, 1982
Submitted in partial fulfillment
of the requirements for the degree of
MASTER OF SCIENCE IN PHYSICAL OCEANOGRAPHY
AND METEOROLOGY
from the
NAVAL POSTGRADUATE SCHOOL
June 1991
ABSTRACT
An experiment by the Naval Postgraduate School and the National Data Buoy
Center was performed in the Gulf of Mexico to characterize the underwater sound
generated by heavy precipitation and to determine if rainfall rates ofheavy precipitation
can be measured using underwater sound. During this stage of the experiment, twenty-
two data sets were recorded with rainfall rates up to 340 mm/hr. For a given rainfall rate,
it is found that sound levels from heavy convective precipitation are higher at the
beginning of the storm and when the rainfall rate is increasing than at the end of the
storm event or when rainfall rates are decreasing. This may be due to changes in the
drop size distribution during the life cycle ofthe storm or to variations in the temperature
difference between the raindrop and the ocean surface. Very heavy rainfall (rate > 150
mm/hr) generates near surface bubble layers or bubble clouds which attenuate sound
energy at higher frequencies (>15 kHz). The distinctive 15 kHz peak in the sound
spectrum for light rain is absent during heavy rain suggesting that the sound production
mm
mechanism previously identified for small drops (0.8 - 1.1 in diameter) is suppressed
by heavy rain even though those small drops are undoubtedly present during heavy
rainfall rates. These data show a very high correlation between underwater sound level
and the logarithm of the rainfall rate except when high wind speeds (> 10 m/s) and high
rainfall rates (> 150 mm/hr) are present. An empirical rainfall rate algorithm for
convective precipitation is proposed suggesting that sound energy is directly proportional
m
to rainfall rate, however any empirical algorithm which does not adjust for changing
storm characteristics should be used with caution.
IV