The 1 % and 1 cm perspective in deriving and validating AOP - BGD

23 Pages·2012·3.54 MB·English

BiogeosciencesDiscuss.,9,9487–9531,2012 Biogeosciences Dis
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doi:10.5194/bgd-9-9487-2012 s
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Thisdiscussionpaperis/hasbeenunderreviewforthejournalBiogeosciences(BG). p
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PleaserefertothecorrespondingfinalpaperinBGifavailable.
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The 1% and 1cm perspective in deriving s
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and validating AOP data products P
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S.B.Hooker1,J.H.Morrow2,andA.Matsuoka3 |
1NASAGoddardSpaceFlightCenter,OceanEcologyLaboratory,Greenbelt, Dis
Maryland20771,USA cu
2BiosphericalInstrumentsInc.,5340RileyStreet,SanDiego,California92110,USA ss
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3Universite´ Laval,AvenuedelaMe´decine,Que´becCity,G1V0A6,Canada n
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Received:29March2012–Accepted:15June2012–Published:27July2012 p
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Correspondenceto:S.B.Hooker([email protected])
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PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
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Abstract D
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A next-generation in-water profiler designed to measure the apparent optical proper- ss
ties(AOPs)ofseawaterwasdevelopedandvalidatedacrossawidedynamicrangeof ion
in-waterproperties.Thenewfree-fallinginstrument,theCompact-OpticalProfilingSys- P
a
p
5 tem(C-OPS),wasbasedonaclusterof19state-of-the-artmicroradiometersspanning er
320–780nmandanewkite-shapedbackplanedesign.Thekite-shapedbackplanein-
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cludestunableballast,ahydrobaricbuoyancychamber,pluspitchandrolladjustments,
to provide unprecedented stability and vertical resolution in near-surface waters. A D
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unique data set was collected as part of the development activity and the first major cu
s
10 fieldcampaignthatusedthenewinstrument,theMalinaexpeditiontotheBeaufortSea sio
inthevicinityoftheMackenzieRiveroutflow.Thedatawereofsufficientresolutionand n
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quality to show that errors – more correctly, uncertainties – in the execution of data a
p
sampling protocols were measurable at the 1% and 1cm level with C-OPS. A sensi- e
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tivityanalysisasafunctionofthreewatertypesestablishedbythepeakintheremote
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sensing reflectance spectrum, R (λ), revealed which water types and which parts of
15 rs D
the spectrum were the most sensitive to data acquisition uncertainties. Shallow river- is
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ine waters were the most sensitive water type, and the ultraviolet and near-infrared u
s
were the most sensitive parts of the spectrum. The sensitivity analysis also showed sio
n
how the use of data products based on band ratios significantly mitigated the influ-
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20 enceofdataacquisitionuncertainties.Theunprecedentedverticalresolutionprovided ap
e
highqualitydataproductsatthespectralendmembers,whichsubsequentlysupported r
an alternative classification capability based on the spectral diffuse attenuation coef- |
ficient, K (λ). The K (320) and K (780) data showed how complex coastal systems
d d d D
can be distinguished two-dimensionally and how near-ice water masses are different is
c
25 from the open ocean. Finally, an algorithm for predicting the spectral absorption due us
s
to colored dissolved organic matter (CDOM), denoted a (λ), was developed us- io
CDOM n
ingtheKd(320)/Kd(780)ratio,whichwasbasedonalinearrelationshipwithrespectto Pa
a (440),withover99%ofthevarianceexplained.Therobustnessoftheapproach p
CDOM e
r
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wasestablishedbyexpandingtheuseofthealgorithmtoincludeageographicallydif- D
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ferentcoastalenvironment,theSouthernMid-AtlanticBight,withnosignificantchange c
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in accuracy (approximately 98% of the variance explained). Alternative spectral end s
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members reminiscent of next-generation (340 and 710nm) as well as legacy satellite n
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5 missions(412and670nm)werealsousedtoaccuratelyderiveaCDOM(440)fromKd(λ) ap
ratios(94%ormoreofthevarianceexplained). e
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1 Introduction D
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Anumberofinternationaloceancolorsatellitesensorsweredesignedandlaunchedin s
s
the last decade and a half to support oceanographic studies and applications includ- io
n
ingthefollowing:theOceanColorandTemperatureScanner(OCTS),thePolarization P
10 a
andDirectionalityoftheEarth’sReflectance(POLDER)sensor,theSea-viewingWide pe
r
Field-of-viewSensor(SeaWiFS),twoModerateResolutionImagingSpectroradiometer
(MODIS)instrumentslaunchedontheEarthObservingSystem(EOS)TerraandAqua |
satellites, and the Medium Resolution Imaging Spectrometer (MERIS). All of these D
is
15 sensorshavecontributedsignificantlytothegeneralproblemofinvertingopticalmea- cu
surementstoderiveconcentrationestimatesofbiogeochemicalparameters,andsome ss
io
continuetoprovideregularcoverageoftheglobalbiosphere. n
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TheSeaWiFSandMODISmissionsareofparticularimportance,becausetheircal- a
p
ibration and validation capabilities were developed in parallel and jointly supported e
r
activitiesthatestablishedmanyoftherequirementsforoceancolorresearch,e.g.,the
20
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atmosphericcorrectionscheme.Aparticularlyimportantjointaccomplishmentwasthe
establishmentofaseparatesiteforvicariouscalibrationdata,whichinvolvedarotating D
is
deploymentofspeciallybuiltMarineOpticalBuoy(MOBY)unitsinaclear-watersiteoff cu
s
thecoastofLanai,Hawaii(Clarketal.,1997). s
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The worldwide deployments of commercial off-the-shelf (COTS) radiometers have n
25 P
beentheprimarysourceofvalidationdataforoceancolordataproducts,becausethey a
p
areoneofthefewmechanismstosamplethedynamicrangeinvolved.TheSeaWiFS er
9489 |
Bio-optical Archive and Storage System (SeaBASS) has provided long-term access D
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tothesedatafortheglobalcommunity(Hookeretal.,1994).COTSinstrumentshave cu
s
also been used for vicarious calibration, which is primarily an open-ocean problem s
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becauseoftheneedforspatialandtemporalhomogeneityduringinsitudatacollection, n
5 atasimilarlevelofefficacytocustomhardwarelikeMOBY(Baileyetal.,2008). Pap
TheabilitytouseCOTShardwareforvicariouscalibrationhasalsobeenconfirmed e
r
bytheBoue´epourl’acquisitiondeSe´riesOptiquesa` LongTerme(BOUSSOLE)project
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in the Ligurian Sea (Antoine et al., 2008). Principal difficulties with buoy solutions for
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AOPmeasurementsarethewave-inducedmotions,biofoulingofthesensors,anddam- is
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10 age from commercial and recreational boaters. Both MOBY and BOUSSOLE rely on us
unique structural choices for wave mitigation (spar and transparent-to-swell designs, sio
respectively),diverstokeepthesensoraperturesclean,andremotelocationstomiti- n
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gate(butnoteliminate)thenegativeaspectsofboattraffic. ap
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The central theme in the discussion presented here is the incremental pursuit of r
15 moreaccuratefieldobservationstoensureaccesstostate-of-the-artadvancesbymak- |
ingthesolutionscommerciallyavailable.Thecurrentchallengeinoceancolorremote
D
sensingistoextendtheaccomplishmentsachievedintheopenoceanandthemargins is
c
of the coastal zone into much shallower waters (McClain et al., 2006), e.g., estuaries us
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and rivers. This requirement is driven by the present focus of satellite observations, io
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20 which are inexorably tied to launching new missions based on novel research topics P
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andensuringthequalityoftheensuingsatellitedata. p
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The long-term NASA programmatic requirements for ocean color remote sensing r
span a range of scales and applications (Hooker et al., 2007): (a) global separation |
of pigments and ecosystem components, (b) high spatial and temporal resolution of D
25 near-shore waters, (c) active assessment of plant physiology and composition, and isc
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(d) determination of mixed layer depths. The corresponding programmatic research s
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questionsspanequallylargescales: io
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– How are oceanic ecosystems and their attendant biodiversity influenced by cli- ap
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mateorenvironmentalchanges,andhowwilltheseevolveovertime? r
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– How do carbon and other elements transition between oceanic pools and pass D
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through the Earth system; and how do biogeochemical fluxes impact the ocean c
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andplanetaryclimateovertime? s
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– How (and why) are the diversity and geographical distribution of coastal marine P
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habitatschanging,andwhataretheimplicationsforhumanhealth? p
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– Howdohazardsandpollutantsimpactthehydrographyandbiologyofthecoastal
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zoneandhumanactivities,andcantheeffectsbemitigated?
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Thesequestionsrequiremoreinterdisciplinaryscienceandgreaternumbersofobser- c
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vations in the land–sea boundary than prior ocean color missions. More importantly, s
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a higher accuracy in field observations is needed, because the types and diversity of n
10
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dataproductsinvolvesignificantlymoreopticallycomplexwatermassesthanbefore. a
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The principal objective of the results presented here is to initiate the preparedness e
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forthelaunchofthenext-generationofoceancolorsatellites(NRC,2007;NASA,2010)
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with the most capable COTS instrumentation in the shortest time possible. The latter
D
15 is required to ensure that the science teams can start collecting the baseline obser- is
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vations needed to begin formulating and testing the myriad details associated with u
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hypotheses,algorithms,anddatabasesforthenewmissions.Becauseoftheempha- sio
sis on the near-shore environment, which is typified by shallow water depths and an n
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opticallycomplexverticalstructure,thereistheaddedrequirementtodemonstratethat a
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20 the new technology can be validated in waters with unprecedented multi-dimensional r
heterogeneity.
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2 Background cu
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Theprincipaldataproductinoceancolorresearchistheradiantenergyemergingfrom n
the sea, the so-called water-leaving radiance, LW(λ), where λ denotes wavelength. Pap
25 Forthepurposesofgroundtruth–morecorrectlysea-truth–observations,LW(λ)can er
9491 |
be derived by extrapolating in-water measurements taken close to the sea surface D
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orobtaineddirectlyfromabove-watermeasurements.Formeaningfulapplications,an c
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extremely high radiometric accuracy is required. The SeaWiFS Project, for example, s
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establishedaradiometricaccuracytowithin5%absoluteand1%relative,andchloro- n
5 phylla(Chla)concentrationtowithin35%overarangeof0.05–50.0mgm−3 (Hooker Pap
andEsaias,1993).Infact,field-to-satellitecomparisons(ormatchups)aremadewith e
r
respect to the total chlorophyll a (TChl a) concentration, denoted [TChla]. Variables
+ |
explicitlyaccountingfortheglobalsolarirradiance,E (0 ,λ),atthetimeofdatacollec-
d D
tion–so-calledapparentopticalproperties(AOPs)–areusedformatch-upanalysis, is
10 because derivations of LW(λ) in identical waters, but different illumination conditions, cus
willdiffer. sio
Commercial sampling systems capable of measuring in-water AOPs in the open n
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ocean with an accuracy in keeping with calibration and validation requirements were ap
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refinedduringthepreparationandlaunchofSeaWiFSandthetwoMODISinstruments r
15 (HookerandMaritorena,2000).Somecommercialin-waterinstrumentswereshownto |
beacceptableinturbidcoastalwatersandatmospheresunderrestrictedcircumstances
D
(Hooker et al., 2004). Above-water methods were considered particularly appropriate is
c
forcoastalwaters,becausetheydidnothavetoresolvetheverticalcomplexityofthe us
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water column – which in coastal waters typically involves one or more optically dif- io
n
20 ferent layers close to the surface – the LW estimate was obtained directly (Hooker Pa
et al., 2002). Another advantage of the above-water approach was being able to ex- p
e
pand it to include atmospheric measurements, which are critical in coastal validation r
exercises,asfirstdemonstratedbyHookeretal.(2000)withtheSeaWiFSPhotometer |
Revision for Incident Surface Measurements (SeaPRISM). Problems associated with D
25 theinevitableplatformperturbationsassociatedwithabove-watersensorsystemswere isc
u
showntobesolvable(HookerandZibordi,2005),whichpermittedanetworkedcapa- s
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bilityofSeaPRISMunitsforremotesensingvalidationincoastalwaters(Zibordietal., io
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2004). The development of a telescoping mast for deploying solar technologies with P
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unobstructedviewing(Hooker,2010)hasvirtuallyeliminatedplatformcontaminationof D
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upwardanddownwardabove-waterobservations(Hlaingetal.,2010). c
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Despiteprogressinmakinghigh-qualitymeasurementsinthecoastalzone,thepri- s
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maryremotesensingperspectivehasbeenontheopenocean(HookerandMcClain, n
5 2000), because most recently the satellite sensors and project offices were designed Pap
aroundthefactthatthemajorityoftheocean–whichistosaythemajorityofthepixels e
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in a global image – is a desert. A single instrument architecture with the necessary
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spectral and sampling resolution to span the dynamic range of near-shore turbid wa-
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tersandatmospherestoopen-oceanbluewaterandblueskywasnotneededandnot is
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10 available. There was also the very significant practical problem that the field sensors us
availableatthetimewerephysicallytoolargetobeusedinashallowriverorestuary; sio
notonlybecauseofself-shadingconcerns,butalsobecauseaninstrumentthatisap- n
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proximately1mlongorlongeranddescendingontheorderof1ms−1 isverydifficult ap
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touseina2–5mdeepriver. r
15 Although free-falling, but tethered, in-water profilers can be floated away to avoid |
platform perturbations associated with the structure the instrumentation is being de-
D
ployed from (e.g., a research vessel), additional problems remain and are a function is
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ofthebasicdesign.Rocket-shapedprofilersusebuoyantfinsandaweightednose to us
s
verticallyorientthelightsensors,butregardlessoftheirlength,aratherhighdescent io
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20 speed is needed to maintain vertical stability to within reasonable thresholds (usually P
towithin5◦).Closetothesurface,whentherightingmomentassociatedwithreleasing ap
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theprofilerisestablished,largeoscillationsarecommonandmuchofthenear-surface r
data are unusable. In a normal sea state with swell and wind waves, the oscillations |
can be accentuated, and the first depths of usable data can be as deep as 3–5m, D
25 dependingonhowthelightsensorsaremountedontheprofiler. isc
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2.1 Methods io
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The significance of not acquiring useful data close to the sea surface is expressed ap
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directly in the processing scheme used to derive data products from the light r
9493 |
measurements.Theprocessorusedhereisbasedonawell-establishedmethodology D
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(Smith and Baker, 1984) that was evaluated in an international round robin (Hooker c
u
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et al., 2001) and shown to be capable of agreement at the 1% level when the pro- s
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cessingoptionswereassimilaraspossible.Completedetailsforthetermsanddepen- n
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5 dencies are available in the Ocean Optics Protocols (hereafter, the Protocols), which ap
initially adhered to the Joint Global Ocean Flux Study (JGOFS) sampling procedures e
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(JGOFS,1991)anddefinedthestandardsforNASAcalibrationandvalidationactivities
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(MuellerandAustin,1992).Overtime,theProtocolswereinitiallyrevised(Muellerand
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Austin,1995),andthenupdatedannually(Mueller2000,2002,2003). is
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10 TheProtocolsaredetailed,soonlyabriefoverviewforobtainingdataproductsfrom us
vertical profiles of upwelling radiance (Lu) plus upward and downward irradiance (Eu sio
and E , respectively) are presented here. In-water radiometric quantities in physical n
d P
units, P (i.e., Lu, Eu, or Ed), are normalized with respect to simultaneous measure- ap
+ e
ments of the global solar irradiance, E (0 ,λ,t), with t explicitly expressing the time r
d
15 dependence,accordingto |
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P(z,λ,t )=P(z,λ,t)Ed(0 ,λ,t0), (1) isc
0 Ed(0+,λ,t) uss
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where P(z,λ,t ) identifies the radiometric parameters as they would have been n
0 P
recorded at all depths z at the same time t , and t is generally chosen to coincide a
0 0 p
withthestartofdataacquisition.Forsimplicity,thevariabletisomittedinthefollowing er
text.Inaddition,anydatacollectedwhentheverticaltiltoftheprofilerexceeds5◦ are
20 |
excludedfromtheensuinganalysis.
D
Afternormalizationandtiltfiltering,anear-surfaceportionofEd(z,λ)centeredatz0 isc
andhavinghomogeneousopticalproperties(verifiedwithtemperatureandattenuation u
s
parameters)extendingfromz1=z0+∆zandz2=z0−∆zisestablishedseparatelyfor sion
25 theblue-greenandredwavelengths;theultraviolet(UV)isincludedintheintervalmost P
a
similartotheUVattenuationscales.Bothintervalsbeginatthesameshallowestdepth, p
buttheblue-greenintervalisallowedtoextenddeeperifthelinearityinln(cid:2)L (z,λ)(cid:3),as er
u
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determined statistically, is thereby improved. The negative value of the slope of the D
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regressionyieldsthediffuseattenuationcoefficient,K (λ),whichisusedtoextrapolate c
d u
thefittedportionoftheE profilethroughthenear-surfacelayertonulldepth,z=0−. ss
Fluctuations caused byd surface waves and so-called lens effects prevent accurate ion
P
5 measurements of Ed(λ) close to the surface. A value just below the surface (at null ap
depth z=0−) can be compared to that measured contemporaneously above the sur- e
r
face(atz=0+)withaseparatesolarreferenceusing
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E (0−,λ)=0.97E (0+,λ), (2) D
d d is
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where the constant 0.97 represents the applicable air–sea transmittance, Fresnel re- u
s
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10 flbeectttearntcheasn, a1n%dftohresiorrlaadriealnecveatrioenflsecatabnocvee3(E0u◦/aEndd),loawnd-tois-mdoedteerrmatienewdintdosapneaecdcsu.rTahcey ionP
distributionofEdmeasurementsatanydepthzinfluencedbywavefocusingeffectsdo ape
notfollowagaussiandistribution,solinearfittingofE inanear-surfacelayerispoorly r
d
constrained, especially if the number of samples is small. The application of Eq. (2) |
15 tothefittingprocessestablishesaboundaryconditionorconstraint forthefit(Hooker D
andBrown,2012). is
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TheappropriatenessoftheE extrapolationinterval,initiallyestablishedbyz andz , s
d 1 2 s
isevaluatedbydeterminingifEq.(2)issatisfiedtowithinapproximatelytheuncertainty io
n
of the calibrations (a few percent); if not, z and z are redetermined – while keeping P
1 2 a
theselecteddepthswithintheshallowesthomogeneouslayerpossible–untilthedis- p
20 e
(cid:2) (cid:3) r
agreement is minimized (usually to within 5%). The linear decay of ln P(z,λ) for all
lightparametersinthechosennear-surfacelayerarethenevaluated,andiflinearityis |
acceptable,theentireprocessisrepeatedonacast-by-castbasis.Subsurfaceprimary D
quantitiesatnulldepth,P(0−,λ),areobtainedfromtheslopeandinterceptgivenbythe isc
25 least-squareslinearregressionofln(cid:2)P(z,λ)(cid:3)versusz withintheextrapolationinterval uss
specifiedbyz1 andz2. ion
Thewater-leavingradianceisobtaineddirectlyfrom Pa
p
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L (λ)=0.54L (0−,λ), (3) r
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wheretheconstant0.54accuratelyaccountsforthepartialreflectionandtransmission D
is
oftheupwelledradiancethroughtheseasurface,asconfirmedbyMobley(1999).To c
u
s
accountfortheaforementioneddependenceofL onthesolarflux,whichisafunction s
W io
of atmospheric conditions and time of day, L is normalized by the (average) global n
W P
5 solarirradiancemeasuredduringthetimeintervalcorrespondingtoz1 andz2: ap
e
L (λ) r
R (λ)= W , (4)
rs E (0+,λ) |
d
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where R is the remote sensing reflectance. Normalized variables are the primary is
rs c
u
inputparametersforinvertingTChlaconcentrationfrominsituopticalmeasurements s
s
aspartofthe“OC”classofalgorithms(O’Reilleyetal.,1998),whichmeanstheyare io
n
10 centralvariablesforvalidationexercises. Pa
An additional refinement includes the bidirectional nature of the upwelled radiance p
e
field, which is to a first approximation dependent on the solar zenith angle. An early r
attempttoaccountforthebidirectionalityofL byGordonandClark(1981),following |
W
Austin(1974),definedanormalizedwater-leavingradiance,(cid:2)L˜ (λ)(cid:3) ,asthehypothet- D
15 icalwater-leavingradiancethatwouldbemeasuredintheabseWnceoNfanyatmospheric iscu
loss with a zenith Sun at the mean Earth–Sun distance. The latter is accomplished s
s
by adjusting Rrs(λ) with the time-dependent mean extraterrestrial solar irradiance, F0 ion
(ignoringalldependenciesexceptwavelengthforbrevity): P
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(cid:2)L (λ)(cid:3) =F (λ)R (λ), (5) r
W N 0 rs
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whereF (λ)isusuallyformulatedtodependonthedayoftheyearandisderivedfrom
20 0 D
look-uptables(Thuillieretal.,2003).Anadditionalcorrectionforaso-calledexact nor- is
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malizedwater-leavingradianceisrequiredforsatelliteandsea-truthmatchups(Mueller u
s
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andMorel,2003),butthatlevelofcompletenessisnotneededhere. io
n
The light absorbance of colored dissolved organic matter (CDOM) was measured P
a
25 using an UltraPath liquid waveguide system manufactured by World Precision Instru- pe
ments, Inc. (Sarasota, Florida). The detailed methodology is described in Matsuoka r
9496 |
etal.(2012),soonlyabriefsummaryispresentedhere.Watersampleswerecollected D
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from a Niskin bottle or a clean plastic container (for surface samples) into pre-rinsed c
u
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glassbottlescoveredwithaluminumfoil.Watersampleswereimmediatelyfilteredafter s
io
samplingusing0.2µmGHPfilters(AcrodiscInc.)pre-rinsedwith200mlofpurewater. n
P
5 Absorbancespectraofthefilteredwaterswerethenmeasuredatseafrom200–735nm ap
with1nmincrementswithreferencetoasaltsolution(thesalinityofthereferencewas e
r
adjustedtothatofthesample(towithin2salinityunits),preparedwithpurewaterand
|
granularNaClpre-combustedinanoven(at450◦Cfor4h).
D
AnUltraPathallowsfouropticalpathlengthsrangingfrom0.05–2.0m(i.e.,0.05,0.1, is
c
10 0.5, and 2.0m) to be selected. In most cases, a 2.0m path length was used, except us
forcoastalwatersattheMackenzieRivermouth,wherea0.1mpathlengthwasused. sio
The absorption coefficient of CDOM, a (λ), in units of per meter, was calculated n
CDOM P
fromthemeasuredvaluesofabsorbance(A),oropticaldensity,asfollows: ap
e
r
A(λ)−A¯(685)
a (λ)=2.303 , (6) |
CDOM l
D
where2.303isafactorforconvertingfromnaturaltobase10logarithms,l istheoptical is
15 c
u
path length (in meters), and for each absorbance spectrum, the 5nm average of the s
s
measured values of A(λ) centered around 685nm, A¯(685), was assumed to be zero io
n
andtheA(λ)spectrumwasshiftedaccordingly(Pegauetal.,1997;Babinetal.,2003). P
a
p
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2.2 Next-generationperspective r
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The emphasis on coastal processes in next-generation planning created a potential
20 D
void in the instrumentation needed to provide sea-truth observations at the neces- is
c
saryqualitylevel.Althoughexistingabove-watersensorscouldprovidetheneededL u
W s
s
measurements directly, they could not provide the desired water column properties. io
n
Existingfree-fallingin-waterinstrumentscouldprovidethelatter,butnotalwaysclose P
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25 tloontgheroscekaets-ushrfaapceedanddevnicoetsal(w1amysinaltetnhgethdeosrirmedorvee)rtthicaatlfreellsqouluictikolny,(baebcoauuts1emthse−y1)wweirteh per
9497 |
aslowdatarate(usually6Hz)foropticallycomplexwaters.Addingtothedifficultywith D
is
alegacy approachwasthatrocketsarethemoststablewhenmovingquickly,soslow- c
u
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ingtheirdescentusuallyledtohighdatalossesfromexcessiveverticaltilts(datathat s
isnotplanartowithin5◦ arerejectedwhenderivingL ). ion
W P
5 Anin-wateralternativewastomountthelightsensorsinawinch-and-cranedeploy- ap
mentsystem,butsuchanapproachhasdifficultymakingunperturbedmeasurements e
r
closetotheseasurface,becauseofthepresenceandmotionoftheshipordeployment
|
platform,exceptunderspecializedcircumstances(Zibordietal.,2002).Therewasalso
theneedtoreducethesizeofthesensorstoreduceself-shadingeffects(importantin Dis
c
10 turbidwaters)andtomakealternativedeploymentplatformsmoreaccessible(e.g.,re- us
motely or autonomously piloted vehicles) to expand the total number of observations sio
beingsubmittedtodatabases. n
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What was needed was much smaller light sensors with a higher sampling rate that ap
e
couldbemountedonanewbackplane thatfellthroughthewaterveryslowly.There- r
15 sulting high vertical resolution would allow the optical complexity of water masses to |
be resolved. That is, thin intrusive layers (perhaps of freshwater origin from rivers or
D
meltingice)wouldbeproperlysampledforthefirsttimeinafreelyfallingpackage,as is
c
wouldtherapidlightvariationsinclearopen-oceanwaters.Bothofthesecomplexities us
s
wereusuallyaliasedinthesamplingbylegacydevices.Topromoteinternationalpart- io
n
20 nerships, a COTS solution available to all scientists was attractive. The basic design P
a
criterionwasaCompact-OpticalProfilingSystem(C-OPS)thatwasequallycapableof p
e
samplingshallow(2m)riversandtheopenocean(hundredsofmeters). r
The first step in developing a new free-falling profiler was to enhance a COTS ra- |
diometer as the starting point for designing the new sensors and testing the pro- D
25 filer in the laboratory and field. The Biospherical Surface Ocean Reflectance Sys- isc
u
tem (BioSORS), an above-water system manufactured by Biospherical Instruments s
s
Inc.(SanDiego,California)with19wavebands,wasusedtotestthenewsizereduction io
n
andcharacterizationconceptswhileretaininganapproximately10-decaderangeinre- P
a
sponsivity(Hookeretal.,2010a).Thelatterwascriticaltothenewapproach,because pe
r
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an anticipated above-water application of the new sensor technology was to sup- D
is
portnext-generationjointocean-atmospheremissions,likeAerosol-Cloud-Ecosystems c
u
s
(ACE) and Pre-Aerosol, Clouds, and Ocean Ecosystem (PACE). A common architec- s
io
tureforbothabove-andin-waterinstrumentswasimagined,becausethiswouldreduce n
P
5 costs and make the new COTS technology more accessible for the global research ap
community. e
r
For next-generation missions, it was anticipated that a more sophisticated above-
|
water version of the new sensor system would function as a radiometer to sample
D
the ocean and as a sun photometer to sample the atmosphere (Hooker et al., 2012). is
c
10 Such a capability had already been established with the aforementioned SeaPRISM us
device, but a more capable system with polarization and a larger spectral range from sio
theultraviolet(UV)totheshort-waveinfrared(SWIR)wasneededfornext-generation n
P
satellitemissions.Atthisearlystage,theemergingfull-designconcepthadtoinclude ap
e
ahyperspectralcomponenttosupportmissionsliketheGeostationaryCoastalandAir r
15 PollutionEvents(GEO-CAPE)andtheHyperspectralInfraredImager(HyspIRI). |
The next-generation missions placed an emphasis on adding high-quality calibra-
D
tion and validation data products from the UV and NIR domains. These spectral end is
c
members posedasignificantsamplingproblemforin-waterdataacquisition,because us
s
the signals are usually highly attenuated and must be accurately recorded very close io
n
20 totheseasurfacewherewavefocusingeffectsdominate.Consequently,althoughthe P
a
newsensordesignstartedoutbasedonanabove-waterperspective,thenextcritical p
e
stagewasassociatedwiththeuniqueaspectsoftheuseofthesensorsaspartofan r
in-waterprofiler. |
D
is
c
3 Akite-shapedprofiler u
s
s
io
ThelessonslearnedwiththeBioSORSsensorswereincorporatedintoanewin-water n
25 P
profilercalledtheBiosphericalProfiler(BioPRO),a19-channel(λ )devicebasedon a
19 p
a proven rocket-shaped design, although the fins were not buoyant (Hooker et al., er
9499 |
2010b).Thetailbuoyancycamefromafoamcollarattachedtothemainbodyattheend D
is
oftheprofiler,andthetwofinsweresolidplastic.Theoveralllengthwasapproximately c
u
s
0.6m,soitwasarathercompactdesign(typicalfree-fallprofilersinuseforopen-ocean s
io
samplingatthetimewereabout1.2–1.8minlength).Usingonlythecollarweight,the n
5 descent speed was about 30cms−1; to achieve greater stability, weight was added, Pap
whichresultedinatypicaldescentspeedof40–60cms−1. e
r
Open-oceantestingofBioPROshowedthesensormodificationsto-dateresultedin
|
excellentperformancewithrespecttoestablishedstandards.Anopenocean(case-1)
D
intercomparisonwithalegacyinstrumentbasedoncommonwavelengthsshowedthe is
10 unbiased percent difference (UPD1) for each channel between the two profilers aver- cus
aged −7.6% to 0.3%, with an overall average of −2.2%, which is to within the cali- sio
brationuncertainty.Thelargestdifferencecorrespondedtothe510nmchannel,which n
P
was a source of bias and a problematic wavelength with a particular class of legacy ap
e
radiometers (Hooker and Maritorena, 2000). A least-squares linear regression of the r
15 datashowedalmostone-to-onecorrespondencetowithin4.2%,withover99%ofthe |
varianceexplained.
D
Thefollow-onstepsforthein-waterdevelopmentprogramcreatedanewfree-falling is
c
instrument called the Submersible Biospherical Optical Profiling System (SuBOPS). us
s
SuBOPScombinedtheincrementalchangesinradiometrywithanewcompactback- io
n
20 plane for mounting the light sensors that could descend more slowly – but also very P
stably (vertical tilts less than 5◦) – than legacy, rocket-shaped devices (Hooker et al., ap
e
2010b). The new backplane used a four-point harness reminiscent of a kite. The ori- r
entationofthelightsensorscouldbequicklyadjustedtocountercabletension(oran |
insitucurrent)andmovableflotationallowedthelightsensorstobetrimmed tomain- D
25 tain a planar geometry. In addition, one or more compressible air bladders contained isc
u
with a floodable hydrobaric buoyancy chamber allowed the instrument to loiter at the s
s
io
1TheUPDisdefinedas200(Y −X)/(Y +X),whereX isthereferenceinstrumentordata, n
P
whichinthiscaseisthelegacyprofiler,buttheotherdatasourceisconsideredequallyvalid. a
p
UPDstatisticsareusedheretodiscernbiases,whichshouldnotbepresent. er
9500 |
seasurfacebeforedescendingandreachingterminalvelocity,whichgreatlyimproved D
is
near-surfaceverticalresolution. c
u
s
Comparisons of SuBOPS with a legacy profiler showed SuBOPS recorded 519, s
io
829,and1138samplesinthefirst5,10,and15m,respectively,ofthewatercolumn, n
P
5 whereas the legacy instrument returned 38, 70, and 102 samples, respectively. With ap
a vertical sampling resolution of approximately 1cm in the upper 5m, SuBOPS cap- e
r
turedthehighfrequencyperturbationsassociatedwithwave-focusingeffectscloseto
|
theseasurfacesufficientlywelltominimizethealiasingnormallyencounteredwiththe
D
legacydevice.Undermanyconditions,thekite-shapedprofileralsoprovidedstabletilts is
c
10 during the retrieval of the instrument after descent was stopped, whereas the legacy us
instrumentrarelypermittedtherecordingofanupcast.Withtheadjustablefeaturesof sio
thenewbackplane,planarorientationofalllightsensorswasalmostalwaystowithin n
P
5◦,exceptduringsignificantlyadversecurrentsituations. ap
e
TheintercomparisonresultsbetweenSuBOPSandBioPROtookplaceinpredomi- r
15 nantlyeutrophic(case-2)waters,becausethefocuswastobegintheprocessofmak- |
ing better AOP observations in coastal waters. The average UPD for channels pre-
D
dominantlycommontothefirstBioPROintercomparisonrangedfrom−3.5%to3.6%, is
c
withanoverallaverageof1.3%,whichistowithinthecalibrationuncertainty.Aleast- us
s
squareslinearregressionofthedatashowedanalmostone-to-onecorrespondenceto io
n
20 within4%,withover95%ofthevarianceexplained. P
a
Followinganincrementalapproachtomanagerisk,thenextlevelofsophisticationin- p
e
volvedevaluatingnewradiometersthatwereemergingfromaseparateSmallBusiness r
Innovation Research (SBIR) development activity based on so-called microradiome- |
ters (Booth et al., 2010). One of the architectural advantages of the microradiometer D
25 approachwastoestablish–forthefirsttimeinoceanographicopticalinstrumentation isc
u
–asingleinstrumentdesignwithinherentflexibilityanddynamicrangeastobescal- s
s
able across all the sampling requirements for both above- and in-water optical mea- io
n
surements (Morrow et al., 2010a). A microradiometer had components that were so P
a
small,theyhadtobemachineassembled.Theouterdiameterof1.1cmwassetbythe pe
r
9501 |
photodetector and after the fore optics and metal shielding were applied, the overall D
is
lengthwas9.6cm.Theautomatedproductionapproach,withconformalcoatingofthe c
u
s
electronics,removedalmostalloftheinstrument-to-instrumentperformancevariability s
io
thathadplaguedlegacyinstruments,whichwerehandmade. n
P
5 In practice, system expansion for a radiance sensor built with microradiometers is ap
onlylimitedbydatarates,becauseeachmicroradiometer,afterapplicationofthefore- e
r
optics is, in fact, a radiance sensor. Irradiance sensors are considerably more con-
|
strained, however, because each microradiometer has to properly view the solitary
diffuser used in the construction of the cosine collector. Although the SuBOPS irradi- Dis
10 ance sensor was the starting point for designing the new C-OPS irradiance diffuser, cus
the21.4%smallersizeofthelatterwithrespecttotheformer,andtherigid,linearform sio
factorofmicroradiometers,posedchallenges. n
P
A microradiometer is a complex assembly of filter, photodetector, acquisition elec- ap
e
tronics, and microprocessor, all packaged in a thin metal cylinder. For multiple wave- r
15 bandinstruments,theindividualmicroradiometerscannotbetiltedtoorientthematthe |
center of the secondary diffuser and still maintain a small sensor diameter, because
D
microradiometersaretoolong.Tosolvethisproblem,aplano-convexlenswasdevel- is
c
oped to control the viewing geometry and center each microradiometer on the same us
areaofthelowerintermediatediffuser(Boothetal.,2010). sio
n
20 Thefirstrealizationoftheoriginalfree-fallingdesignconceptwastheC-OPS,which P
a
combined all the lessons learned to-date into a single device (Morrow et al., 2010b). p
e
Part of the attraction of sensor systems built from microradiometers includes their r
adaptability to sensor networking, for example, acquiring data from an ancillary sen- |
sor like a global positioning system (GPS) device or controlling an accessory device D
25 suchasashadowbandattachmentforthesolarreference(Bernhardetal.,2010).The isc
u
lattercanbeusedtoimprovetheself-shadingcorrectionbyprovidingmeasurementsof s
s
theskyirradiance.Figure1presentstheC-OPSinstrumentinitsmostadvancedstate io
n
including the use of a custom-blended, low-density polyurethane resin for buoyancy, P
a
p
e
r
9502 |
whichwasspeciallyformulatedtoproducearigidfoamthatismachinablewhileretain- D
is
ingaveryhighcrushandwaterresistance. c
u
s
Intermsofnext-generationobjectives,aprincipaladvantageofmicroradiometersis s
io
theirflexibilityinprovidingamultitudeofsystemconfigurationsandupgradepathsfor n
P
5 shallowandopticallycomplexwaters(Morrowetal.,2010a).Anotherbenefitofthemi- ap
croradiometertechnologyistheopportunityforanunprecedentedamountofsimplicity e
r
in assembly and repair. Both of these accomplishments were expected to reduce the
|
cost of acquiring and maintaining sensors with a state-of-the-art observational capa-
D
bility. Prior to C-OPS, changing, replacing, or repairing a filter or filter-photodetector is
c
10 combinationrequireddisassemblyoftheentireelectro-opticssectionoftheinstrument, us
whichwasatime-consumingandtediousprocedurewithaninherentriskofunintended sio
damagetoassociatedcomponents.Significanteffortwassubsequentlyrequiredtotest n
P
thesubassembliesandthenrecalibratethesensor. ap
e
In contrast, changing a filter or replacement of an entire microradiometer within r
15 C-OPS requires only a few hours to unplug the component and replace it before re- |
calibrating. In addition to the savings in time (and money), the potential for uninten-
D
tionalnegativeconsequencesisminimizedwiththemicroradiometerdesign,because is
c
somuchoftheelectronicsismodular.Consequently,therearesignificantlyfewercon- us
s
nectors and cables that must be dealt with. This also means instruments can be ini- io
n
20 tially populated with fewer microradiometers if resources are limited, and then easily P
a
expandedovertimeasresourcesorscienceobjectivesevolve. p
e
The C-OPS instrument was commissioned in mesotrophic (case-1) coastal waters r
andevaluatedineutrophic(case-2)coastalwatersbyintercomparingitwiththeSuB- |
OPSinstrument(Morrowetal.,2010b).TheC-OPSsolarreferencewasdifferentthan D
25 SuBOPS,becauseirradiancesensorsbasedonmicroradiometersuseaplano-convex isc
lenstoprojectthefluxexitingthesecondarydiffusertoallowaparallel,radialarrange- us
s
mentofmicroradiometers.Consequently,theintercomparisonsbetweenthetwoinstru- io
n
mentsystemsincludedabove-andin-watertrials. P
a
p
e
r
9503 |
Theaverageunbiasedpercentdifferenceforasubsetofninechannelsbetweenthe D
is
two in-water systems ranged from −7.0% to 6.5%, with an overall average of 1.8% c
u
s
(case-1)and−0.8%(case-2),whichistowithinthecalibrationuncertainty.Theleast- s
io
squares linear regressions of the data showed almost one-to-one correspondence, n
P
5 to within 2% for both water types, with 95% or more of the variance explained. The ap
above-waterreferenceshadaUPDrangeof−2.5to3.9%,withanoverallaverageof e
r
1.3% (for the same wavelengths as the in-water results), which is also to within the
|
calibration uncertainty. A least-squares linear regression of the data showed one-to-
D
onecorrespondencetowithin3.4%,withover99%ofthevarianceexplained. is
c
10 With the addition of the UV and NIR end-member bands to the in-water intercom- us
parisons, the UPD range continues to increase and reaches −50.7% at 780nm. The sio
reason for this increase is the presence of near-surface layers in the case-2 environ- n
P
ment. Although the two profilers were very similar in their capabilities (e.g., they both ap
hadverticaltiltstowithin1.5◦),theC-OPSprofilerhadthemostadvancedbackplane, er
15 whichallowedittoloiteratthesurfaceforalongerperiodoftimethanSuBOPS.Conse- |
quently,moreC-OPSdatawerecollectedinthepartofthewatercolumnthatwasthe
D
mostsensitivetodifferencesinverticalsamplingresolution,whichmeansthatthever- is
c
ticalextentoftheC-OPSextrapolationinterval,z1–z2,wasusuallysmallerforC-OPS uss
thanforSuBOPS,whilestillretainingalargenumberofdatapoints. io
n
20 ThedifferenceinsurfaceloiteringresultedintheC-OPSextrapolationintervalshav- P
a
ingalmost70%moredata(onaverage)thanSuBOPS,whentheintervalswerecom- p
e
parable;whentheintervalsweredifferent,theverticalextentoftheC-OPSextrapola- r
tionintervalwasabout29%less(onaverage).Byeithermeasure,theverticalresolu- |
tionofC-OPSwassubstantiallybetterthanC-OPS.Forhighlyattenuatedwavelengths D
25 (e.g., the open ocean NIR and the coastal ocean UV), this exposed an important dif- isc
u
ference between the two profilers and created the concept of estimating the degra- s
s
dation in producing data products from profiles wherein the data sampling was not in io
n
keeping with the optical complexity of the water column as a function of wavelength. P
a
Thefactthatthevalidationintercomparisonsweresuccessfullyconductedinthemost pe
r
9504 |
challenging(case-2)watersdemonstratedthatAOPmeasurementscouldbevalidated D
is
acrossawidedynamicrangeinwaterproperties.Thisresultsuggestedthatthesensi- cu
s
tivitiesforallspectraldomains(UV,blue,green,red,andNIR)–intermsofsampling s
io
resolution problems – could potentially be investigated in the field with a single high- n
P
5 resolutioninstrument(C-OPS). ap
e
r
4 The1%and1cmperspective |
D
Tomanagethedynamicrangeoftheinsitudata,theAOPprofilesareseparatedinto isc
u
threegroupsaccordingtothelocationofthepeakintheR (λ)spectrum:blue,green, s
rs s
and red, with the latter used if the peak is in the near-infrared (NIR) domain. These io
n
three categories roughly correspond to deep open-ocean, shallow coastal, and very P
10 a
shallowestuarine(orriver)waters.TheC-OPSinstrumentwasdeployedfromtheso- pe
r
called barge during the Malina field campaign within these three categories of water
types, and was used to collect data close to the ice edge on several occasions. The |
nominalverticalresolutionofC-OPSsamplingintheupper5mofthewatercolumnwas D
is
15 1cm, which permitted well-resolved sensitivity analyses to be performed on a variety cu
ofdeploymentpracticesthatcandegradethequalityoftheobservationsand,thus,the ss
io
dataproductsderivedfromthedata. n
P
Thefirstanalysisconsideredhereistheinfluenceofartificialverticaldisplacements a
p
onthedata.Anexamplesourceforsuchadisplacementwouldbeincorrectlydetermin- e
r
ingtheoffsetdistancebetweenthelightsensorapertureandthepressuretransducer
20
|
(Fig. 1). Calibration and validation uncertainties require this type of uncertainty, as
measuredbytherelativepercentdifference(RPD2),tobeunbiasedandlessthan1%. D
is
c
2TheRPDisdefinedas100(Y −X)/X,whereX isthereferencedata,whichinthiscase us
s
hasnoartificialverticaldisplacement.RPDstatisticsareusedheretoshowthelevelofbias, io
n
whichshouldbepresent,onceanartificialdisplacementisapplied. P
a
p
e
r
9505 |
4.1 Verticaldisplacements D
is
c
u
Ttioonboefgainntihnecraenaasliynsgiss,iztheeoRfaPnDairntifithceiadldeitseprmlacineamtioennto,δfR,rrsa(nλg)inisgcforonmsid1e–r3e2dcams.aTfhuensce- ssion
displacementsrepresentpotentialerrorsinLu(z,λ)andEd(z,λ)duetothefactthatthe Pa
p
5 depthz foreachlightmeasurementisdeterminedbythepressuretransduceratadif- er
ferent horizontal plane then each of the optical sensors, which also differ from each
|
other.TheRPDvaluesarecomputedbyfirstfollowingthemethodologyinSect.2.1to
establishthereferenceextrapolationintervals,whereinδ=0cm.Next,δ isincreased D
is
andthedataarereprocessedusingthereferenceextrapolationintervals,butwiththe cu
s
displacement applied to the original data. A 1% threshold is set for calibration and s
10 io
validation activities or algorithms requiring absolute radiometry (e.g., next-generation n
P
missionplanningforPACEandACE),sobiasestowithin1%canbeconsiderednegli- a
p
gible. e
r
The results for the blue water type (Fig. 2a) show all displacements have a dis-
|
cerniblebias,whichmeanstheprocessingscheme(andreferencedata)issufficiently
15
D
sensitivetodetectdisplacementsatthe1cmscale.Forthe1and2cmdisplacements, is
c
however,allbiasesarelessthan1%,andforthe4cmdisplacementonlythe780nm u
s
channelstartstoexceed1%.Forallotherdisplacements,thebiasesfirstincreasein sio
n
theNIRandUVdomains,andthensubsequentlyintheblueandredregions.Notably,
P
20 theblue-greendomainremainssubstantiallytowithin1%evenwithadisplacementas ap
e
largeas32cm(theapproximatelengthofaC-OPSradiometer). r
In the green water type (Fig. 2b), the 2cm displacement is barely contained within |
the 1% threshold, and all the displacements first show the greatest sensitivity in the
D
UV followed by the NIR. The reversal to a lower RPD value at 320nm is caused by is
c
25 thelowradiancesatthisvalueforwhichthedisplacementscancausenon-physicalre- us
s
sults,whichareflaggedbytheprocessorandignored(and,thus,notaveragedintothe io
n
statistics).Thesmallestamountofstructureforeachdisplacementcurveisinthevicin- P
a
ityofthespectralpeak(560nm).The32cmvaluemayseemexcessiveforarealistic p
e
r
9506 |