SeaWiFS Channels Research Articles

Chlorophyll-a Concentration Retrieval in the Optically Complex Waters of the St. Lawrence Estuary and Gulf Using Principal Component Analysis

Empirical methods based on band ratios to infer chlorophyll-a concentration by satellite do not perform well over the optically complex waters of the St. Lawrence Estuary and Gulf. Using a dataset of 93 match-ups, we explore an alternative method relying on empirical orthogonal functions (EOF) to develop an algorithm that relates the satellite-derived remote sensing reflectances to in situ chlorophyll-a concentration for the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Results show that an accuracy of 41% at retrieving chlorophyll-a concentration can be reached using the EOF method compared to 140% for the widely-used Ocean Chlorophyll 4 (OC4v4) empirical algorithm, 53% for the Garver-Siegel-Maritorena (GSM01) and 54% for the Generalized Inherent Optical Property (GIOP) semi-analytical algorithms. This result is possible because the EOF approach is able to extract region-specific radiometric features from the satellite remote sensing reflectances that are related to absorption properties of optical components (water, coloured dissolved organic matter and chlorophyll-a) using the visible SeaWiFS channels. The method could easily be used with other ocean-colour satellite sensors (e.g., MODIS, MERIS, VIIRS, OLCI) to extend the time series for the St. Lawrence Estuary and Gulf waters.

Using SeaWiFS Measurements to Evaluate Radiometric Stability of Pseudo-Invariant Calibration Sites at Top of Atmosphere

The Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) data from 1997 to 2001 are adopted to monitor the radiometric stability of six pseudo-invariant calibration sites (PICSs) at the top of atmosphere (TOA). Cloud-free and homogeneous observations of the spectral TOA reflectance ρ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TOA</sup> at eight SeaWiFS channels over these sites are fitted to the Ross-Li bidirectional reflectance distribution function (BRDF) model, and the time series of BRDF-normalized spectral TOA reflectance R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TOA</sup> is presented and analyzed afterward. Overall, good stability during the evaluated period is exhibited as more than half of the derived trends are statistically insignificant, whereas root mean square (RMS) of the BRDF modeling residuals reveal spectral dependence of the PICSs' stability at TOA, i.e., the uncertainty of R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TOA</sup> appears to be larger at shortwave visible (SV) channels (~2.5%) compared with that of red/NIR bands (~1%). In addition, the early mission data adopted in our study shows favorable reliability thus is recommended to be applied for similar purposes.

Simultaneous retrieval of aerosol and ocean properties by optimal estimation: SeaWiFS case studies for the Santa Barbara Channel

In this work, some sea viewing wide field‐of‐view sensor (SeaWiFS) images over the Santa Barbara Channel (SBC) are analysed. Pixel‐by‐pixel measurements of radiances at eight SeaWiFS channels and analytic Jacobians are simulated using a coupled atmosphere–ocean radiative transfer model. The inverse algorithm is based on optimal estimation with loosely constrained a priori data. The five‐element state vector has two aerosol (optical depth at 865 nm, bimodal fraction of particles) and three marine (chlorophyll concentration, detrital/dissolved‐matter absorption at 443 nm, and backscattering coefficient at 443 nm) parameters. The retrieval is stable and well posed; the results are smoother and show less spread than those derived from the standard SeaDAS v4.8 algorithm. For a 28 February 2003 SeaWiFS image, the average radiance residual is less than 1% for seven SeaWiFS channels, and less than 2% for the 765 nm channel. For a series of SBC SeaWiFS match‐up cases over a 4‐year period, estimated water‐leaving radiances agree well with field measurements.

Retrievals of a size parameter for phytoplankton and spectral light absorption by colored detrital matter from water-leaving radiances at SeaWiFS channels in a continental shelf region off Brazil

Many efforts are currently oriented toward extracting more information from ocean color than the chlorophyll a concentration. Among biological parameters potentially accessible from space, estimates of phytoplankton cell size and light absorption by colored detrital matter (CDM) would lead to an indirect assessment of major components of the organic carbon pool in the ocean, which would benefit oceanic carbon budget models. We present here 2 procedures to retrieve simultaneously from ocean color measurements in a limited number of bands, magnitudes, and spectral shapes for both light absorption by CDM and phytoplankton, along with a size parameter for phytoplankton. The performance of the 2 procedures was evaluated using different data sets that correspond to increasing uncertainties: (1) measured absorption coefficients of phytoplankton, particulate detritus, and colored dissolved organic matter (CDOM) and measured chlorophyll a concentrations and (2) SeaWiFS upwelling radiance measurements and chlorophyll a concentrations estimated from global algorithms. In situ data were acquired during 3 cruises, differing by their relative proportions in CDM and phytoplankton, over a continental shelf off Brazil. No local information was introduced in either procedure, to make them more generally applicable. Over the study area, the absorption coefficient of CDM at 443 nm was retrieved from SeaWiFS radiances with a relative root mean square error (RMSE) of 33%, and phytoplankton light absorption coefficients in SeaWiFS bands (from 412 to 510 nm) were retrieved with RMSEs between 28% and 33%. These results are comparable to or better than those obtained by 3 published models. In addition, a size parameter of phytoplankton and the spectral slope of CDM absorption were retrieved with RMSEs of 17% and 22%, respectively. If these methods are applied at a regional scale, the performances could be substantially improved by locally tuning some empirical relationships.

Preliminary results for SeaWiFS vicarious calibration coefficients in the Baltic Sea

The technique of vicarious calibration is used in connection with an atmospheric correction to improve the Sea viewing Wide Field of view Sensor (SeaWiFS) normalized water-leaving radiance by the first determination of mean vicarious calibration coefficients from in situ measurements in the Baltic Sea. A necessary adjustment of the SeaWiFS pre-flight calibration slope was found to be +3.5%, +0.3%, −1.7%, −0.4%, +0.8% and −1.3% for the first six SeaWiFS channels. The derived mean vicarious calibration coefficients are higher than the coefficients in the standard SeaWiFS Data Analysis System (SeaDAS) software but with similar shape and good agreement with other research results. The coefficients were used to obtain better normalized water-leaving radiance from SeaWiFS measurements in the Baltic Sea. The deviations of calculated to measured radiances in the open Baltic Sea are between 3% and 47% in the channels 412 to 670 nm, with the trend of higher deviations in the blue channels. The objective of radiance determination in all SeaWiFS channels with a maximum uncertainty of 5% in clear water regions is probably not reachable in the turbid water of the Baltic Sea.

SeaWiFS data analysis and match-ups with in situ chlorophyll concentrations in Danish waters

For the year 1999 all Sea viewing Wide Field of view Sensor (SeaWiFS) scenes of the Danish waters from the North Sea to the Baltic Sea were browsed, and a total of 47 SeaWiFS scenes with reasonably low cloud cover and, therefore, potential in situ match-ups were found and processed. The in situ data used as match-ups were collected on routine monitoring cruises by Danish and Swedish environmental authorities. A few stations in the North Sea, Skagerak and the western Baltic Sea were sampled, while most stations were located in Kattegat and the inner Danish waters. A turbid water SeaWiFS atmospheric correction algorithm was applied, since the standard SeaWiFS algorithm for chlorophyll-a (CHL) has been shown to be fairly inaccurate in turbid coastal waters. This is due to both inaccurate atmospheric and to relatively high and variable abundance of yellow substance. The application of the turbid atmospheric correction substantially improved the SeaWiFS CHL estimates. Regressions between SeaWiFS estimates using the OC2 and OC4 algorithms used in the SeaDAS software (versions 3.3 and 4.0, respectively) and in situ CHL values were made as well, and regression with a number of other possible reflectance ratios with SeaWiFS channels. The best correlation was found to be R2=0.54 using a double-ratio algorithm using both R510/R555 and R443/R670, while the OC4v4 had the second best correlation of R2=0.39. Among other single ratios, the R510/R555 had the highest correlation with CHL, which was expected since this is also the ratio that OC4v4 most often switches to in the waters investigated here. The range of CHL concentrations in this study was rather limited (all but three points from 0.5–3 mg m−3) so there is a need for inclusion of more data to expand the concentration range. This should be possible using also data from 2000, 2001 and onwards and, hereafter, a more ‘stable’ empirical algorithm can be derived for the Danish waters.

Atmospheric correction of SeaWiFS ocean color data in the Southern Hemisphere

An atmospheric correction algorithm for ocean color data in an extremely clear atmospheric condition is proposed. The correction is based on using the data for all eight SeaWiFS channels instead of using just two of the near-infrared channels. The upwelling radiation from a model atmosphere–ocean was calculated for each of the eight wavelengths of the SeaWiFS bands as a function of aerosol optical thickness. Next, a best-fit optical thickness is grossly determined using the generated look-up-table of the upwelling radiation. Then, a ratio of radiance at 443 nm to that at 560 nm taken by SeaWiFS is compared with the ratio derived using the LUT, to derive a suitable chlorophyll density. The new algorithm was applied to SeaWiFS data over the ocean off Cape Howe (−37.8° S, 150.7° E) and Brisbane (−26.6° S, 154.7° E) in East Australia in an extremely clear atmospheric condition. The data were taken in July 20, 2000 and June 8, 1998, respectively. A comparison of SeaDAS with our new algorithm was undertaken with regard to chlorophyll density. The general patterns of chlorophyll density agreed well. However, the new atmospheric correction code provided reasonable chlorophyll densities for some pixels which failed atmospheric correction using the SeaDAS code. A comparison of SeaDAS derived chlorophyll with sea-truth was carried out. The SeaDAS derived chlorophyll densities are higher than those of sea-truth, if the densities are greater than 0.4 mg/m −3. In contrast the new algorithm gave results which agreed well with the sea-truth. Furthermore, a comparison of SeaDAS derived aerosol optical thickness with that based on the present radiative transfer model was carried out. The data were taken off Tasmania, Australia (−48.9° S, 146.5° E) in October 16, 1999. Again, the general pattern of aerosol optical thickness agreed well. More precisely, optical thickness in a tubid atmosphere derived by SeaDAS code is 10% higher, whereas in clear atmospheres it is about a half, when compared to those of the present method.

SeaWiFS atmospheric correction by an approximate model and vicarious calibration

An approximate atmospheric model accounting for Rayleigh- and aerosol multiple scattering, and Rayleigh-aerosol coupling, is proposed for the atmospheric correction of SeaWiFS data. The accuracy of predicting water-leaving radiance is shown to improve by the use of vicarious calibration factors trimming the absolute radiometric calibration inaccuracy of the space-borne sensor and the model inaccuracy. Making use of data from three field campaigns carried out in 1998 at a calibration/validation site in the North Adriatic Sea, adjustment factors in the range of - 3% for the SeaWiFS channels between 412 and 765 nm have been computed for the pre-launch sensor calibration constants. The utilization of these adjustment factors in the atmospheric correction process is discussed making use of 12 match-ups exhibiting almost coincident in situ and remote sensing data of the study area. Results show differences generally smaller than 10% in the 490 over 555 nm ratio of satellite-derived and in situ normalized water-leaving radiances.

Observations of diffuse upwelling irradiance and chlorophyll in case i waters near the canary islands (Spain)

Two drifting buoys equipped with downward looking 4-channel colour sensors were deployed south-west of Gran Canaria island in the vicinity of a cyclonic eddy. Diffuse upwelling irradiance was measured in four channels (corresponding to some of the SeaWIFS channels) centred on wavelengths 440, 490, 570 and 670 nm. Chlorophyll concentrations were obtained at the start and end of each buoy deployment from profiles of fluorescence calibrated against extracted chlorophyll samples. A weighted chlorophyll concentration was calculated which corresponded to the depth detected by the colour sensor. The ratio of blue (440 nm) to green (570 nm) wavelength diffuse upwelling irradiance was found to vary in an ‘n’ shape throughout the day, and the size of the ‘n’ varied between days and buoy deployments. Possible causes for the observed variations and the problems encountered in calibrating the ratio of blue to green light with chlorophyll concentration are discussed. The surface reflectance ratio of blue to green irradiance was consistent with results in the literature for oceanic waters, with very low near-surface chlorophyll concentrations.

Detection of phytoplankton pigments from ocean color: improved algorithms

Passive ocean-color data at 32 wavelengths in the visible domain and laser-induced fluorescence line heights of chlorophyll and phycoerythrin, measured simultaneously from an aircraft in the New York Bight area, are used to examine the problem of developing algorithms for pigment retrieval from ocean-color data that would be capable of distinguishing between chlorophyll and phycoerythrin. Using factor analysis, it is shown that it is indeed possible to develop such algorithms. Furthermore, the wavelengths used in the algorithms can be reduced from 32 to 6 (similar to the SeaWiFS channels) without much loss in information. These multiwavelength algorithms yield significantly higher correlation coefficients for chlorophyll compared with the conventional blue-green ratio used for retrieval of this pigment. The Coastal Zone Color Scanner wavelengths appear to be inadequate for quantitative retrieval of the phycoerythrin signal.