The investigation focuses on two key issues in satellite ocean color remote sensing, namely the presence of whitecaps on the sea surface and the validity of the aerosol models selected for the atmospheric correction of SeaWiFS data. Experiments were designed and conducted at the Scripps Institution of Oceanography to measure the optical properties of whitecaps and to study the aerosol optical properties in a typical mid-latitude coastal environment. CIMEL Electronique sunphotometers, now integrated in the AERONET network, were also deployed permanently in Bermuda and in Lanai, calibration/validation sites for SeaWiFS and MODIS. Original results were obtained on the spectral reflectance of whitecaps and on the choice of aerosol models for atmospheric correction schemes and the type of measurements that should be made to verify those schemes. Bio-optical algorithms to remotely sense primary productivity from space were also evaluated, as well as current algorithms to estimate PAR at the earth's surface

Deschamps, P.-Y.

Frouin, R.

English

NASA Technical Reports Server

NASA-CR-Z05679
INVERSION SCHEMES TO RETRIEVE ATMOSPHERIC AND OCEANIC
PARAMETERS FROM SEAWIFS DATA, NAGW-3498: FINAL REPORT
This investigation (PI. R. Frouin, Co-I: P.-Y. Deschamps) was conducted during
the period 1/ 1/93-12/31/96 with financial support from NASA' s Ocean Biology Program in
the amount of $330,535. Resources from the University of Lille, namely instrumentation
and the services of two engineers, Drs. Jean-Yves Balois and Pierre Lecomte, were utilized
for experiments at the Scripps Institution of Oceanography Pier, La Jolla. During part of
the grant period, i.e. from September 94 through August 96, the PI was on leave of
absence from the Scripps Institution of Oceanography, detached to NASA Headquarters
where he managed NASA's Ocean Biology Program and served as Program Scientist for
SeaWiFS and MODIS (ocean aspects). The personnel partly supported by the grant
included, in addition to the PI and a programmer analyst, Mr. John McPherson, two post-
graduate researchers, Drs. Beatrice Berthelot and Eric Pouliquen, and two graduate
students, Ms. Myriam Schwindling and Mr. Bertrand Fougnie.
1. Results
Though the original objective of the investigation was to develop new inversion
schemes to retrieve atmospheric and oceanic parameters from SeaWiFS data, including
neural nets, it was realized that the SeaWiFS project would be better served by focusing on
two key issues in satellite ocean color remote sensing, namely the presence of whitecaps on
the sea surface and the validity of the aerosol models selected for the atmospheric correction
of SeaWiFS data. Therefore, experiments were designed and conducted at the Scripps
Institution of Oceanography Pier, La Jolla to measure the optical properties of whitecaps
and to study the aerosol optical properties in a typical mid-latitude coastal environment.
CIMEL Electronique sunphotometers, now integrated in the AERONET network, were
also deployed permanently in Bermuda and in Lanai, calibration/validation sites for
SeaWiFS and MODIS. Original results were obtained on the spectral reflectance of
whitecaps (Frouin et al., 1996) and on the choice of aerosol models for atmospheric
correction schemes and the type of measurements that should be made to verify those
schemes (Swindling et al., 1994, 1997).
Bio-optical algorithms to remotely sense primary productivity from space were also
evaluated (Berthelot and Deschamps, 1994), as well as current algorithms to estimate PAR
at the earth's surface (Frouin and Pinker, 1995). The PI chaired the "Atmospheric
Correction over the Oceans" session of a NASA-sponsored workshop on "Aerosols and
Atmospheric Correction" held on April 15-19, 1996 in Washington, DC, which reviewed
the issues/problems that remain to be addressed/solved before the atmospheric correction
can be qualified as generally accurate over the range of conditions expected to be
encountered over the oceans (Tanr_ et al., 1997). The major results and conclusions are
summarized below.
a. The spectral reflectance of sea foam measured at the Scripps Institution of
Oceanography Pier was found to decrease substantially in the visible and near-infrared,
contrary to findings of previous studies, theoretical as well as experimental. Values in the
visible (0.44 gin) were reduced by typically 40% at 0.85gin, 50% at 1.02 I-tin, and 85% at
1.65gm. The spectral effect was explained by the nature of foam, which is composed of
large bubbles of air separated by a thin layer of water (foam stricto-sensu) and of bubbles
of air injected in the underlayer. The presence of bubbles in the underlayer enhances water
absorption and, thus, reduces reflectance in the near-infrared. For ocean color remote
sensing, affected by the presence of foam and aerosols, the consequences of neglecting the
spectral dependence of foam are dramatic. With only a small amount of foam, in the
presence of aerosols or not, and thus irrespective of aerosol type, the errors in the retrieved
https://ntrs.nasa.gov/search.jsp?R=19980007211 2020-06-16T00:38:42+00:00Z
water reflectance at 0.44 gm are above 0.01, which does not meet the accuracy goal of
0.001 for biology applications. The measurements also indicated that foam significantly
affects the retrieval of aerosol turbidity at 0.85 and 1.02 lain for wind speeds above 10 m/s,
but impacts minimally turbidity estimates at 1.65 gm.
b. Sun transmittance and sky radiance were measured spectrally at the Scripps
Institution of Oceanography Pier, La Jolla during the winters of 1993 and 1994. Sun
transmittance was also measured aboard R/V Wecoma and on Catalina Island during the
1994 CALCOFI winter cruise. The data were analyzed 1) to verify whether the aerosol
models selected for satellite ocean color remote sensing are adequate and 2) to identify what
type of atmospheric optics measurements should be performed to verify atmospheric
correction algorithms for satellite ocean color radiances. Aerosol optical thicknesses at
0.87gm were generally low in La Jolla, with most values below 0.1 after correction for
stratospheric aerosols. Values were lower offshore (R/V Wecoma, Catalina Island), and no
systematic correlation was found between aerosol characteristics and meteorological
conditions. Sometimes, however, small (resp. high) Angstrom coefficients were associated
with onshore airflow from the ocean (resp. offshore airflow from land). At such low
optical thicknesses, variability in aerosol scattering properties cannot be determined, and a
mean background model, specified regionally under stable stratospheric component, may
be sufficient for ocean color remote sensing from space. For optical thicknesses above 0.1,
two modes of variability characterized by Angstrom exponents of 1.2 and 0.5 and
corresponding to Tropospheric and Maritime models, respectively, were identified in the
measurements. The aerosol models selected for ocean color remote sensing allowed one to
fit, within measurement inaccuracies, the derived values of Angstrom exponent and
"pseudo" phase function (the product of single scattering albedo and phase function), key
atmospheric correction parameters. Additional aerosol models bring tittle more.
Importantly, the "pseudo" phase function can be derived from measurements of the
Angstrom exponent. Shipbome sunphotometer measurements at the time of satellite
overpass appear sufficient to verify the atmospheric correction of satellite ocean color
radiances.
e. In-situ bio-optical measurements from several oceanographic campaigns were
analyzed to derive a direct relationship between water column primary production, P, ocean
color as expressed by the ratio of reflectances R1 at 0.44gin and R2 at 0.55 gin, and PAR.
The study was restricted to Case I waters, for which the following algorithm was
proposed: log(P) = -4.286 -1.39 log(R1/R2) +0.621 log(PAR) with P in gC/m2/day and
PAR in J/m2/day. Using this algorithm, the rms. accuracy on P is 0.17 on a logarithmic
scale, i.e. a factor of 1.5. The requirements for central wavelength, spectral bandwidth,
and radiometric noise level were also investigated for the spectral bands to be used by an
ocean color space mission dedicated to estimating global primary production and the
associated carbon fluxes. Nearly all the useful information is provided by two spectral
bands centered at 0.44 and 0.55 gm, but the P accuracy appears to be weakly sensitive to
spectral bandwidth, which may therefore be enlarged by several tens of nanometers. The
sensitivity to radiometric noise, on the contrary, is strong, and a noise equivalent
reflectance of 0.005 degraded the P accuracy by a factor of 2.
d. Current satellite algorithms to estimate PAR at the earth's surface were
reviewed. PAR can be obtained directly from top-of-atmosphere solar radiance, which is
used to determine the transmissivity of the atmosphere. Since clouds do not absorb
significantly at PAR wavelengths, the radiative transfer modeling is generally simplified
compared to that for total insolation. The inaccuracies reported, about 10 and 6% on daily
and monthly time scales, respectively, are useful for modeling oceanic and terrestrial
primary productivity. The large short-term variability in the ratio of PAR and insolation,
essentially due to clouds, is reduced at those time scales, suggesting that reasonably
accurate PAR climatologies may be obtained from available insolation climatologies
(satellite or other). The proposed algorithms, however, require further validation.
Improvementsarealsoneededin situationsof cloudheterogeneityandin the presenceof
snowor iceat thesurface.
e.Theremainingissuesfor satelliteoceancolor remotesensingincludethe effects
of absorbingaerosols,the impactof aerosolverticalstructure,the influenceof thin clouds
in thesensor'sfield of view, theeffectof stratosphericaerosols,the validity of theaerosol
models, the influence of whitecapson the ocean surface, the sensor sensitivity to
polarization,the influenceof instrumentstraylight, the impactof angulardistribution of
water-leavingandglitter radiances,theseparationof caseI andcaseII waters,theeffectsof
sensorcalibrationerrors,andthevalidationof the retrievedwater-leavingradiances.The
mostcritical issues,however,areabsorbingaerosolsandwhitecaps.They canaffectlarge
oceanicareasandin theirpresencecurrentatmosphericorrectionschemeswill not perform
adequately.For absorbingaerosols,apromisingapproachmightbeto usespectralbandsin
the ultraviolet (as molecular scatteringincreaseswith decreasingwavelength, aerosol
absorptionalso increases).Ultravioletwavelengthsmight also improve the atmospheric
correctionmoredirectly, by constrainingthe aerosolpath radianceextrapolatedfrom red
andnear-infraredwavelengths.For whitecaps,morein-situ measurementsarein order, to
determinetheir optical propertiesand how these propertiesdepend on environmental
factors.Ideally,theeffectivewhitecapreflectanceshouldbedeterminedfor eachpixel. This
mightbepossibleusingappropriatespectralbandsin thenear-infrared.
2. Publications
Schwindling,M., P.-Y. Deschamps,andR. Frouin, 1994:Ground-basedoptical
measurementsfor thevalidationof oceancolor from space.Eos Trans., 75, 194.
Berthelot, B., and P.-Y. Deschamps, 1994: Evaluation of bio-optical algorithms to
remotely sense marine primary production from space. J. Geophys. Res., 99,
7979-7989.
Frouin, R., and R. Pinker, 1995: Estimating photosynthetically Active Radiation (PAR) at
the Earth's surface from satellite observations. Rein. Sen. Environ., 51, 98-107.
Frouin, R., M. Schwindling, and P.-Y. Deschamps, 1996: Spectral reflectance of sea foam
in the visible and near-infrared: in-situ measurements and remote sensing
implications. J. Geophys. Res., 101,14,361-14,371.
Kaufman, Y. J., D. Tanr6, H. R. Gordon, T. Nakajima, J. Lenoble, R. Frouin, H.
Grassl, B. M. Herman, M. D. King, and P. Teillet, 1997: Passive remote sensing
of tropospheric aerosol and atmospheric correction for the aerosol effect. J.
Geophys. Res.,102, 16,815-16,830.
Gregg, W. W., W. E. Esaias, G. C. Feldman, R. Frouin, S. B. Hooker, C. R. McClain,
and R. H. Woodward, 1997: Coverage opportunities for global ocean color in a
multi-mission era. IEEE Trans. Geoscience and Remote Sensing, accepted.
Schwindling, M. P.-Y. Deschamps, and R. Frouin, 1997: Verification of aerosol models
for satellite ocean color remote sensing, J. Geophys. Res., submitted.


Inversion Schemes to Retrieve Atmospheric and Oceanic Parameters from SeaWiFS Data

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