AndreaAntonini(1),AlessandroMazza(1,2),SamanthaMelani(1,2),FrancescoSaba:ni(2)andAlbertoOrtolani(1,2)Affilia:ons:(1)LaMMAConsor:um(2)CNR-IBIMET

mailto:antonini@lamma.rete.toscana.itweb:www.lamma.rete.toscana.it

PRELIMINARYRESULTS

Alowcostacous:cdisdrometerasci:zensensorforrainfallmeasurements

INTRODUCTIONInthelastdecadestheincreasingoccurrenceofextremeweathereventsinurbanareashashighlightedsomeweaknessandgapsoftheobservingnetworksystemsusedbycivilprotec:onorganiza:ons.Morespecifically,theprecipita:onparameterischaracterisedbyveryhighspa:alandtemporalvariabilitysothatthemeasurementnetworksfailtomonitoritwiththenecessaryprecisiontoprovidemeaningfulinforma:onaturbanscale.Furthermore, the measurement networks are rarely able to dis:nguish the type of precipita:on with resolu:ons that significantly can impact on the urban areas.Polarimetricweather radars are instruments that canextract informa:onabout the typeofprecipita:on,but theyareexpensiveandoBen characterizedbydifficul:esduringinstalla:onprocedures(obtainingauthoriza:ons,riskforpopula:onexposuretoelectromagne:cfields,etc.).Disdrometersareabletocharacterisetheprecipita:ontypealsoonthedifferentheightlevelsofthever:calatmosphericprofileabovetheirinstalla:onsites.

The second processing algorithm is in the frequency domain: Short Time Fourier Transform (STFT) wasperformed. The preliminary STFT analysis in the frequency domain shows very clearly the impact of thehydrometeors,asthesignalconsistsofawidespectraldynamic.Theselectedcasestudy,rela:vetoashort:me period in which eight hydrometeors impact the sensor, shows that the eight drops can be clearlyrecognized in the frequencydomainbutnoteasily in the:medomain. Thisbehaviourpersists even in thequan:ta:veanalysisof thesignal,as the frequencydomain ismoresensi:veandcanmoredirectlyextractinforma:on. For this case, a short :me interval (1 s) has been chosen to avoid too many impacts ofhydrometeors,acircumstancethatwouldmakeuncleartheexample.

8thIPWGand5thIWSSMJointWorkshopBologna,3-7October,2016

Theinstrumentisanacous:cdisdrometerworkingwiththesameprincipleasanordinarymicrophonethatconvertssoundwavesinelectricsignals.Apiezodetector,fixedtotheboNomlayeroftheacous:cdisdrometer,convertsthevibra:onscausedbythedropsimpac:ngtheinstrumenttoanelectricsignal,whichistheinputfordataloggerandprocessing.Thedisdrometer (seeFigure)was realisedbyassembling:asheetofglassmountedonawood frame;apiezoelectricelement(usedasacontactmicrophone)fixed to theboNom layerof theglass;abipolarcableconnected to thebipolaroutputof thesensor. In theproposedprototype implementa:on, thesoundcardofapersonalcomputerwasusedas thesignalacquisi:onsystem. The limited sampling frequency of the sound card (44kHz) is sufficient to extract the needed spectral and amplitudeinforma:on:edtoprecipita:onoccurrence.

THEACOUSTICDISDROMETER

THEMETHODOLOGYThesignalprocessingiscomposedoftwoanalysisalgorithms;thefirstoneisinthe:medomain,wherethesignalamplitudecanbeeasilymanagedandthewaveformscausedbytheraindroparecharacterisedbyanini:al peak followedby dampingoscilla:ons due to the relaxa:onof thepiezo-element to the equilibriumstate.Thehydrometeorsimpac:ngthesensoraredis:nguishablebydisplayingtheiramplitudeversus:me,astheamplitudeoftheini:alpeakismuchgreaterthanthebackgroundnoise.Thepeakamplitudeiscorrelatedtothesizeofthe impac:ngdrop,aswellasthetransient:me.Thenumberofraindropsfalling inacertain:me interval is correlated to the rain rate in the same:me interval. The chosen:me interval is 1minute.Someanalysislimita:onsareduetothebackgroundnoiseamplitudeintroducedbythecableandthesensorand the external acous:c noise (i.e., air condi:oning machines in proximity of the sensors, traffic noises,airplanes,etc.)

SENSITIVITYTESTSSome laboratory tests were conducted inorder to inves:gate acous:c disdrometersensi:vity to raindrop diameter. Raindrops of1, 3 and 5 mm diameters were falling downfromaheightof18m(theroofofabuilding)inorder to reach their terminal velocity beforethe impact with the instrument, thussimula:ngarealrainfalldrop.

Raindropdiameter[mm]

TIME N°ofdrops Meandura:on

[s]

NormalizedMean

amplitude

1 16:05:35 13 0,017 0,133 16:01:11 56 0,039 0,435 15:44:52 27 0,046 0,76

The results show clearly the capability of thesystemindis:nguishingdifferentdropsizes.

The algorithm implemented for drop analysisprovides as output the drop dura:on andmaximumamplitude,togetherwiththenumberofdetecteddropsina:meframeof55s.

IntheTable,thesignalduetothevariousdropdiametersdifferssignificantlyfordura:onandamplitude.

1mm

3mm

5mm

CONCLUSIONSandFUTUREWORKSThepreliminaryresultsareencouragingversusareliableuseofthislowcostinstrument;itssensi:vitytodropdiameterispromisingindetec:ngdifferentprecipita:onregimes.Verypreliminarycomparisonwithraingauge(:ppingbucketresolu:on0.2mmsampling:me1m)havebeenmadeforcalibra:onpurposes.Anextendeddatasetwithvariousrainrateintensi:esisneededforobtainingacalibra:onrela:onship.Futuredevelopmentsareaimedtorefinethealgorithmsusingdifferentpiezoelectricsensorsindifferentsitesundervariousprecipita:onregimes.Finally,alsohailpar:cleswillbeinves:gated.

Somesta:s:calanalysesduringthefirsttwomonthsof 2016 were performed in order to verify thesystemrainratees:ma:oncapability,usinga:ppingbucketraingaugeasreference.The figures show the histograms of the number ofdetecteddrops,dropsignaldura:onandnormalizedamplitude for a rainrate of 1.2 mm/h (a :ppingbucketof0.2mmin1minute).Thehistogramscanprovideinforma:onaboutmostrecurrentvaluestobeusedforcalibra:onpurposes.

Asfurtherparameterthe sum of absolutevalues of samples isprovided.